Valerio Basile
email: v.basile@rug.nl

Johan Bos
email: johan.bos@rug.nl

Towards Generating Text from Discourse Representation Structures

We argue that Discourse Representation Structures form a suitable level of languageneutral meaning representation for micro planning and surface realisation. DRSs can be viewed as the output of macro planning, and form the rough plan and structure for generating a text. We present the first ideas of building a large DRS corpus that enables the development of broad-coverage, robust text generators. A DRS-based generator imposes various challenges on micro-planning and surface realisation, including generating referring expressions, lexicalisation and aggregation.

Introduction

Natural Language Generation, NLG, is often viewed as a complex process comprising four main tasks [START_REF] Bateman | Natural Language Generation[END_REF]: i) macro planning, building an overall text plan; ii) micro planning, selecting referring expressions and appropriate content words; iii) surface realisation, selection of grammatical constructions and linear order; and iv) physical presentation, producing final articulation and layout operations. Arguably, the output of the macro planning component in an NLG system is some sort of abstract, language-neutral representation that encodes the information and messages that need to be conveyed, structured by rhetorical relations, and supported by information that is presupposed to be common ground.

We argue that the Discourse Representation Structures (DRSs) from Discourse Representation Theory [START_REF] Kamp | A Theory of Truth and Semantic Representation[END_REF] form an appropriate representation for this task. This choice is driven by both theoretical and practical considerations:

• DRT, being a theory of analysing meaning, is by principle language-neutral;

• Many linguistic phenomena are studied in the framework provided by DRT;

• DRT has a model-theoretical backbone, allowing applications to perform logical inferences with the aid of theorem provers.

As a matter of fact, DRT has means to encode presupposed information in a principled way [START_REF] Van Der Sandt | Presupposition Projection as Anaphora Resolution[END_REF], and connections with rhetorical relations are spelled out in detail [START_REF] Asher | Reference to Abstract Objects in Discourse[END_REF]. Moreover, the formal integration of DRS with named entities, thematic roles and word senses is natural.

These are, mostly, purely theoretical considerations. But in order to make DRSs a practical platform for developing NLG systems a large corpus of text annotated with DRSs is required. Doing this manually is way too costly. But given the developments in (mostly statistical) parsing of the last two decades we are now in a position to use state-ofthe-art tools to semi-automatically produce gold (or nearly gold) standard DRS-annotated corpora.

Such a resource could form a good basis to develop (statistical) NLG systems, and this thought is supported by current trends in broad-coverage NLG components [START_REF] Elhadad | An overview of SURGE: a reusable comprehensive syntactic realization component[END_REF][START_REF] White | Towards broad coverage surface realization with CCG[END_REF], that take deep semantic representations as starting points for surface realisation. The importance of a multi-level resource for generation is underlined by [START_REF] Bohnet | Broad coverage multilingual deep sentence generation with a stochastic multi-level realizer[END_REF], who feel the lack of such a resource is hampering progress in the field.

In this paper we show how we are building such a corpus (SemBank, Section 2), what the exact nature of the DRSs in this corpus is, and what phenomena are covered (Section 3). We also illustrate what challenges it poses upon micro planning and surface realisation (Section 4). Finally, in Section 5, we discuss how generating from DRSs relates to the traditional NLG pipeline. 2005), FrameNet [START_REF] Baker | The Berkeley FrameNet project[END_REF], the Penn Discourse TreeBank [START_REF] Prasad | The Penn Discourse TreeBank as a resource for natural language generation[END_REF], and several resources developed for shared tasks such as CoNNL and SemEval. Annotated corpora that combine various levels of annotation into one formalism hardly exist. A notable exception is OntoNotes [START_REF] Hovy | Ontonotes: the 90% solution[END_REF], combining syntax (Penn Treebank style), predicate argument structure (based on PropBank), word senses, and coreference. Yet all of these resources lack a level comprising a formally grounded "deep" semantic representation that combines various layers of linguistic annotation.

Filling this gap is exactly the purpose of Sem-Bank. It provides a collection of semantically annotated texts with deep rather than shallow semantics. Its goal is to integrate phenomena instead of covering single phenomena into one formalism, and representing texts, not sentences. SemBank is driven by linguistic theory, using CCG, Combinatory Categorial Grammar [START_REF] Steedman | The Syntactic Process[END_REF], for providing syntactic structure, employing (Segmented) Discourse Representation Theory [START_REF] Kamp | A Theory of Truth and Semantic Representation[END_REF][START_REF] Asher | Logics of conversation[END_REF] as semantic framework, and first-order logic as a language for automated inference tasks.

In our view, a corpus developed primarily for research purposes must be widely available to researchers in the field. Therefore, SemBank will only consists of texts which distribution isn't subject to copyright restrictions. Currently, we focus on English newswire text from an American newspaper whose articles are in the public domain. In the future we aim to cover other text genres, possibly integrating resources from the Open American National Corpus [START_REF] Ide | The manually annotated sub-corpus: a community resource for and by the people[END_REF]. The plan is to release a stable version of SemBank in regular intervals, and to provide open access to the development version.

The linguistic levels of SemBank are, in order of analysis depth: part of speech tags (Penn tagset); named entities (roughly based on the ACE ontology); word senses (WordNet); thematic roles (Verb-Net); syntactic structure (CCG); semantic representations, including events and tense (DRT); rhetorical relations (SDRT). Even though we talk about different levels here, they are all connected to each other. We will show how in the following section.

Size and quality are factors that influence the usefulness of annotated resources. As one of the things we have in mind is the use of statistical techniques in NLG, the corpus should be sufficiently large. However, annotating a reasonably large corpus with gold-standard semantic representations is obviously a hard and time-consuming task. We aim to provide a trade-off between quality and quantity, with a process that improves the annotation accuracy in each periodical stable release of SemBank.

This brings us to the method we employ to construct SemBank. We are using state-of-the-art tools for syntactic and semantic processing to provide a rough, first proposal of semantic representation for a text. Among other tools, the most important are the C&C parser [START_REF] Clark | Parsing the WSJ using CCG and Log-Linear Models[END_REF] for syntactic analysis, and Boxer [START_REF] Bos | Wide-Coverage Semantic Analysis with Boxer[END_REF] for semantic analysis. This software, trained and developed on the Penn Treebank, shows high coverage for texts in the newswire domain (up to 98%), is robust and fast, and therefore suitable for this task.

The output of these tools are corrected by crowd- WordNet (Fellbaum, 1998) pred(X,loon,n,2) rhetorical relations SDRT [START_REF] Asher | Logics of conversation[END_REF] rel(K1,K2,elaboration) sourcing methods, comprising (i) a group of experts that are able to propose corrections at various levels of annotation in a wiki-based fashion; and (ii) a group of non-experts that provide information for the lower levels of annotation decisions by way of a Game with a Purpose, similar to the successful Phrase Detectives [START_REF] Chamberlain | Addressing the Resource Bottleneck to Create Large-Scale Annotated Texts[END_REF] and Jeux de Mots [START_REF] Artignan | Multiscale visual analysis of lexical networks[END_REF].

Discourse Representation Structures

A DRS comprises two parts: a set of discourse referents (the entities introduced in the text), and a set of conditions, describing the properties of the referents and the relations between them. We adopt well-known extensions to the standard theory to include rhetorical relations [START_REF] Asher | Reference to Abstract Objects in Discourse[END_REF] and presuppositions ( [START_REF] Van Der Sandt | Presupposition Projection as Anaphora Resolution[END_REF]. DRSs are traditionally visualised as boxes, with the referents placed in the top part, and the DRS conditions in the bottom part. The convention in SemBank is to sort the discourse referents into entities (variables starting with an x), events (e), propositions (p), temporalities (t), and discourse segments (k), as Figure 2 shows.

The DRS conditions can be divided into basic and complex conditions. The basic conditions are used to describe names of discourse referents (named), concepts of entities (pred), relations between discourse referents (rel), cardinality of discourse referents denoting sets of objects (card), or to express identity between discourse referents (=). The complex conditions introduce embedded DRSs: implication (⇒), negation (¬), disjunction (∨), and modalities (2, 3). DRSs are thus of recursive nature, and the embedding of DRSs restrict the resolution of pronouns (and other anaphoric expressions), which is one of the trade mark properties of DRT.

The aim of SemBank is to provide fully resolved semantic representations. Obviously, natural language expressions can be ambiguous and picking the most likely interpretation isn't always straight-forward: Some pronouns have no clear antecedents, word senses are often hard to distinguish, and scope orderings are sometimes vague. In future work this might give rise to adding some underspecification mechanisms into the formalism.

DRSs are formal structures and come with a model-theoretic interpretation. This interpretation can be given directly [START_REF] Kamp | From Discourse to Logic; An Introduction to Modeltheoretic Semantics of Natural Language, Formal Logic and DRT[END_REF] or via a translation into first-order logic [START_REF] Muskens | Combining Montague Semantics and Discourse Representation[END_REF]. This is interesting from a practical perspective, because it permits the use of efficient existing inference engines developed by the automated deduction community. Applying logical inference can play a role in tasks surrounding NLG (e.g., summarisation, question answering, or textual entailment), but also dedicated components of NLG systems, such as generating definite descriptions, which requires checking contextual restrictions [START_REF] Gardent | Generating definite descriptions: Non-incrementality, inference and data[END_REF].

Figure 1 illustrates how SemBank provides the compositional semantics of each sentence in the text in the form of a CCG derivation. Here each token is associated with a supertag (a lexical CCG category) and its corresponding lexical semantics, a partial DRS. The CCG derivation, a tree structure, shows the compositional semantics in each step of the derivation, with the aid of the λ-calculus (the @ operator denotes function application).

Table 1 shows how the various levels of annotation are integrated in DRSs. Thematic roles (Verb-Net) are implied by the neo-Davidsonian event semantics employed in SemBank, and are represented as two-place relations. The named entity types form part of the basic DRS condition for names, and Word senses (WordNet) are represented as a feature on the one-place conditions for nouns, verbs and modifiers. Rhetorical relations are already part and parcel of SDRT. Hence, SemBank provides all these different layers of information within a DRS. Figure 2 shows an SDRS for a small text of SemBank.

Challenges for Text Generation

We believe that taking DRS as the basis for NLG will introduce not only variants of known problems, but will impose many new challenges. Here we focus on just three of them: generating referring expressions, lexicalisation, and aggregation.

