Proposition for propagated occupation grids for non-rigid moving objects tracking Benjamin Lefaudeux IMARA -INRIA Rocquencourt benjamin.lefaudeux@inria.fr

Gwennael Gate

Aldebaran Robotics Email: ggate@aldebaran-robotics.com Fawzi Nashashibi IMARA -INRIA Rocquencourt CAOR -Mines Paristech Email: fawzi.nashashibi@inria.fr Abstract-Autonomous navigation among humans is, however simple it might seems, a difficult subject which draws a lot a attention in our days of increasingly autonomous systems. From a typical scene from a human environment, diverse shapes, behaviours, speeds or colours can be gathered by a lot of sensors ; and a generic mean to perceive space and dynamics is all the more needed, if not easy. We propose an incremental evolution over the well-known occupancy grid paradigm, introducing grid cell propagation over time and a limited neighbourhood, handled by probabilistic calculus. Our algorithm runs in real-time from a GPU implementation, and considers completely generically space-cells propagation, without any a priori requirements. It produces a set of belief maps of our environment, handling occupancy, but also items dynamics, relative rigidity links, and an initial object classification. Observations from free-space sensors are thus turned into information needed for autonomous navigation.

I. OCCUPANCY GRIDS -EXTENDED

A. General considerations 1) Perception of non-structured moving objects: Amid a vast set of localisation and perception algorithms currently developed, among which various SLAM variants have taken the greater share for the last years, some challenges are still not addressed in the perception field. Detection and tracking of moving objects, without pre-requisites on rigidity, structured environment or immobility remains a tough question, and is not yet addressed by many approaches. Different SLAM-based approaches have been used for the last years, some of them being very successful in building a map of the environment from a non-associative sensor, while dealing with the identification of some moving objects amid the map. FastSLAM, using particle filtering and initially presented by Thrun et. al ( [START_REF] Thrun | Fastslam: An efficient solution to the simultaneous localization and mapping problem with unknown data association[END_REF]) or other iterative optimisation techniques (ICP, [START_REF] Wang | A Hierarchical Object Based Representation for Simultaneous Localization and Mapping A Hierarchical Object Based Representation for Simultaneous Localization and Mapping[END_REF] [START_REF] Steux | CoreSLAM: a SLAM Algorithm in less than 200 lines of C code[END_REF]) are very powerful algorithms which can deal with some level of movement amid tracked points, as long as fixed points of the map are enough to estimate robot pose and environment mapping. This proved effective, for example by Wang et. al ([4]), which showed that moving points could be separated from a still structure with a good accuracy, and tracked accordingly. Overall scene requisites are however still strong in this setup, and would probably not be enough for reliable moving object perception and tracking in a highly unstructured environment, without a majority of solid points to observe, which can easily happen outdoor in densely populated areas. Another possibility is exploitation of detection before tracking, using information from visual or speed measures sensor, in order to remove moving objects from the traditional SLAM calculus. This has notably been demonstrated by Agrawal ([5]), which proved effective to detect individuals while still needing fixed points as a reference. Contrary to SLAM, our aim is thus not primarily to localise oneself in the environment along with building a map of its static features, but instead to be able to reliably detect and monitor position and dynamics of surrounding moving objects, possibly outdoor and without much visible solid infrastructure. Those observations will drive the following developments, and while the proposed algorithm is nowhere near best SLAMs in terms of accuracy, we believe it could be useful in typical human environments.

2) Interests and limits of the simple grid-based approach:

Occupancy grids are very common in robotics, since their first introduction by Moravec and Elfes ([6]), initially related to sonar based mapping. Principles are simple and effective, relying on the information storage and information source spatial localisation similarities, thus allowing to keep any spatial relations between cells at a minimal cost. This is still used nowadays, although not being any more the only processing step of the algorithm, and can still be viewed as a very-capable mean of storing spatially related information or sharing information between several subjects. It is also specially well-suited to deal with promising new massivelyparallel computing capabilities. Occupancy grids were indeed present in most of the latest SLAM propositions (based on filtering or optimisation), and in many of broadly speaking perception systems (Badino et al [START_REF] Badino | Free space computation using stochastic occupancy grids and dynamic programming[END_REF] showed for example a free-space perception system based on vision and occupancy grids in 2007, as did more recently by Yguel et al. [START_REF] Yguel | Grid based fusion of off-board cameras[END_REF]).