Generating referring expressions

Viewed from a formal perspective, DRT is said to be a dynamic theory of semantics: the interpretation of an embedded DRS depends on the interpretation of the DRSs that subordinate it -either be sentenceinternal structure, or by the structure governed by rhetorical relations. A case in point is the treatment of anaphoric expressions including pronouns, proper names, possessive constructions and definite descriptions.

In DRT, anaphoric expressions are resolved to a suitable antecedent discourse referent. Proper names and definite descriptions are too, but if finding a suitable antecedent fails then a process usually referred to as presuppositional accommodation introduces the semantic material of the anaphoric expression on an accessible level of DRS ( [START_REF] Van Der Sandt | Presupposition Projection as Anaphora Resolution[END_REF]. The result of this process yields a DRS in which all presupposed information is explicitly distinguished from asserted information. This gives rise to an interesting challenge for NLG.

A DRS corresponding to a discourse unit will contain free variables for semantic material that is presupposed or has been linked to the preceeding context. On encountering such a free variable denoting an entity, the generator has a couple of choices in the way it can lexicalise it: as a quantifier, pronoun, proper name, or definite description. Even though the DRS context may provide information on names and properties assigned to this free variable, we expect it will be non-trivial to decide what properties to include in the corresponding expression. Text coherence probably plays an important role here, but whether thematic roles and rhetorical relations will be sufficient to predict an appropriate surface form remains a subject for future research. It is also interesting to explore the insights from approaches dedicated to generating referring expressions using logical methods [START_REF] Kees Van Deemter | Generating referring expressions that involve gradable properties[END_REF][START_REF] Gardent | Generating definite descriptions: Non-incrementality, inference and data[END_REF] with robust surface realisation systems.

Aggregation

Coordinated noun phrases are known to be potentially ambiguous between distributive and collective interpretations. A simple DRT analysis for the distributive interpretation yields two possible ways to generate strings: one where the noun phrases are coordinated within one sentence, and one where the noun phrases involved are generated in separate sentences. For instance, the DRSs corresponding to "Deep Purple and Pink Floyd played at a charity show" (with a distributive interpretation) and "Deep Purple played at a charity show, and Pink Floyd played at a charity show", would be equivalent. This is due to copying semantic material in the compositional process of computing the meaning of the coordinated noun phrase "Deep Purple and Pink Floyd". (Note that the collective reading, as in "Deep Purple and Pink Floyd played together at a charity show" would not involve copying semantic material, and would result in a different DRS, with a different interpretation.) It is the task of the aggregation process to pick one of these realisations, as discussed by [START_REF] White | Efficient realization of coordinate structures in combinatory categorial grammar[END_REF]. Doing this from the level of DRS poses an interesting challenge, because one would need to recognise that such an aggregation choice is possible in the first place. Alternatively, instead of copying, one could use an explicit operator that signals a distributive reading of a plural noun phrase, for instance as suggested by [START_REF] Kamp | From Discourse to Logic; An Introduction to Modeltheoretic Semantics of Natural Language, Formal Logic and DRT[END_REF]. Arguably, this is required anyway to adequately represent sentences such as "Both Deep Purple and Pink Floyd played at a charity show".

Lexicalisation

The predicates (the one-place relations) found in a DRS correspond to concepts of an hierarchical ontology. Time expressions and numerals have canonical representations in SemBank. The representation for noun and verb concepts are based on the synonym sets provided by WordNet (Fellbaum, 1998). A WordNet synset can be referred to by its internal identifier, or by any of its member word-sense pairs. For instance, synset 102999757 is composed of the noun-sense pairs strand-3, string-10, and chain-10. The lexicalisation challenge is to select the most suitable word out of these possibilities. Local context might help to choose: a "string of beads" is perhaps better than a "chain of beads" or "strand of beads". As another example, consider the synset {loon-2,diver-3} representing the concept for a kind of bird. American birdwatchers would use the noun "loon", whereas in Britain "diver" would be preferred to name this bird.

Discussion

In the DRT-based framework that we propose for generating text, the issue arises where in the traditional NLG pipeline DRSs play a role. In the introduction of this paper we suggested that DRSs would be output by the macro planner, and hence fed as input to the micro planner. On the one hand this makes sense, as in a segmented DRS all content to be generated is present and the rhetorical structure is given explicitly. But then the question remains whether the theoretical distinction between micro planning and surface realisation really works in practice or would just be counter-productive. Perhaps a revised architecture tailored to DRS generation should be tried instead. This issue is closely connected to the level of semantic granularity that one would like to see in a DRS. We illustrate this by four examples:

• pronouns -we have made a particular proposal using free variables, but we could also have followed [START_REF] Kamp | From Discourse to Logic; An Introduction to Modeltheoretic Semantics of Natural Language, Formal Logic and DRT[END_REF], introducing explicit referents for pronouns;

• distributive noun phrases -as the discussion in Section 4.2 shows, it is unclear which represention for distributive noun phrases would be most suitable for the purpose of sentence planning;

• sentential and verbal complements -should there be a difference in meaning representation between "Tim expects to win" and "Tim expects that he will win"?

• active vs. passive voice -should a meaning representation reflect the difference between active and passive sentences?

At this moment, it is not clear whether one wants a more abstract DRS and give more freedom to sentence planning, or a more specific DRS restricting the number of sentential paraphrases of its content. Perhaps even an architecture permitting both extremes would be feasible, where the task of micro planning would be to add more constraints to the DRS until it is specific enough for the surface realisation component to generate text from it. It is even thinkable that such a planning component would take over some tasks of the macro planner, making the distinction between the two fuzzier.

A final point that we want to raise is a possible role that inference can play in our framework. DRSs can be structurally different, yet logically equivalent. This could influence the design of a generation system and have a positive impact on its output. For instance, it would be thinkable to equip the NLG system with a set of meaning-preserving tranformation rules that change the structure of a DRS, consequently producing different surface forms.

Conclusion

SemBank provides an annotated corpus combining shallow with formal semantic representations for texts. The development version is currently available online with more than 60,000 automatically annotated texts; the release of a first stable version comprising ca. 1,000 texts is planned later this year. We expect SemBank to be a useful resource to make progress in robust NLG. Using DRSs as a basis for generation poses new challenges, but also could offer fresh perspectives on existing problems in NLG.

Figure 1 :

 1 Figure 1: Screenshot of SemBank's visualisation tool for the syntax-semantics interface combining CCG and DRT.

Figure 2 :

 2 Figure 2: SDRS for the text "David Bowie grew up in Kent. He played the saxophone. He was a singer in London blues bands", as shown in SemBank.

Table 1 :

 1 Illustration of linguistic information integration in SemBank

	Level	Theory/Source	Internal DRS Encoding
	semantics named entity thematic roles word senses	DRT (Kamp and Reyle, 1993) ACE VerbNet (Kipper et al., 2008)	drs(...,...) named(X,'Clinton',per) rel(E,X,'Agent')

The Groningen SemBankVarious semantically annotated corpora of reasonable size exist nowadays: PropBank(Palmer et al.,

Acknowledgments

We are grateful to Michael White, who provided us with useful feedback to the idea of using a DRS corpus for developing and training text generation systems. We also would like to thank the three anonymous reviewers of this article. They gave extremely valuable comments that considerably improved our paper.


Valerio Basile
email: v.basile@rug.nl

Johan Bos
email: johan.bos@rug.nl

Kilian Evang
email: k.evang@rug.nl

Noortje Venhuizen
email: n.j.venhuizen@rug.nl

A platform for collaborative semantic annotation

Data-driven approaches in computational semantics are not common because there are only few semantically annotated resources available.

We are building a large corpus of public-domain English texts and annotate them semi-automatically with syntactic structures (derivations in Combinatory Categorial Grammar) and semantic representations (Discourse Representation Structures), including events, thematic roles, named entities, anaphora, scope, and rhetorical structure. We have created a wiki-like Web-based platform on which a crowd of expert annotators (i.e. linguists) can log in and adjust linguistic analyses in real time, at various levels of analysis, such as boundaries (tokens, sentences) and tags (part of speech, lexical categories). The demo will illustrate the different features of the platform, including navigation, visualization and editing.

Introduction

Data-driven approaches in computational semantics are still rare because there are not many large annotated resources that provide empirical information about anaphora, presupposition, scope, events, tense, thematic roles, named entities, word senses, ellipsis, discourse segmentation and rhetorical relations in a single formalism. This is not surprising, as it is challenging and time-consuming to create such a resource from scratch.

Nevertheless, our objective is to develop a large annotated corpus of Discourse Representation Structures [START_REF] Kamp | From Discourse to Logic; An Introduction to Modeltheoretic Seman-tics of Natural Language, Formal Logic and DRT[END_REF], comprising most of the aforementioned phenomena: the Groningen Meaning Bank (GMB). We aim to reach this goal by: 1. Providing a wiki-like platform supporting collaborative annotation efforts;

2. Employing state-of-the-art NLP software for bootstrapping semantic analysis;

3. Giving real-time feedback of annotation adjustments in their resulting syntactic and semantic analysis;

4. Ensuring kerfuffle-free dissemination of our semantic resource by considering only public-domain texts for annotation.

We have developed the wiki-like platform from scratch simply because existing annotation systems, such as GATE [START_REF] Dowman | Web-assisted annotation, semantic indexing and search of television and radio news[END_REF], NITE [START_REF] Carletta | The NITE XML toolkit: flexible annotation for multi-modal language data[END_REF], or UIMA [START_REF] Hahn | An annotation type system for a data-driven NLP pipeline[END_REF], do not offer the functionality required for deep semantic annotation combined with crowdsourcing.

In this description of our platform, we motivate our choice of data and explain how we manage it (Section 2), we describe the complete toolchain of NLP components employed in the annotationfeedback process (Section 3), and the Web-based interface itself is introduced, describing how linguists can adjust boundaries of tokens and sentences, and revise tags of named entities, parts of speech and lexical categories (Section 4).

Data

The goal of the Groningen Meaning Bank is to provide a widely available corpus of texts, with deep semantic annotations. The GMB only comprises texts from the public domain, whose distribution isn't subject to copyright restrictions. Moreover, we include texts from various genres and sources, resulting in a rich, comprehensive corpus appropriate for use in various disciplines within NLP.