In those examples, grids are however only used for information representation and storage, most of the processing being due to external algorithms. Sensor filtering over time is indeed rarely considered within the grid formalism (grid update rule often relies on mere accumulation). Links between cells are also rarely considered while updating grids, spatial and temporal independence between measures being at the heart of the initial simplicity of the method. Obviously a big step from accurate world description, this is basically of little consequences provided the aim is, as it was in the original Moravec article [START_REF] Moravec | High resolution maps from wide angle sonar[END_REF], the cartography of still environment. In case we chose the occupancy grid formalism to track moving objects, this is however a major limitation, as one could expect the displacement of physically related points to be correlated.

3) Optimal update to occupancy grids: Considering a hypothetical occupancy grid propagation, one could wonder why an optimal Bayesian propagation would not be possible. The probability of any possible cell displacement could be computed, thus computing the most-probable map prediction from a given set of measure, allowing both temporal filtering and a link between new measures and known information. Trouble is, computing the formal probability (provided all the needed information is known) is extremely demanding, if one is to consider every possible move. A complete probability calculus, considering a set of S possible state in any cell of a N * M grid, would imply the consideration of each and every possible map, which is S N * M . Taking the smallest possible state sampling (2 states), and a ridiculously small map of 10*10 cells, and the amount of maps to compute to get to the full solution is already beyond any reasonable range (2 100 ). The key to this "absurd" complexity when keeping every probability on track, which could seem somewhat odd from a human perception (we don't usually need much time to get a good grasp on a typical scene involving moving objects), surely relies in a lot of useless probabilities being taken into account, even possibly beyond the causality principle. Anything at a given place have little to no chance of influencing the very next future of a very remote location, and this is the idea behind dealing with neighbourhood-restricted probabilities. Another possible approach, demonstrated by Coue et. al in [START_REF] Coue | Bayesian Occupancy Filtering for Multitarget Tracking: An Automotive Application[END_REF], would be the use of a Bayesian network coupled with a motion model for dynamic objects mapping. In this case, no actual association needs to be done, this being handled by the Bayesian framework computing transition probabilities between states. The main difference with our approach is that they do not attempt to compute the most probable next place for a given occupancy, but rely on a given motion model (constant velocity in this example). This is very fine in most cases, and definitely is an improvement over static occupancy maps, but we believe that this can lead to the wrong prediction in some cases, among which heavy occlusion or colliding courses. We however certainly share a lot of the abstraction presented in this article, although a few more notions are present in our algorithm.

B. Tackling inter-dependence within computing boundaries

We propose the use of propagated occupancy grids, able to deal with some of the interactions between cells, in a common prediction/measure Bayesian cycle. Our algorithm aims at taking into account both temporal and spatial relations between measures, while keeping computing costs low enough to conceive a real-time use and concurrent use to other more specialised algorithms.

Firstly, we introduce the probability for every cell to move to its neighbourhood, given previous knowledge of the scene (occupation, speed, classification) and specific heuristics (separate cells cannot converge, nor can cells from the same object diverge). Secondly, we compare this prediction to a new measure, and compute the most probable estimate given prediction and latest measure. Thirdly, we update associated knowledge used in the prediction step, namely occupancy of every cell of the grid, speed, relation between cells (in a neighbourhood) or object classification used for different sensor models. Those principles were presented by Gate in [START_REF] Gate | An approach for robust mapping, detection, tracking and classification in dynamic environments[END_REF], initially on a standard CPU implementation, and showed very promising results despite a high computing cost making it prohibitive for any real-time application.

1) Initial definitions : :

The probability mass functions (pmf ) modelled in the following algorithm concern a set of notions that we'd like to define : -Mapping (occupancy) probability M k (x i ) ∈ [0, 1] of the cell x i in the spatial environment E at the time k, provided the measures Z 0:k = {z 0 , ..z k } :

P (M k (x i ) = 1|Z 0:k ) ∀x i ∈ E (1)
-Vehicle localisation (including position and speed in E ×V ), at the iteration k. This is not yet addressed by the algorithm, and in the examples below every speed and position is relative to the vehicle.

P (L k = l j |Z 0:k ) ∀l j ∈ (E × V ) (2)
-Association, ie the probability for a given cell from the iteration k-1 to be associated with another given cell at the iteration k. In our case, only associations coming from a restricted neighbourhood are taken into account, which cuts the number of evaluated map candidates from an exponential dependence on the number of cells to a more reasonable dependence on neighbourhood scale.

P (X next k-1 (x i ) = x j |M k-1 (x i ) = 1, Z 0:k ) ∀(x i , x j ) ∈ E 2 (3)
-Velocity probability of a cell, given its occupancy and previous measures :

P (V k (x i ) = v|M k (x i ) = 1, Z 0:k ) ∀(x i , v) ∈ E × V (4)
-Detection probability, to handle the probability that two given cells x i and x j are part of the same object. The neighbourhood constraint limiting interactions to a finite part of the map is used once more, to limit the intricateness and heavy computing cost. Affiliation to a given object can however be "propagated" further than one cell's neighbourhood, although our span is limited and this could prove to be a problem. Detection probability, D k (x i , x j ) ∈ [0, 1] is 1 if x i x j are from the same object.

P (D k (x i , x j ) = 1|M k (x i ) = 1, M k (x j ) = 1, Z 0:k ) ∀(x i , x j ) ∈ E 2 (5)
Several criteria could be used to determine this probability from measures, among which a constant relative configuration, fit with a given shape, or a set of distinct characteristics (speed, shape, colour,..). In this first implementation, a simple geometrical criteria is used : D k (x i , x j ) = 1 if the distance between x i and x j is conserved over iterations, with a Gaussian decrease elsewhere (detailed below).

-Classification designs the probability of this set of cells to be part of a given class of objects (car, pedestrian, still object,..).

We attempt to model this by matching extended characteristics of a set of cells (beyond geometrical characteristics for example) to a model. This conditions the update rule of association calculus, envisaged future positions of a cell being for example adapted from its class motion model. Classification probability are simultaneously kept from a different set of classes, a cell being capable of a partial fit with different classes. This ensures a more robust classification, initially prone to errors.

P (C k (x i ) = c j |M k (x i ) = 1, Z 0:k ) ∀(C × E) (6)
2) Update rules -proposed algorithm : : Having set these definitions, the proposed update rules, implemented as such in the parallel we present as a last-part example, are as follows : -Associations are updated in a several pass mechanism, making an extra initial assumption of independence between cells behaviour, which we attempt to correct in a second part. This behaviour was already present in the [START_REF] Gate | An approach for robust mapping, detection, tracking and classification in dynamic environments[END_REF] proposition, and is a key to the possible use of massively parallel computing. Approximations are obviously primordial in our attempt to make the calculus feasible in real-time, but we believe most of the interactions between cells are still modelled with this proposition.

P local (X next k-1 (x j )|M k-1 (x j ) = 1, Z 0:k ) = η • P local (z k |X next k-1 (x j ), M k-1 (x j ) = 1, Z 0:k-1 ) Correction • P local (X next k-1 (x j )|M k-1 (x j ) = 1, Z 0:k-1 ) P rediction (7)
η is here a normalisation constraint, to ensure that possible moves sum up to one for any given cell.

First we then compute the local associations prediction, which is to say that macroscopic interactions are not yet taken into account : cell previous speed and class are used to predict the asserted new positions. This could be seen, similarly to SLAM particle filters ( [START_REF] Thrun | Fastslam: An efficient solution to the simultaneous localization and mapping problem with unknown data association[END_REF]), as a new set of particles generated for every cell of the grid iteratively, representing this cell's occupancy possible next moves, depending on previous knowledge and motion model. In our case, initial predictions are weighted by a Gaussian, whose standard deviation is function of the identified class of the object (thus representing the possible uncertainty in an object next move, this being different for a pedestrian or an identified bus). The centre of the Gaussian weight is relative to the object previous speed estimation.