The documents in the current version of the GMB are all in English and originate from four main sources: (i) Voice of America (VOA), an online newspaper published by the US Federal Government; (ii) the Manually Annotated Sub-Corpus (MASC) from the Open American National Corpus [START_REF] Ide | The manually annotated sub-corpus: a community resource for and by the people[END_REF]; (iii) country descriptions from the CIA World Factbook (CIA) (Central Intelligence Agency, 2006), in particular the Background and Economy sections, and (iv) a collection of Aesop's fables (AF). All these documents are in the public domain and are thus redistributable, unlike for example the WSJ data used in the Penn Treebank [START_REF] Miltsakaki | The Penn Discourse Treebank[END_REF].

Each document is stored with a separate file containing metadata. This may include the language the text is written in, the genre, date of publication, source, title, and terms of use of the document. This metadata is stored as a simple feature-value list.

The documents in the GMB are categorized with different statuses. Initially, newly added documents are labeled as uncategorized. As we manually review them, they are relabeled as either accepted (document will be part of the next stable version, which will be released in regular intervals), postponed (there is some difficulty with the document that can possibly be solved in the future) or rejected (something is wrong with the document form, i.e., character encoding, or with the content, e.g., it contains offensive material).

Currently, the GMB comprises 70K English text documents (Table 1), corresponding to 1,3 million sentences and 31,5 million tokens. 3 The NLP Toolchain

The process of building the Groningen Meaning Bank takes place in a bootstrapping fashion. A chain of software is run, taking the raw text documents as input. The output of this automatic process is in the form of several layers of stand-off annotations, i.e., files with links to the original, raw documents. We employ a chain of NLP components that carry out, respectively, tokenization and sentence boundary detection, POS tagging, lemmatization, named entity recognition, supertagging, parsing using the formalism of Combinatory Categorial Grammar [START_REF] Steedman | The Syntactic Process[END_REF], and semantic and discourse analysis using the framework of Discourse Representation Theory (DRT) [START_REF] Kamp | From Discourse to Logic; An Introduction to Modeltheoretic Seman-tics of Natural Language, Formal Logic and DRT[END_REF] with rhetorical relations [START_REF] Asher | Reference to Abstract Objects in Discourse[END_REF].

The lemmatizer used is morpha [START_REF] Minnen | Applied morphological processing of english[END_REF], the other steps are carried out by the C&C tools [START_REF] Curran | Linguistically Motivated Large-Scale NLP with C&C and Boxer[END_REF] and Boxer [START_REF] Bos | Wide-Coverage Semantic Analysis with Boxer[END_REF].

Bits of Wisdom

After each step in the toolchain, the intermediate result may be automatically adjusted by auxiliary components that apply annotations provided by expert users or other sources. These annotations are represented as "Bits of Wisdom" (BOWs): tuples of information regarding, for example, token and sentence boundaries, tags, word senses or discourse relations. They are stored in a MySQL database and can originate from three different sources: (i) explicit annotation changes made by experts using the Explorer Web interface (see Section 4); (ii) an annotation game played by nonexperts, similar to 'games with a purpose' like Phrase Detectives [START_REF] Chamberlain | Addressing the Resource Bottleneck to Create Large-Scale Annotated Texts[END_REF] and Jeux de Mots [START_REF] Artignan | Multiscale visual analysis of lexical networks[END_REF]; and (iii) external NLP tools (e.g. for word sense disambiguation or co-reference resolution).

Since BOWs come from various sources, they may contradict each other. In such cases, a judge component resolves the conflict, currently by preferring the most recent expert BOW. Future work will involve the application of different judging techniques.

Processing Cycle

The widely known open-source tool GNU make is used to orchestrate the toolchain while avoiding unnecessary reprocessing. The need to rerun the toolchain for a document arises in three situations: a new BOW for that document is available; a new, improved version of one of the components is available; or reprocessing is forced by a user via the "reprocess" button in the Web interface. A continually running program, the 'updat- ing daemon', is responsible for calling make for the right document at the right time. It checks the database for new BOWs or manual reprocessing requests in very short intervals to ensure immediate response to changes experts make via the Web interface. It also updates and rebuilds the components in longer intervals and continuously loops through all documents, remaking them with the newest versions of the components. The number of make processes that can run in parallel is configurable; standard techniques of concurrent programming are used to prevent more than one make process from working simultaneously on the same document.

The Expert Interface

We developed a wiki-like Web interface, called the GMB Explorer, that provides users access to the Groningen Meaning Bank. It fulfills three main functions: navigation and search through the documents, visualization of the different levels of annotation, and manual correction of the annotations. We will discuss these functions below.

Navigation and Search

The GMB Explorer allows navigation through the documents of the GMB with their stand-off annotations (Figure 1). The default order of documents is based on their size in terms of number of tokens. It is possible to apply filters to restrict the set of documents to be shown: showing only documents from a specific subcorpus, or specifically showing documents with/without warnings generated by the NLP toolchain.

The Explorer interface comes with a built-in search engine. It allows users to pose single-or multi-word queries. The search results can then be restricted further by looking for a specific lexical category or part of speech. A more advanced search system that is based on a semantic lexicon with lexical information about all levels of annotation is currently under development.

Visualization

The different visualization options for a document are placed in tabs: each tab corresponds to a specific layer of annotation or additional information. Besides the raw document text, users can view its tokenized version, an interactive derivation tree per sentence, and the semantic representation of the entire discourse in graphical DRS format. There are three further tabs in the Explorer: a tab containing the warnings produced by the NLP pipeline (if any), one containing the Bits of Wisdom that have been collected for the document, and a tab with the document metadata.

The sentences view allows the user to show or hide sub-trees per sentence and additional information such as POS-tags, word senses, supertags and partial, unresolved semantics. The derivations are shown using the CCG notation, generated by XSLT stylesheets applied to Boxer's XML output. An example is shown in Figure 2.

The discourse view shows a fully resolved semantic representation in the form of a DRS with rhetorical relations. Clicking on discourse units switches the visualization between text and semantic representation. Figure 3 shows how DRSs are visualized in the Web interface.

Editing

Some of the tabs in the Explorer interface have an "edit" button. This allows registered users to manually correct certain types of annotations. Currently, the user can edit the tokenization view and on the derivation view. Clicking "edit" in the tokenization view gives an annotator the possibility to add and remove token and sentence boundaries in a simple and intuitive way, as Figure 4 illustrates. This editing is done in real-time, following the WYSIWYG strategy, with tokens separated by spaces and sentences separated by new lines. In the derivation view, the annotator can change part-of-speech tags and named entity tags by selecting a tag from a drop-down list (Figure 5). As the updating daemon is running continually, the document is immediately reprocessed after editing so that the user can directly view the new annotation with his BOW taken into account. Re-analyzing a document typically takes a few seconds, although for very large documents it can take longer. It is also possible to directly rerun the NLP toolchain on a specific document via the "reprocess" button, in order to apply the most recent version of the software components involved. The GMB Explorer shows a timestamp of the last processing for each document.

We are currently working on developing new editing options, which allow users to change different aspects of the semantic representation, such as word senses, thematic roles, co-reference and scope.

Demo

In the demo session we show the functionality of the various features in the Web-based user interface of the GMB Explorer, which is available online via: http://gmb.let.rug.nl.

We show (i) how to navigate and search through all the documents, including the refinement of search on the basis of the lexical category or part of speech, (ii) the operation of the different view options, including the raw, tokenized, derivation and semantics view of each document, and (iii) how adjustments to annotations can be realised in the Web interface. More concretely, we demonstrate how boundaries of tokens and sentences can be adapted, and how different types of tags can be changed (and how that affects the syntactic, semantic and discourse analysis).

In sum, the demo illustrates innovation in the way changes are made and how they improve the linguistic analysis in real-time. Because it is a web-based platform, it paves the way for a collaborative annotation effort. Currently it is actively in use as a tool to create a large semantically annotated corpus for English texts: the Groningen Meaning Bank.

Figure 1 :

 1 Figure 1: A screenshot of the web interface, displaying a tokenised document.

Figure 2 :

 2 Figure 2: An example of a CCG derivation as shown in GMB Explorer.

Figure 3 :

 3 Figure 3: An example of the semantic representations in the GMB, with DRSs representing discourse units.

Figure 4 :

 4 Figure 4: Tokenization edit mode. Clicking on the red '×' removes a sentence boundary after the token; clicking on the green '+' adds a sentence boundary.

Figure 5 :

 5 Figure 5: Tag edit mode, showing derivation with partial DRSs and illustrating how to adjust a POS tag.

Table 1 :

 1 Documents in the GMB, as of March 5, 2012

	Documents	VOA MASC CIA	AF	All
	Accepted	4,651	34	515	0	5,200
	Uncategorized 61,090	0	0 834 61,924
	Postponed	2,397	339	3	1	2,740
	Rejected	184	27	4	0	215
	Total	68,322	400	522 835 70,079


Jean-Philippe Moreux
email: jean-philippe.moreux@bnf.fr

Innovative Approaches of Historical Newspapers: Data Mining, Data Visualization, Semantic Enrichment Facilitating Access for various Profiles of Users

Keywords: heritage newspapers, document analysis, OCR/OLR, metadata, data mining, data visualisation, semantic enrichment, named entities recognition, digital mediation, digital humanities

In this age of Big Data this paper describes how digital librairies can apply at large scale innovative approaches to better valorize and bring better experiences of old newspapers.

On the first hand, the state-of-the-art OLR (optical layout recognition) technique in one of the largest heritage press digitization projects in Europe (Europeana Newspapers, www.europeananewspapers.eu, 2012-2015) was used in a data mining experiment. Data analysis was applied to quantitative metadata derived from a 850K pages subset of six XIX th -XX th c. French newspaper titles from the BnF collection. The METS/ALTO XML data was analyzed with data mining and data visualization techniques that show promising ways for the production of knowledge about historical newspapers that are of great interest for library professionals (digitization programs management, curation and mediation of newspaper collections) and for end-users, particularly the digital humanities community.

On the other hand, the Retronews web portal showcases how advanced semantic annotation techniques can improve the retrieval efficiency on a digital newspapers collection; thus the rediscovery and reappropriation of these documents by various types of users: teachers, students, researchers, general public.