P local (X next k-1 (x i ) = x j |M k-1 (x j ) = 1, Z 0:k-1 ) = Ψ(x j , x i , V k-1 (x j ), C k-1 (x j )) (8)
which could be rewritten as, with g the standard Gaussian expression and σ its standard deviation dependent on the identified class :

Ψ(x j , x i , V k-1 (x j ), C k-1 (x j )) = g(x j + V k-1 (x j ) • dT -x i , σ C k-1 (xj ) ) (9)
Last measure is then taken into account to produce an estimated local association, still without macroscopic constraints to alter these predicted associations. Predictions are weighted according to the sensor model occupancy new measure, while keeping the normalized sum of all possible displacements :

P local,weighted (X next k-1 (x i ) = x j |M k-1 (x j ) = 1, Z 0:k ) = γP local (X next k-1 (x i ) = x j |M k-1 (x j ) = 1, Z 0:k-1 ) • P (M k (x j ) = 1|Z k ) (10)
with γ such as a k ∈A P local,weighted (a k ) = 1 (A being the set of investigated associations).

Associations initially local estimation are then altered according to additional constraints : unlikely moves are penalised according to different heuristics (different cells cannot converge to the same place, cells from the same rigid object cannot diverge). Rigidity and non inter-penetration constraints are modelled by the potential function Φ association , which is currently based on two Gaussian-window weight functions.

P (X next k-1 (x j ) = x|M k-1 (x j ) = 1, Z 0:k-1 ) ≃ a k ∈A    1≤j≤N P local (X n k-1 ext(x j )|M k-1 (x i ) = 0, Z 0:k )    • Φ association (a k , E, Z 0:k ) ( 11
)
were A is again the set of all possible associations. The process here described can be differently factorised, but was split into several summations in an attempt to increase its readability.

-Mapping is computed taking into account the two cases : in the cell is a newly observed presence, or the displacement (possibly null) of a previously seen cell. In the first case, the sensor model is the only input taken into account, in the second case contributions from all the possible associations are summed up to compute the predicted occupancy. Cell interactions have in this case already been taken into account in the association computation. The two cases are dissociated by a random variable S k , which can take two values : 0 if the cell has never been seen, 1 if the cell has already been seen. Its probability is computed with association computation results : to sum up, if the considered cell corresponds to a local association maximum, P (S k (x i ) = 1|Z 0:k ) takes the value γ ∈ [0, 1], else it takes the value 1γ. The value of γ is chosen depending on the "renewal" rate of the map, that is to say "how often do we think a new object can appear from nowhere" ? A value around 0.5 has proven to work well in practice.

P (M k (x i )|Z 0:k ) = P seen (M k (x i )|S k x i = 1, Z 0:k ) • P (S k (x i ) = 1|Z 0:k ) + P unseen (M k (x i )|S k x i = 0, Z 0:k ) • P (S k (x i ) = 0|Z 0:k ) (12)
with the corresponding calculus (A being the cell neighbourhood) :

P seen (M k (x i )|Z 0:k ) = j∈A {P (X next k-1 (x j ) = x i |M k-1 (x j ) = 1, Z 0:k-1 ) • P (M k-1 (x j ) = 1|Z 0:k-1 )} (13)
P unseen is in this case typically related to the sensor occupancy model.

-Velocities are computed taking into account the same two possibilities, depending if the observed cell is considered a new one, or the association of an already-observed cell to a new position : -considering the velocity of already-observed cells, velocity is simply computed from the associations, summing up speed values steming from all the possible contributors.

-considering appearing cells, the probability distribution of velocities had been proposed by Gate in [START_REF] Gate | An approach for robust mapping, detection, tracking and classification in dynamic environments[END_REF] as follows, and kept in this proposition :

P (V k (x i )|S k (x i ) = 0, Z 0:k ) = 1 card(V ) (14)
Merging of the two possibilities is done similarly to eq. 12.

In current implementation, we only retain the most probable a posteriori value, and a confidence value, instead of retaining the whole probability distribution.

-Detection update needs a broader view from previous information, in order to be able to detect structures and links between updated cells. In this paper we propose a simple (and fast) mechanism to handle this detection, but we expect to adopt a maybe more "large scale" approach in the future. The mechanism proposed in this initial algorithm consists in measuring the auto-correlation between consecutive cells associations after filtering, in the vicinity of their neighbourhood. We then measure the compatibility of their predicted moves with the "rigid body" hypothesis. With the approach used in eq. 12, we dissociate in this calculus cells which are believed to have been seen before from "new" ones, for which no rigidity information can be guessed.