INTRODUCTION

Libraries are full of digital data and everyday they produce new data: bibliographic metadata are created or updated in catalogs describing collections [START_REF] Cukier | Big Data: A Revolution That Will Transform How We Live, Work, and Think[END_REF], [START_REF] Green | The Interplay of Big Data, WorldCat, and Dewey[END_REF]; usage data on libraries and their audience are collected; digital documents are produced by the digitization of content stored in heritage libraries.

But can library data and metadata fit with the concept of big data? Are they legitimate targets for data mining? Their relatively small volume (12 millions of records for BnF catalog) does not encourage some caution? The criterion of the volume is irrelevant, if we believe Viktor Mayer-Schoenberger and Kenneth Cukier: "(…) big data refers to things one can do at a large scale that cannot be done at a smaller one, to extract new insights or create new forms of value (…)" [START_REF] Cukier | Big Data: A Revolution That Will Transform How We Live, Work, and Think[END_REF]. On a large scale, but set against the activity ("my big data is not your big data" [START_REF] Green | The Interplay of Big Data, WorldCat, and Dewey[END_REF]), with methods different from those satisfying the nominal business needs, and with the aim to "create something new": new links (author, place, date, etc.) are built on top of catalogs (OPAC) [START_REF] Teets | Libraries' Role in Curating and Exposing Big Data[END_REF]; libraries management can be backed by the analysis of attendance and reading data [START_REF] Lapôtre | Faire parler les données des bibliothèques : du Big Data à la visualisation de données -Let the data do the talking: from Big Data to Dataviz[END_REF]; a history of newspapers' front pages can be written on data extracted from digital libraries (DLs) [5], [START_REF] Sherratt | 4 million articles later…[END_REF]. E.g., does it make sense to data mine quantitative metadata of the daily newspapers digitized and refined during the Europeana Newspapers project [7]? What lessons can be learned from the Retronews portal, which makes extensive use of semantically enriched data? We attempt to answer these questions by first presenting the process of creating new metadata; then some methods of analysis, interpretation and reuse of these metadata; and finally data quality issues.

THE EUROPEANA NEWSPAPERS LIVE CASE

Creating new Metadata

Six national and regional newspapers (1814-1945, 880K pages, 140K issues) of BnF collections are part of the data set OLR'ed (Optical Layout Recognition) by the project Europeana Newspapers. The OLR refinement consists of the description of the structure of each issue and article (spatial extent, title and subtitle, etc., using METS/ALTO formats [START_REF] Neudecker | Europeana Newspapers, A Gateway to European Newspapers Online[END_REF]) and the classification of content types (MODS format). Based on this finding, from each digital document a set of bibliographical and descriptive metadata related to content and layout is derived, both at issue and page levels (date of publication, number of pages, articles, words, illustrations, etc.). XSLT or Perl scripts (Fig. 2) are used to extract some metadata from METS manifest (e.g. number of articles) or OCR files (e.g. number of words). The complete set of derived data contains about 5.5M atomic metadata values expressed with XML, JSON or CSV formats.

WHEN THE DATA TALK

Producing Knowledge

Some data describe a reality of which the analyst has prior knowledge or intuition. This is the case of statistical information helping to pilot digitization or curation actions. The data set could then be a representative sample of the collection, because the information sought are mostly statistical measures.

Digitization programs: What is the density in articles of these newspapers (Fig. 3)? What is the potential impact on OLR processing costs? Image bank: What titles contain illustrations (Fig. 4)? What is the total number of images one can expect? The collected statistical measures help to enrich this knowledge with actual data (mean, total, maximum, distribution…) of great value for digitization program managers but librarians as well, in the case where such techniques were applied to the DL collection as a whole.

Discovering knowledge through visualization

Data visualization allows researchers (digital humanities, history of press, information science) to discover meaning and information hidden in large volumes of data. Moreover, OLR content types classification feature helps researchers to spot on specific types of content.

The History of the Press

Front page: The role of the image in the daily press is a classic research subject [5], [START_REF] Joffredo | La fabrication de la presse[END_REF] that data mining analysis and visualization tools can enrich with micro-facts as well as macrotrends. Thus, the singular curve describing a supplement of Le Petit Journal illustré (Fig. 5) highlights the appearance of the full front page illustration on Nov. 29, 1890. Activity: The content classification performed during OLR refinement allows an analysis in terms of types of content (text, table, ad…). Figure 7 shows the impact of the Great War on the activity and assesses the period of return to pre-war level activity (roughly 10 years). [START_REF] Langlais | La formation de la chronique boursière dans la presse quotidienne fran aise (1 01-1870). Métamorphoses textuelles d'un journalisme de données -The Stock exchange section in the French daily (1801-1 70)[END_REF]: The content classification performed during OLR can help researchers focusing on specific newspapers content, e.g. Stock Market section (quotes and analysis). The quantitative metadata are of a great help because "tables" in newspapers are predominantly used in such quotes (Fig. 9).

Stock market section in Daily

The History of Newspaper's titles

Data visualization on a complete data set (one data per issue) makes possible to focus on a specific daily title.

Digital archeology of papermaking and printing: Page format information can be retrieved from the digitized images. It provides researchers the complete printing history of a title.

Engaging new Audience with Datavizualisation

Data visualization facilitates rediscovery and reappropriation of the digital documents described by these data. Such methods and tools can help DLs to improve the access to their collection, in addition to the classic keyword spotting and page flip mode.

Querying the Metadata

Improving the effectiveness of the analysis can be achieved with dedicated tools or processes (ETL frameworks, APIs to access content [START_REF] Sherratt | 4 million articles later…[END_REF], [START_REF] Beranger | Big Data -Collecte et valorisation de masses de données[END_REF], XML or no-SQL databases, statistical environments like R…). BaseX (basex.org) is one of these simple and elegant solutions to agglomerate all the individual metadata files in a unique database and to query it using XPath/Xquery languages. As part of a digital mediation action devoted to a press title or to the complete data set, a basic FLWOR query will identify all "graphical" pages, that is to say both those poor in words (here based from an average) and including at least one illustration: This query retrieves hundred of pages from the whole data set (comics, portraits, press cartoon, maps, ads…), which would have been extremely laborious to manually identify. BnF digital curators and mediators have expressed their interest in this approach and a XQuery HTTP API have been set up on the BaseX database, which helps them to identify graphical "nuggets" through the BnF newspapers collection. Fig. 14 shows an iconographical research related to the murder of Gaston Calmette: relevant illustrations are easily retrieved. Similar queries can be written to dig into the data and find specific types of content previously identified with dataviz, e.g. the pages censored during the Great War (see Fig. 12), which have a slightly smaller word counts than the pages average. This method leads to a 45% recall rate and a 68% precision rate (based on a ground truth carried on the JDPL front pages for 1915). Obviously a medium performance, showing the limits of a statistical approach when applied to a word based metric biased by layout singularities (titles, ads, etc.) and proved to be ineffective on illustrated pages. However a successful method if completeness is not required, like in a mediation context (see the resulting Gallica blog post) or for fuzzy search on length of documents in terms of word counts (see [START_REF] Lease Morgan | Use and understand: the inclusion of services against texts in library catalogs and discovery systems[END_REF]).

Advanced Search Modes for Newspapers

DLs' items are not anonymous text. They all have specific form factors, they are all part of the long history of publishing. Consequently, feeding the DL search engine with layout and structural metadata could allow users to perform advanced mixed queries taking into account this fact. Trove (http://trove.nla.gov.au) is an emblematic example of this approach. Fig. 17 shows Trove advanced search form dedicated to structural and layout information search criteria (category of content, word count, illustration count). And what about books? (one could ask). Books' OCR also contains meaningfull quantitative information [START_REF] Lease Morgan | Use and understand: the inclusion of services against texts in library catalogs and discovery systems[END_REF]: word, table, map, ornament, drop cap… Using the same approach, users could perform complex illustration retrieval tasks on illustrated content (maps, pictures).

Data QA

The quality of derived data affects the validity of the analysis and interpretation [START_REF] Lapôtre | Faire parler les données des bibliothèques : du Big Data à la visualisation de données -Let the data do the talking: from Big Data to Dataviz[END_REF], [START_REF] Jeanneret | Complexité de la notion de trace. De la traque au tracé[END_REF]. Irregular data in nature or discontinuous in time may introduce bias. A qualitative assessment should be conducted prior to any interpretative analysis.

Newspapers are characterized by the relative homogeneity of their shape over time, which induces consistency and constant granularity of the derived metadata (issue, page, article...). Moreover, its large size and the option to apply the analysis to the entire data set and not a subset of it also guarantees its representativity [START_REF] Aiden | Uncharted: Big Data as a Lens on Human Culture[END_REF].

The data itself can sometimes contribute to their own QA. A synthetic calendar display of available data for a title (JDPL, Fig. 19) shows rare missing issues, which suggests that the digital collection is representative of the reality [START_REF] Dunning | Representation and Absence in Digital Resources: The Case of Europeana Newspapers[END_REF].

Or, before starting a study on stock market section ( [START_REF] Langlais | La formation de la chronique boursière dans la presse quotidienne fran aise (1 01-1870). Métamorphoses textuelles d'un journalisme de données -The Stock exchange section in the French daily (1801-1 70)[END_REF], §2.2.2) based on the content typed "table", one can empirically validate this hypothesis by the sudden inflections recorded in 1914 and 1939 for all titles (Fig. 20), being known and established the historical fact of the temporary halt of trading during the two World Wars. Furthermore, care must be taken to inform users of the data set characteristics: production method, known deficiencies, through or under-representation issues, etc.

THE RETRONEWS PORTAL

Retronews (www.retronews.fr) is a BnF public/private partnership web portal project, launched in 2016. The overall objective of the project is to facilitate access to information contained in a corpus of three centuries of heritage press.

Creating new Data

Retronews foundations are based on four concepts (named entities, themes, events, topics) from which the text corpus is enriched with semantic annotations:

 Named entities (the NE's 3 categories are: person, place, organization): The NE recognition is driven by linguistic grammar-based techniques and authorities' records (BnF, VIAF, dictionaries of famous people from the XVII th up to the XIX th c.)  Themes (14 top level themes, 231 second level themes): Derived from the IPTC classification and refurbed for the heritage press. A lexica has been created for each theme, and the text corpus indexed relatively to these lexica.  Events (147): Closed list of historical events defined by the editorial team, each event associated with a lexica (Wikipedia).

 Topics (20,000): The topic modelling uses Wikipedia articles titles and a list of the most frequent queries expressed by users on Gallica press content.

When the Data Talk

Advanced Search Modes for Newspapers

Adding a pinch of semantic flavor helps to get closer to natural language queries. If we reformulate the previous example ( §2.2.5, "Caillaux trial"), taking advantage of the semantic features available in Retronews (Fig. 21) like named entities recognition or topic modelling, we now gain a richer expressivity: Retronews has implemented a faceted search functionality to allow end-user to express queries on the semantic concepts already mentioned as well as on a layout criteria ("front page only"). Fig. 22 shows the "Caillaux trial" query expressed with these features and a page extracted from the result list, showing how the semantic classification data are rendered: NE occurrences colored on the image itself; semantic classification data (themes and NE) listed under the page.

Engaging new Audience with Data Vizualisation

The semantic annotated corpus are legitimate candidates for innovative visualization modes. The next version of Retronews will implement classic text mining tools (terms frequency, concordance, proximity, comparison of speech, named entities graph, etc.).

Data visualization artefacts like timelines are also perfectly suited for historical daily. Fig. 23 shows the Retronews newspapers timeline produced by digital mediators for editorial purposes.

Thanks to data mining and data visualization techniques, extracted facts can also enrich individual daily timelines. Fig. 24 shows the Journal des Débats politiques et littéraires timeline, mixing the main events of the daily history (extracted from bibliographic data but also thanks to the data mining analysis) and historical events, some of the latest sometimes interfering with the first (e.g. changes of format or paging during wartime).

Data QA

The different semantic annotation techniques used have posed particular challenges when applied to historical news content:

 OCR Text Recognition: The quality of text produced by the OCR has a direct influence on the results of semantic extraction tasks (NE, topic modelling, etc.). Now the corpora has an average quality due to its nature (degraded documents) or its mode of production (scan from microfilm).  OCR Segmentation: Some semantic extraction tasks operate at the informational entity of the paragraph or article's level (topic, events, themes). But this level of structure may not exist (OCR without OLR) or be unreliable (OCR detects paragraphs but is mistaken).  Reference corpus and real corpus not synchronous: Some contemporary information resources used to facilitate the semantic extraction tasks (Wikipedia, IPTC) applied with difficulty to a heritage corpus.

Moreover, the classic difficulties of semantic annotation have occurred, particularly noise (over-sensing), silence (sub-detection) and disambiguation. All of these difficulties have been taken into account:

 NE Recognition: Noise has been reduced by working on syntactic tagging, by inserting a rules engine taking into account the context and by filtering NEs using alignment with the authorities' records.  Themes Extraction: Editorial work on the IPTC classification to make it relevant for heritage dailies; fine tuning of the sub-detected and over-detected themes; filtering of the number of detected themes per page. Some barriers (beyond the state of the art) persist, such as the theme detection of an illustrated front page (cf. Fig. 5).  Events Extraction: A date filter has been add to reduce noise; the lexica have been manually enriched to cope with the anachronism issue (Wikipedia/heritage daily).  Topic Modelling: The 20,000 entries list extracted from Wikipedia article titles has been manually cleaned up to remove contemporary topics.

Finally, the semantic annotations are not perfect (they can't be), neither the information retrieval functionalities (we cannot index everything with everything), but the overall benefit for the end-users is quite significant.

CONCLUSION

The first live case exposed (Europeana Newspapers data set) shows that even meaningless descriptive metadata can give new insights into the history of the press and into history itself through the use of basic data mining methods and tools. This surprising finding is explained by the target corpus, daily press, ideal subject for OLR structural enrichment and hence, the production of consistent metadata over a large period of time.

As a digital library, to create and disseminate such metadata (by download or by any other means: API, web app, etc.) gives researchers a field of study ready to use and easy to use: a corpus of 1TB of METS/ALTO files leads to a set of metadata weighing a few MB, in formats (CSV, JSON) suitable for statistical analysis.

Moreover, we showned that this method also enhances the information retrieval capacities of DLs' end-users and helps them to cope with amounts of information ever larger, from innovative perspectives.

Its results could be followed up in various ways:

 Apply the same data mining process to the other Europeana Newspapers OLR'ed data sets to expand the scope of the analysis to the entire European press and to the ongoing BnF press digitization program, which also uses OLR [START_REF]Référentiel d'enrichissement du texte[END_REF].  Experiment with other types of materials having the desired consistency characteristic and a temporal dimension (e.g. long life magazines or revues, early printed books).  Provide the derived data sets to researchers. Such data, possibly crossed with the OCRed text transcription, usually provide a fertile ground for research hypotheses [START_REF]The Comédie-Française Registers Project[END_REF].  Generalize the principle of derived data set for researchers to text data sets, based on the themes (Ad, Weather, Judicial sections…) marked up during the OLR process.  Assess the opportunity of setting up a data mining framework in the BnF to be feed with Gallica's collections.

The last three issues will be addressed during the BnF research project "Corpus" (2016-2018), which aims to study the data mining and text mining services a library can provide for researchers.

The second live case (Retronews) demonstrates the potential of semantic refinements on information retrieval functionalities. Retronews faceted search considerabily increases the capacity of users to find facts and things in the newspapers' content.

We believe that these two use cases have amply demonstrated that DLs can benefit from the digital humanities methods and tools (data and text mining, dataviz, automatic language processing), and in turn provide better service to all users, including the DH community.

Fig. 1 .

 1 Fig. 1. OLR process 2.1.1 From the Digital Documents to the Derived Data OCR and OLR files are full of interesting objects marked up into the XML:  OCR (ALTO) is a source for quantitative metadata: number of words, illustrations & tables; paper format…

Fig. 2 .

 2 Fig. 2. Derived data production process This operating principle has many advantages:  It makes light derived data sets, rather than heavy XML corpora available to endusers.  It's not rocket science and it's fast (30,000 pages/hour with an optimized NoXML parsing Perl script).

Fig. 3 .

 3 Fig. 3. Average number of articles per issue

Fig. 4 .

 4 Fig. 4. Average number of illustrations for 1,000 pages Invited to comment on these results, the collections curators easily establish links with the documentary reality they know: "Of course, Le Matin is a daily which was published during the golden age of modern newspapers (1890-1914) and emblematic of the age's innovations: it is highly structured and illustrated." (Fig. 3: brown curve; Fig. 4: 6k illustrations for 1k pages) "The Journal des Débats politiques et littéraires (JDLP) founded in 1789 is an heir of the first newspapers (gazettes): it retains throughout its history a rubric based layout, and in which the illustration is rare." (cf. Fig. 3: orange curve, Fig. 4: only 225 illustrations for 1k pages)

Fig. 5 .Figure 6

 56 Fig. 5. Average number of illustrations on front page (Le Petit Journal illustré)

Fig. 6 .

 6 Fig. 6. Average number of illustrations per page (Le Petit Parisien)

Fig. 7 .

 7 Fig. 7. Types of content per issue (Le Matin)Layout: Form factors and layout of Dailies have varied considerably over time. Fig.3allows us to locate a major transition in the 1880s, with two families of newspapers, the "old", poorly structured into articles (Le Gaulois, Journal des Débats politiques et littéraires) and the "modern" (Le Matin, Le Petit Parisien, Ouest-Éclair, Le Petit Journal illustré) borned with a structured layout. Combining in a bubble chart (Fig.8) the three form factors of "modernity" which are the average number of articles per page (x), illustrations per page (y) and illustrations on front page (z) illustrate this typology.

Fig. 8 .Fig. 9 .

 89 Fig. 8. Newspaper modernity classification

Fig. 10 .

 10 Fig. 10. Page format (JDPL, complete data set)

Fig. 11 .

 11 Fig. 11. Number of illustrations per issue (JDPL, complete data set)

Fig. 12

 12 Fig. 12 shows an interactive web chart of the word density per page over the complete data set of the Journal des débats politiques et littéraires (1824-1944, 45K issues). Data singularities demonstrated by the chart can prompt users to discover and browse the collection differently:  The significant breaks in the scatter plot chart are linked to the successive changes in layout and/or format (as studied by historians of the press [12]), motivated by technical innovations on papermaking and printing (e.g.: Dec. 1, 1827: 3 columns, 330450mm; Oct. 1, 1830: 4 col.; March 1, 1837: 400560mm) or historical events (Aug. 4, 1914: move to 2 p. and 3 col. then back to 6 col. on Aug. 8).  Outliers can also reveal treasures, likes this 24 words/p. issue (May 2, 1889, Paris Universal Exposition's map) or examples of censorship during the WW1 (22 May, 1915).

Fig. 12 .

 12 Fig. 12. Average number of words per page (JDPL, complete data set)

  <result> {let $textDensity := 0.25 let $threshold := avg(//page/nbString) * $textDensity for $page in //page[blockIllustration>1] where $page/nbString < $threshold let $tokens := fn:tokenize($page/../../metad/date, "\.") return <illustration> <date>{$tokens[3]}-{$tokens[2]}-{$tokens[1]}</date> <pageNo>{$page/fn:count(preceding-sibling::page) + 1}</pageNo> </illustration>} </result>

Fig. 13 .

 13 Fig. 13. Samples from the results: Ouest-Éclair, Le Petit Journal, Le Petit Parisien, Le Matin, Le Gaulois, Le JDPL

Fig. 14 .

 14 Fig. 14. Image search API results with date and front page criteria

Fig. 15 .

 15 Fig. 15. Censored issues: query results & GT (Journal des débats politiques et littéraires, 1915)

Fig. 16

 16 Fig. 16 lists a couple of queries leveraging some of the rich informational objects which have been previously discussed (illustrated article, article title, article including tables) and the other available information sources (catalog, OCRed text).

Fig. 16 .

 16 Fig. 16. Mixed queries using various information sources: famous Caillaux trial (left); metal prices quotes (right)

Fig. 17 .

 17 Fig. 17. Trove advanced search form (extract)

Fig. 18 .

 18 Fig. 18. Mixed queries on books collection: looking for illustrations of planet Mars

Fig. 19 .

 19 Fig. 19. JDPL missing issues (1814-1944)

I

  Fig. 21. Advanced query system on a semantically refined corpus

Fig. 22 .