P (D k (x i , x j ) = 1|M k (x i ) = 1, M k (x j ) = 1, Z 0:k ) = a k ,a ′ k ∈A k ×A ′ k {P (X next (x i ) = a k |Z 0:k ) • P (X next (x j ) = a ′ k |Z 0:k )} (15) with a k , a ′
k being in fact the same associations in each cell respective referential (we go through every possible association for the (x i , x j ) cells. Although not very illustrative, eq. 15 calculus is fast to compute, but relationships further than a cell neighbourhood are not taken into account (typically a few meters radius). This could prove insufficient for the tracking of big objects, and other methods could be investigated in the future. A simple k-means clustering could for example be used outside of the prediction/update cycle to emphasize object detection for an external navigation task.

-Classification updates can similarly be done using every gathered information (mapping, velocity, rigidity links,..) correspondence to a given sensor model, which would on the other hand improve prediction steps of the algorithm. This is not yet present in our implemented algorithm, and presented results can thus be seen as perfectible. There is however no theoretical constraints on this calculus, which should be in place in our implementation algorithm in a short time for several classes of objects (pedestrian, cars,..). The performance impact is to be investigated, but should not theoretically prevent the algorithm to run in real-time, every added class acting in this organisation as another "layer" of probabilities to be computed. Worst impact could thus be a linear cost in terms of the number of identified classes, which should not be increased inconsiderately.

II. PRACTICAL IMPLEMENTATION AND RESULTS

A. Performance considerations

As stated in eq. I-A3, complexity constraints on calculus are not to be neglected in occupancy grid update rules, many thinkable algorithms being simply not realistic in terms of computing needs. The complexity of the mechanism we propose can be summarised as follows :

-for the sake of simplicity, we state a N * N grid, every cell being able to move in a M * M neighbourhood.

-considering the propagation of one cell, every possible move of every of its neighbours (O(M 4 )) are to be investigated for each individual envisaged propagation (O(M 2 )), which translates in O(M 6 ) complexity.

-cells updates being independent except for conformation rules already taken into account in the previous step, overall cell considerations is finally of O(N 2 * M 6 ) complexity for the full update step.

Heavy approximations are of course still present when compared to an full propagation calculus, namely that each and every cell moves are initially computed individually, although being later filtered to take some interactions into account. Computation is thus still not intricate, which keeps the complexity "low", although O(N 2 * M 6 ) remains a heavy burden for any realistic dimensions. Several points can be emphasised from this simple complexity calculus : Firstly, although the computation remains slow by all means, the relative independence of most calculus make it a plausible candidate for a parallel implementation, which is nowadays common and lifts some of the computing time constraints. Secondly, digging into specific complexity aspects, our algorithm is also linear in complexity as regards the number of cells for a given neighbourhood (which translates into a squared dependence as regards the size of the grid, naturally). This seems quite a burden, but one must remember the usual exponential complexity of algorithms meaning to explore any possible point move onto a map. This relative lightness in the grid size complexity is a key benefit for sharing applications : extending the size of the grid translates into linear increase in complexity instead of an exponential increase in case of a propagation calculus on the whole map. Extending the domain of tracked speed is however more complicated : tracked speed depends on the span of possible moves taken into account at every iteration, literally V max = M/2 * ∆ τiteration with ∆ the spatial extension relative to one grid cell. Considering a given iteration maximum computing time (limited for example by the Lidar frequency for real-time operations), the maximum tracked speed is rapidly capped by the computing power at disposal relatively to the maximum computing time. We'll see with our preliminary results that this translates to very acceptable maximum speeds for our initial implementation on current hardware.

B. Some results

As usual when dealing with grid-based algorithms, sensor occupancy models are a key factor in our proposition. A standard Lidar occupancy model is used in this initial implementation, computed on GPU. Occupancy of areas in the shadow of laser impacts are chosen to 0.5 out of 1, neutral in our occupancy ratio. Laser impacts are otherwise set to an occupancy probability of 1, while empty spaces between the vehicle and impacts are set to 0, as it is common using Lidars. Real data gathered in an urban environment are used. Logging and replay framework is RTMaps software from Intempora. Exhaustiveness and generic nature of the algorithm is important : every mapped cell of the environment is considered equal, and no a priori is ever made on geometrical bounding, preferred positions, structures, stationary or moving parts. Although still quite demanding on computing resources, the algorithm works in real-time on current state-of-the-art hardware. The first scene computed below needs 120ms to compute on a GF100 GPU from nVidia counting 448 cores, which translates without additional work to below 100 ms on higher-end offerings currently available. As regards raw performance, this algorithm is also an initial draw, and pure implementation could certainly be greatly improved as it is often the case with hardware-sensitive programming.