 22 Fig. 22. Retronews faceted search: date, theme, NE (person)



  OLR (METS) is a valuable source too for high level informational objects: number of articles, titles…; identification of sections (groups of articles); content types classification (ads, judicial, stock market…)

Acknowledgments

The author thanks all partners of the Europeana Newspapers project; the Retronews team; Frederick Zarndt and Caroline Kageneck for proofreading this article. Scripts, data sets and charts are freely available: http://altomator.github.io/EN-data_mining.


Davide Mano

Stefano Zolli

Tanin Pigu

Jerome David Salinger's Nine

Philip Stories

Roth's Portnoy

Michal Zamir

Sami Berdugo

Eshkol Nevo

and himself. "We are not the established, famous, celebrated older generation of Israeli authors", Gavron explained, "we offer a different tone; call it lighter, more cutting edge, or daring: whatever it is, we don't seem to carry the same weight on our shoulders", and "while the previous generation demonstrated a social coherence, we offer a more fragmented, varied outlook for our country".

Gavron's fourth novel Tanin Pigu'a was published in Israel in 2006. It tells in first person and in alternating chapters the two stories of the Israeli-Palestinian conflict during the period of the terrorist attacks of the second intifada. Eitan Enoch (nicknamed Croc) works at Time's Arrow, a high-tech company in Tel Aviv which deals with optimizing the use of time (with voice messages or by setting traffic lights). He miraculously manages to survive three attacks, making himself into a sort of national hero who is invited to talk shows on TV. Fahmi Sabih, who is same age as Croc, is a Palestinian from Murair village, whose father dreams of a future for him at the Bir Zeit University; contrary to these wishes Fahmi is instead forced to follow in the footsteps of his brother Bilhal, a terrorist who lives in Amari refugee camp and is one of the organizers planning the attacks in which Croc is involved. The novel starts with the Israeli protagonist who is riding to work on the number five minibus: A week after the publication of the novel's first chapter in the newspaper Yedioth Ahronoth, before the book came out in bookstores, Assaf Gavron and Zmora Bitan Publishing House were sued by a female driver of a minibus who claimed Gavron wrote about her. The book was revised in some parts and publication postponed.

Once published, the interest that Israeli readers gave to Tanin Pigu'a was still not high: the book sold less than one third the number of copies compared to his previous novel Moving, the best seller story about three Israeli boys working for a moving company in New York.

The Israeli public kept away from a novel that reproduced, in the form of fiction, the tragedy of the attacks and the subsequent traumas experienced. In addition, the refusal to empathize with the hard life of the Palestinian "enemy", imprisoned in the villages and grappling with the Israeli check points, was evident. In a recent interview Gavron said: "When I wrote the book, in Israel we were living a very hard period, it was the time of the attacks. It was clear that people didn't want to read things like that. Another problem, in my opinion, was that the Palestinian character was described with great empathy. To my great joy, however, outside of Israel this was my most successful novel".

As a matter of fact, Tanin Pigu'a attracted much more attention in Europe and US than in Israel. Countries where translations were released include Germany, Netherlands, Italy, the UK, the US and Canada, and will also comprise France and Greece. This paper is thus intended to analyse the reception of the novel outside Israel. In a period comprised between 2008 and 2011, we will consider the cases of Germany, Italy, the UK and the US.

*

Tanin Pigu'a came out in Germany in March 2008 under the title Ein Schönes Attentat (A beautiful attack) and was translated from Hebrew to German by Barbara Linner. This was the first Western-language edition of a book by Assaf Gavron. The editorial contacts with Germany began almost by chance: during a period in the States in a residence for writers, Gavron was introduced by a writer from Berlin to a German agent who showed a great deal of interest in his new novel. Gavron prepared an extract in English translation, worked on it with the agent in Berlin and within a month Tanin Pigu'a was sold to Random House. Beginning with this contact with the German agent, the other European and American editions of the book emerged.

In Germany Tanin Pigu'a was a big success with critics and audiences and in 2010 a second edition came out in paperback. "The country where the book has sold more is Germany, there is selling very well, more than other countries, more than Israel", Gavron says. In Germany there have been many positive reviews of the book: in newspapers, radio, websites. The German literary critic Sigrid Löffler wrote: "This novel is cleverly constructed, very contemporary in its subject matter and, because of Gavron's critically equitable narrative posture, exceptionally interesting." According to Radio Gong the opinion is even more enthusiastic: this "is the best modern novel to come out in Israel in the last years". Gavron explained the interest in the book by foreign publishers as follows: "I think that in Europe, for example in Germany and perhaps also in Italy, it was the political balance, the fact that there were the two parts, the two voices, that there was no preferential treatment accorded to one of the two fronts".

In the context of Israeli-German cultural ties Gavron received a fellowship for 2010 from the Deutschen Akademischen Austausch Dienst (German Academic Exchange Service) in Berlin. During his stay, while also writing a new novel, he toured various German towns for some readings: "Germans love to read and hear the writers. The readings are organized by the publisher and the public pays. It is a good opportunity for a writer to meet readers, booksellers and even journalists". Moreover, in an interview for a German website dedicated to the dialogue with the Islamic world, Gavron told Ariana Mizna that "people in Germany for instance reacted very positively to the book; they say that the book gave them a real insight to the lives of people here. They have seen the headlines down through the years, but are now getting to know two sides without prejudices. So I think literature can influence, and it can educate as well".

In Germany, after Tanin Pigu'a, other two of his novels were published: Hidromania (in March 2009) and Moving (German title Alles Paletti, in September 2010). Even more significantly, the movie studio "Arden Film" in Berlin decided to buy Tanin Pigu'a's copyright in view of a movie adaptation. More recently, Gavron has been appointed a member of the jury for the "Best First Feature Prize" at the "Berlinale International Film Festival 2011".

*

The 2009 Italian edition originated in the wake of the success attained by the book in Germany. Italian writer Helena Janeczek read the novel in German and proposed it to Mondadori Publishing House. As she explained: "What struck me most was that the book focused on the Israeli-Palestinian conflict and that, while doing so, it took full advantage of the tools of the novel, representing ordinary lives, using the mechanism of empathy with the main characters and creating a well-structured plot to make the tragic matter of the narration captivating. I also appreciated the use of irony, that in this case doesn't intend to create distance, but it rather seems to reflect warmness, once more empathy".

The Italian edition was included in the Mondadori's top series "Strade Blu" and was characterised by the publisher's decision to opt for a different title, far from the Hebrew one: "La mia storia, la tua storia" (My story, your story). This new title puts the emphasis on the bipartisan configuration of the book, but neglects a key-word, "attack", present in the Hebrew (pigu'a) and in most Western translations (attentat, attack).

A further difference between the Italian and the German case is about the reception of the book: while in Germany promotion had been effective and contributed to the author's popularity, in Italy this significant encounter with publishers and readers didn't occur for various reasons. Janeczek explained a broader problem concerning the place of literature in Italian media: "The press agent of Mondadori told me that newspapers often reacted to the presentation of the book with unfair remarks such as: once more with this story of Israelis and Palestinians! It seems that the subject matter didn't attract the Italian media... Even more than this -Janeczek pointed out -in the present state of our cultural pages, books lacking top promotion are usually penalised as well as new foreign authors".

The few number of reviews published in the Italian newspapers seems to confirm this lukewarm reception. The review published in the daily La Stampa by Mario Baudino is the most receptive: Baudino sees the originality of the book "in the fact that an Israeli author succeeds in making his worst enemy talk". Baudino defines Gavron as "a skilful constructor of plots" and his novel as "a book that upsets because of its absolute desire for sincerity". On the contrary, the short review published in the daily La Repubblica by Susanna Nirenstein seems to bear crucial importance to the fact that from Gavron's novel one can draw a positive picture of a pluralist Jewish State and a free Israeli society, in strong contrast with a sick Palestinian world embodied by terrorism "that poisons villages and creates crazy anthills". But these political remarks can in no way be ascribable to the perspective adopted by Gavron, who is far from advancing such unconditional judgements.

In contrast with the general reception, it is significant to note how in Italy the novel has gained particular success among readers from the Italian Jewish milieu, to the extent that it has been one of the three finalist books in the Italian Adei-Wizo Literary Prize in 2010 and since then it has merited further attention in the Italian Jewish press.

*

The 2010 British and American editions of Tanin Pigu'a are the fruit of the same work of redaction. The English translation was prepared by Gavron himself, who was asked for the first time to render a Hebrew text into a foreign language. Quoted in the credits is also British author James Lever, who contributed to the final editing.

The British edition appeared in February 2010 for Fourth Estate, while the North-American was released in April for Harper Collins and Harper Canada. Different marketing plans made the book presented in the UK under the title "Croc-Attack!" (strictly faithful to the Hebrew), while in the US and Canada the chosen heading was "Almost Dead" (actually a very effective title in its double reference to the main characters' condition). As in Germany, the book was launched through a series of public meetings and readings at universities, literary festivals and bookshops in London, New York, Cambridge and Los Angeles.

British reviewers often focused on the Israeli character, while the American placed more emphasis on the presence of the Palestinian counterpart. British reviews were often signed by journalists with a comprehensive knowledge of Israeli reality: as a result, their comments gave importance to details. Reversely, the American reviewers proposed a broader reflection with the purpose of provoking a new debate on terrorism, starting from the author's suggestions.

The idea of an Israeli-Palestinian conflict fostered by multiple opposite narratives is regarded as true by British journalist Edward Stourton, who praises the author for offering a picture even more "real" than that of a reportage. In her review Zena Alkayat recalls how the same urgency of writing soon after the events drove New York writer Jay McInerney to write his book "The Good Life" immediately after September 11. According to Los Angeles Times' editor David L.

Ulin, the presence of the Palestinian character besides the Israeli one is an invitation for the reader to reconsider "the very nature of terrorism" from both perspectives and personal sufferings. As Ulin pointed out, Eitan and Fahmi "are framed as symbols for causes that may not be their own. Each tries to find a passage through the madness but remains buffeted by circumstance".

In the UK the success of sales has brought to the re-edition of the book after about one year from the first edition. In a different way, as a result of the attention given to the novel in the US, the author has been invited as "artist in residence" at Chapman University in Orange, CA, in the context of the Schusterman Visiting Artist Program.

*

In summary, according to what emerged from our investigation, Assaf Gavron's Tanin Pigu'a has enjoyed in Europe and in America a largely positive reception that has contributed to making this novel an interesting literary case. However, the story of the book's reception that we have outlined here has brought to light considerable differences from country to country. In Germany, where the book was first launched and where Gavron's novel gained the greatest success, the fact that stands out most is the inclusion of this emerging Israeli author in the arena of German cultural events.

On the contrary, in Italy the novel didn't attain particular prominence in the media, with the exception of the Italian Jewish milieu, where it gained recognition.

The Italian edition was confronted with a certain reluctance manifested by the daily press towards the "age-old" and "overexposed" subject of the Israeli-Palestinian conflict. As a result, few reviews found their place in the Italian cultural pages. British criticism, more than anywhere else, has put emphasis on Gavron's writing and the authenticity of his language. The political balance that characterises the book remains here also, as in the German and Italian cases, at the core of the enthusiasm manifested by critics. However, another matter to be appreciated is the inclusion of multiple individual narratives. The American press has pointed out how this powerful construction has exposed the "real" nature of a conflict in which the opposing fronts are not so far one from the other, but they tend to get close and confused, to intertwine with one another. Without ever running the risk to take the side of suicide bombers nor to justify their extreme act, the novel gives voice to an ordinary and anti-heroic humanity, often shadowed.

In conclusion, we can agree with Shoshana Kordova who in her article for Ha'aretz explained the success of the book outside Israel with the fact that "for those who haven't lived through the period it portrays, the very existence of a single work encompassing both Israeli and Palestinian perspectives constitutes grounds for profuse literary praise". If in Europe and America the book received positive reviews and registered good sales, on the contrary in Israel the Hebrew first edition sold much less than expected and local press tended to reject the book, probably because of its topical interest. Israelis, who tragically lived the period of the second intifada, probably chose not to relive those traumatic days in fiction. The critic as well as the reader mostly rejected the "crocodile of the attacks" (tanin ha-pigu'im) created by Assaf Gavron, one of their newer leading writers.

"

  We were heading down Dizengoff Street when an elderly lady turned to me. Quietly 't think at that moment that his look meant anything. (…) 'Everyone's under pressure, eh?' the other guy at the back said. He had a little goatee and big aviator sunglasses with mirrored lenses. His hair was the colour of honey, held back with plenty of gel, darkening into curls at the back. Cool, at least in his own eyes. Pleasant smile.

	Giora, I know now. Giora Guetta, from Jerusalem. I know plenty of things now. (…) 'This
	paranoia …' I said. 'People are completely crazy.' 'He looks OK to you, right?' I looked
	once more towards the dark guy. I wasn't sure. Who could be sure?"
	she said: 'Doesn't that man look suspicious?' (…) With her eyes she indicated a dark guy
	at the front. We were sitting at the back. He was wearing a grey wool hat and holding a
	suit in a suit bag. (…) 'Come on, don't exaggerate,' I said, 'He looks fine to me.' But I kept

looking at him. (…)

The old lady got off at Jabotinsky Street. She didn't trust our judgement. On her way out she gave the dark guy a long look. He looked back at her. I didn


Amelia Carolina Sparavigna

Flood Basins Created by Ephemeral Rivers of Namibia in the Satellite Image Time Series of Google Earth

Keywords: Satellite Image Time Series, Google Earth, Water Reservoirs, Environment

Here we show that, in the satellite image time series of Google Earth, it is possible to observe how some ephemeral rivers of Namibia can be dammed by the dunes of the Skeleton Coast, creating some flood reservoir basins.

Google Earth is a virtual planet which is showing its life through the time series of satellite images (SITS). These SITS can be a quite interesting tool for studying the development of anthropized areas such as the evolution of natural environments. In some previous papers for instance, we have investigated using the Google Earth SITS the motion of sand dunes [START_REF] Sparavigna | A study of moving sand dunes by means of satellite images[END_REF][START_REF] Sparavigna | A case study of moving sand dunes: The barchans of the Kharga Oasis[END_REF][START_REF] Sparavigna | Analysis of the motion of some Brazilian coastal dunes[END_REF] and the decreasing level of Dead Sea [START_REF] Sparavigna | Sinkholes of Dead Sea in Satellite Image Time Series[END_REF]. Here we will show some SITS of ephemeral rivers of Namibia, which are occasionally flowing towards the Atlantic Ocean, crossing the Skeleton Coast [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF][START_REF] Goudie | The ephemeral rivers and runes of the Skeleton Coast[END_REF][START_REF] Morin | Flood routing and alluvial aquifer recharge along the ephemeral arid Kuiseb River, Namibia[END_REF][START_REF] Schachtschneider | The relationship between fog, floods, groundwater and tree growth along the lower Kuiseb River in the hyperarid Namib[END_REF][START_REF] Botes | Ephemeral rivers and their development: testing an approach to basin management committees on the Kuiseb River, Namibia[END_REF][START_REF] Jacobson | Ephemeral rivers and their catchments: sustaining people and development in western Namibia[END_REF][START_REF] Crerar | An unexpected factor affecting recharge from ephemeral river flows in SWA/Namibia[END_REF][START_REF] Svendsen | Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia[END_REF]. Using Google Earth, it is possible to observe how the ephemeral rivers can be dammed by the dunes of the Skeleton Coast, creating some flood reservoir basins.

The Skeleton Coast is characterized by a wide dune belt, 6-22 km wide [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF]. As observed in [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF], this dune belt dams the W-SW flowing ephemeral rivers on their route towards the ocean. Of these rivers, the southern Koigab, Uniab and Hunkab, are characterized by infrequent ephemeral flows, whereas the northern Hoanib and Hoarusib, are fed by large catchment areas. In this manner, these rivers are characterized by higher discharge and more frequent flows, because they are influences by the intertropical convergence zone having more monsoonal rainfalls [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF]. In the Figure 1, a Google Earth map is used to show the rivers (note that the map is rotated). It is necessary to tell that the river damming due to sand dunes is different according to latitude. The dune belt increases in width and height and changes from the low barchanoid dunes in the south to the large transverse dunes in the north. Since the morphology of the dune field is changing, it happens as a consequence that the northern rivers are effectively dammed at the eastern margin of the erg, causing flood reservoir basins there [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF]. In contrast, southern rivers are dammed within the erg, because the most pronounced and commonly encountered barrier is given by the dunes at its western border [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF].

In the abovementioned paper [START_REF] Krapfa | Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)[END_REF], Landasat images were used. Here, let us see what can be observed on Google Earth. In the Figure 2, for instance, we have a river which is able of opening its route to the ocean among the dunes. It is the Uniab River. The original Google Earth image was adjusted by GIMP. However, we have the possibility to observe, for a northern river, where we find large transverse dunes, the damming of rivers. In the Figure 3, we can see the Hunkab ephemeral river, in images dated 3/2011 and 5/2012. This river is dammed by the dunes. In the left panel we can see a flood reservoir. As we can see from the Figure 4, the reservoir is stretching itself parallel to the dunes, more than one kilometre long. Another example of reservoir in shown in the Figure 5. In fact, Google Earth SITS of this region is containing a limited number of images, therefore the satellite images given in Figures 345are probably not coincident to the maximum of flood. However, to roughly estimate the effect of the flood, we could study the perturbation of the sand dunes about the reservoirs and the patterns which had been created by flowing water. In this manner, it could be possible to estimate how large was the area involved in flooding. Let us conclude, remarking that a large research work is and was made on these rivers. It is important, because flowing ephemeral rivers and their catchments are water resources for people living in this region who is currently using them [START_REF] Jacobson | Ephemeral rivers and their catchments: sustaining people and development in western Namibia[END_REF].

Figure 1 :

 1 Figure 1: Maps of rivers in Namibia, Skeleton Coast.

Figure 2 :

 2 Figure 2: Uniab ephemeral river among the dunes.

Figure 3 :

 3 Figure 3: Hunkab ephemeral river dammed by the dunes, as shown by Google Earth SITS. Note the flood reservoir basin in the left panel. It is also shown in the following Figure 4.

Figure 4 :

 4 Figure 4: Flood reservoir basin Hunkab river.

Figure 5 :

 5 Figure 5: Flood reservoir basin of Hunkab ephemeral river in Google Earth Namibia.


Amelia Carolina Sparavigna

Rock glaciers on Mars viewed with Google Earth

Keywords: Mars, Google Earth, Satellite Imagery

Several glacier-like landforms have been identified on Mars. We can view some of them with Google Earth and its virtual Mars planet. In the images of Ismeniae Fossae glaciers, the flow lines of these landforms are clearly visible too.

Ice is a substance which is common in the solar system. Even Mercury, the planet closest to the Sun, is hosting ice, as observed by a NASA spacecraft that provided the first optical images of it in the shadows of a crater [START_REF] Chabot | Images of surface volatiles in Mercury's polar craters acquired by the MESSENGER spacecraft[END_REF]. Ice is also covering moons of Jupiter and Saturn and comets are made of it, mixed with rocks, dust and frozen gases such as carbon oxides, methane and ammonia [2]. Besides water ice, the solar system possesses many other types of frozen substances, not only in the comets. The poles of Mars for instance contain frozen carbon dioxide, the deposition of which, during a pole's winter, is caused by low temperatures. In fact, 25-30% of the Martian atmosphere is deposited into slabs of CO2 ice (dry ice) [START_REF] Mellon | The presence and stability of ground ice in the southern hemisphere of Mars[END_REF].

Mars has two permanent polar ice caps, which consist primarily of water ice [START_REF] Kieffer | Martian north pole summer temperatures -Dirty water ice[END_REF][START_REF] Bibring | Perennial water ice identified in the south polar cap of Mars[END_REF]. However, besides in its polar caps, the planet has ice in several glaciers [START_REF] Vv | Glaciers on Mars, Wikipedia and references therein[END_REF][START_REF] Squyres | Geomorphic evidence for the distribution of ground ice on[END_REF][START_REF] Head | Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for Late Amazonian obliquity-driven climate change[END_REF]. A particular concentration of them had been found in the Ismenius Lacus region [START_REF] Vv | Glaciers on Mars, Wikipedia and references therein[END_REF]. At mid-Martian latitudes, the current models of Martian atmosphere are showing that ice should not be stable if exposed [START_REF] Williams | Stability of mid-latitude snowpacks on Mars[END_REF]: it is supposed therefore, that most glaciers must be covered with a layer of rubble or dust that can prevent the transfer of water vapour in the sublimation of ice into atmosphere [START_REF] Head | HRSC Co-Investigator Team, Tropical to mid-latitude snow and ice accumulation, flow and glaciation on Mars[END_REF]. The atmospheric pressure on Mars is so low that ice simply evaporates, if it is not well protected under a thick layer of dust. As told in [START_REF] Vv | Glaciers on Mars, Wikipedia and references therein[END_REF], because Martian glaciers are contain substantial proportions of debris, they could be better described as rock glaciers [START_REF] Head | Cold-based mountain glaciers on Mars: western Arsia Mons[END_REF][12][START_REF] Haeberli | Permafrost creep and rock glacier dynamics[END_REF][START_REF] Vv | Rock Glacier[END_REF].

Satellites orbiting over the planet had been used to identify the Martian rock glaciers [START_REF] Whalley | Rock glaciers and protalus landforms: Analogous forms and ice sources on Earth and Mars[END_REF]: with the radar measurements made by the NASA Mars Reconnaissance Orbiter, the presence of ice had be determined and measured. In this manner, a research team of the University of Copenhagen has recently estimated the total volume of water ice present in massive belts of glaciers at the mid-latitudes of Mars, concluding that this volume is a very important water reservoir of this planet [START_REF] Karlsson | Volume of Martian midlatitude glaciers from radar observations and ice flow modelling[END_REF]. The belts of glaciers are located around Mars between the latitudes of 30 and 50 degrees, on both northern and southern hemispheres. Some of the Martian glaciers can be viewed with Google Earth and its virtual Mars planet (maps of Google Earth are of a higher resolution than those on the browser version of Google Mars). On the Mars virtual globe, we can find some very-high-resolution images coming from the Mars Reconnaissance Orbiter's HiRISE camera (HiRISE means High Resolution Imaging Science Experiment). To see the glaciers, let us consider the region of Ismeniae Fossae. In the Figure 1A, we can see an elevation map (Google Maps) of this region and, in 1B, the same region in Google Earth. The Context Camera (CTX) on board NASA's Mars Reconnaissance Orbiter (MRO) spacecraft has taken the images we find in 1B (Location: 40.59°N 42.08°E, acquired on January 28, 2007; Location: 40.46°N 43.48°E, acquired on February 8, 2007; Location: 41.77°N 44.11°E, acquired on November 19, 2006). HiRISE images are also available. In the Figure 2, we can see details from the image taken by the Mars Reconnaissance Orbiter at location 40.59°N 42.08°E, on January 28, 2007. The visibility of flow lines is enhanced using Gimp Retinex [START_REF] Sparavigna | Gimp Retinex for Enhancing Images from Microscopes[END_REF][START_REF] Vv | Retinex[END_REF]. In both panels of Figure 2, we have the merging of two valleys in which ice, according to the local slope, is flowing.

The streamlines we clearly see in the images are interpreted to represent ice-flow features, as told in [START_REF] Baker | Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae[END_REF]. In any case, using Google Earth, we can easily make comparison to glaciers on Earth: in the Figure 3 for instance, the flow lines of glaciers in Alaska are shown.

Figure 1 :

 1 Figure 1: A) Elevation maps (Google Maps) of Ismeniae Fossae region. B) The same region in

Figure 2 :

 2 Figure 2: Two details from the image taken by the Context Camera (CTX) on board NASA's Mars Reconnaissance Orbiter (MRO) spacecraft. Location: 40.59°N 42.08°E, acquired on January 28, 2007. The visibility of flow lines is enhanced using Gimp Retinex [17,18]. Note the behaviour of glaciers at the merging of two valleys. Glaciers are flowing according to the local slope.

Figure 3 :

 3 Figure 3: Two images of glaciers in Alaska for comparison. The visibility of flow lines is enhanced using Gimp Retinex [17,18].


Amelia Carolina Sparavigna

Sinkholes of Dead Sea in Satellite Image Time Series

Keywords:

Here we investigate in satellite image time series of Google Earth a phenomenon linked to the decreasing level of the Dead Sea, the appearance of sinkholes.

In 1930, its surface was 1,050 km 2 and its level was 390 m below sea level. From this year, the Dead Sea has been monitored continuously [START_REF] Vv | Overview of Middle East water resources -Dead Sea[END_REF]. In recent decades, the Sea has been rapidly shrinking because of diversion of incoming water from the Jordan River. The southern end of the Sea is fed by a canal maintained by a company which is converting the sea's raw materials. To preserve the Sea, in December 2013, Israel, Jordan and the Palestinian Authority signed an agreement for laying a water pipeline to link the Red Sea with the Dead Sea [4]. In January 2015 it was reported that the level of water is now dropping by three feet a year [1].

The Dead Sea level drop is causing the appearance of large sinkholes along the western shore. It is linked to freshwater that dissolves salt layers, creating subsurface cavities that collapse forming the sinkholes [START_REF] Klein | Contribution to the knowledge in the fluctuations of the Dead Sea level[END_REF]. In Ref. [START_REF] Yechieli | Sinkhole "swarms" along the Dead Sea coast: reflection of disturbance of lake and adjacent groundwater systems[END_REF], it was told than in 2006, more than a thousand sinkholes had developed along the western coast of the Sea since the early 1980s, all occurring within a narrow strip less than 1 km wide. The researchers observed that the appearance of sinkholes was a highly dynamic phenomenon that was accelerating in the recent years of their observations. The sinkholes had been produced by the salt dissolution by groundwater, a cause evidenced by direct observations in test boreholes. However, as told in [START_REF] Yechieli | Sinkhole "swarms" along the Dead Sea coast: reflection of disturbance of lake and adjacent groundwater systems[END_REF], the entire phenomenon was involving the Sea is a hydrological chain reaction, which "starts by intensive extraction of fresh water upstream of the Dead Sea, continues with the eastward retreat of the lake shoreline, which in turn modifies the groundwater regime, finally triggering the formation of sinkholes".

The sinkholes of the Dead Sea are the subject of several researches, among them [START_REF] Closson | Subsidence and sinkhole hazard assessment in the southern Dead Sea area, Jordan[END_REF][START_REF] Wust-Bloch | Pre-collapse identification of sinkholes in unconsolidated media at Dead Sea area by 'nanoseismic monitoring' (graphical jackknife location of weak sources by few, low-SNR records)[END_REF][START_REF] Closson | Structural control of sinkholes and subsidence hazards along the Jordanian Dead Sea coast[END_REF][START_REF] Atzori | InSAR?based modeling and analysis of sinkholes along the Dead Sea coastline[END_REF][START_REF] Nof | Sinkhole precursors along the Dead Sea, Israel, revealed by SAR interferometry[END_REF][START_REF] Yechieli | Sinkhole formation and subsidence along the Dead Sea coast, Israel[END_REF]. In 2015, in [START_REF] Yechieli | Sinkhole formation and subsidence along the Dead Sea coast, Israel[END_REF], the authors tell that more than 4,000 sinkholes have formed since the 1980s (four times the number given in 2006). Their formation rate accelerated in recent years to > 400 sinkholes per year. Moreover, an association between sinkhole sites and land subsidence is revealed by interferometric synthetic aperture radar (InSAR) measurements [START_REF] Yechieli | Sinkhole formation and subsidence along the Dead Sea coast, Israel[END_REF].

The highly dynamic phenomenon of sinkhole, such as the drop of the dead Sea level can be easily observed in the satellite images of Google Earth. In fact, this virtual globe is offering for some large areas of the Dead Sea, several images with high resolution, arranged in SITS (a SITS, Satellite Image Time Series, is a set of images taken from the same scene at different times). SITS offers opportunities for understanding how a location is changing, and, in some cases, also predicting the future changes. Therefore, it is a fundamental tool for environmental monitoring and analysis of land-cover dynamics, such as for monitoring the land use by human activities [START_REF] Yang | Using a time series of satellite imagery to detect land use and land cover changes in the Atlanta, Georgia metropolitan area[END_REF][START_REF] Serra | Monitoring farmers' decisions on Mediterranean irrigated crops using satellite image time series[END_REF][START_REF] Seto | Quantifying spatiotemporal patterns of urban land-use change in four cities of China with time series landscape metrics[END_REF][17]. In some previous papers, for instance, we have shown examples of SITS to investigate the motion of sand dunes [START_REF] Sparavigna | A study of moving sand dunes by means of satellite images[END_REF][START_REF] Sparavigna | Moving dunes on the Google Earth[END_REF][START_REF] Sparavigna | The GNU Image Manipulation Program applied to study the sand dunes[END_REF][START_REF] Sparavigna | A case study of moving sand dunes: The barchans of the Kharga Oasis[END_REF][START_REF] Sparavigna | Analysis of the motion of some Brazilian coastal dunes[END_REF].

Let us consider here some sinkholes near Ein Gedi, as given in SITS by Google Earth. Ein Gedi is an oasis, near Masada and the Qumran Caves. The series is given in the Figures 1234 We can also use the images of May 2002 and July 2010 for having a composed image for an easy comparison (see Figure 5). The image had been obtained using GIMP, the GNU Image Manipulation Program. In it, it is impressing how the edge of the Sea moved. However, the drop of the Sea is continuing. In the Figure 6, we can see in a HERE map, a more recent image (2014) of the same location. The satellite images are clearly showing that the Dead Sea is drying up. Of course, it is also evident from them, that the number of sinkholes is rapidly increasing, making the shores of the Sea a more dangerous place today than in the past. If the satellite images had a high enough resolution, a monitoring of sinkholes could be made precisely and directly from local population,

  , from May 2002 to July 2010. Note that the edge of the sea moved of about 145 meters in eight years.

Fig. 1

 1 Fig.1

Fig. 2

 2 Fig.2

Fig. 3

 3 Fig.3

Fig. 4

 4 Fig.4

Fig. 5 :

 5 Fig.5: Using Figs.1 and 4, combined by means of GIMP, the GNU Image Manipulation Program, we can easy see how the edge of the Dead Sea moved in eight years.

Fig. 6 :

 6 Fig.6: A HERE map gives us a more recent image (2014) of the same location of Fig.5.

through services such as Google Earth. Shores of Dead Sea then are places where a direct monitoring from above of the local environment could help in a safe management of human activities.