1) Algorithm memory: In this example, we emphasise a temporary occlusion situation, where a pedestrian shadow hides another previously seen pedestrian (figure eq. 1). Only the Lidar sensor is used in this case, camera captures being presented for illustrative purpose, along with bounding boxes. On the pmf representations (figures eq. 2 and eq. 3), the point of view is from above, in a common "bird-view" perspective. All the boxes are drawn for illustrative purpose, we don't present here the output of a detection algorithm. The resolution of grid mapping is 15cm, speeds up to 4.5m/s being theoretically tracked. This last value can be improved without any computing cost by simultaneously degrading the spatial resolution and increasing the range of the measures, which could be a dynamic trade-off depending on the vehicle speed. The shadowed pedestrian is still clearly visible on the We thus stress the importance of sensor filtering, taking into account possible spatial and temporal correlation. In this case, consequences of the pedestrian not being visible without filtering has no practical consequences, but this is not always the case and we believe that such filtering would be compulsory for autonomous vehicle navigation in an urban area.

2) Dynamics estimation: This example plans to emphasise dynamics estimation capabilities of the algorithm, along with segmentation of the scene. Although this is already possible via bounding boxes in the case of clearly separated persons, those matchings often miss when persons are too close to each other, or when groups change in size due to some people joining and leaving. The exhaustive approach that we carry on a per-cell basis provides an estimation of the probable speeds. All boxes presented on figures eq. 4 eq. 5 eq. 6 are here on illustrative purpose, and do not come from a detection algorithm. In green are the crossing pedestrians, which may have been difficult to track on sensor data alone (cf figure eq. 5), due to the pushchair and their proximity. In blue is the road sign, which is obviously standing still, but which shows the residual speed of the vehicle (end of the braking sequence) and noise on Lidar data. It is barely visible on the speed map, due to the very low residual speed of the car. Segmentation of the scene after the algorithm process between moving and still parts proves effective. In red is the car coming on the other way, which is also going slow due to the crossing pedestrians, and have very few Lidar impacts (laser beam was oriented upwards, maybe too much). Tracking on geometrical grounds on Lidar data alone may have been difficult in this case.

C. Future works

Alternative source of occupancy, velocity or classification are planned, mainly based on vision processing. Stereo-vision is for example a proven source of free-space measure, as shown by Moravec once again in [START_REF] Moravec | Robot Spatial Perceptionby Stereoscopic Vision and 3D Evidence Grids[END_REF], or more recently Badino et al. using dense disparity calculus and occupancy grids ( [START_REF] Badino | Free space computation using stochastic occupancy grids and dynamic programming[END_REF] and articles following). Motion detection and evaluation have also be proven to be a valuable output from stereo-vision capture, Argrawal et al. demonstrating in [START_REF] Agrawal | Real-time detection of independent motion using stereo[END_REF] that platform ego-motion could also be removed from the initial optical flux in order to track moving objects. Initial theoretical work on this use would be from Adiv et. al ([12]), although this field has received a lot of attention in the past years, notably since dense stereo-vision processing is now possible in realtime. To finish with, the state of the art as regards visionbased SLAM (notably Davison [13] [14])) leads us to believe that laser-based sensors could possibly be replaced in the near future for most perception tasks, this being a strong incentive for us to develop visual inputs. Another source of possible improvements would be an evolution to handle collaborative perception, by means of merging grid-based beliefs in our algorithm.

III. CONCLUSION

Perception of moving objects, such as pedestrians, with minimal requirements on their size, moves or behaviour is a difficult task ; which will nevertheless be a key to enable autonomous navigation in urban environments, or even comprehensive assistance on current automotive devices. We proposed a novel technique based on an extension of the traditional occupancy grids registration, using probabilist propagation and extensive consideration of displacements and interactions over a restricted neighbourhood. An evolution from an initial ambitious proposition, our work shows promising results while being capable of real-time execution, although still being a work-in-progress. An interesting evolution would be its extension towards collaborative perception, which should be easier than for some other approaches due to the grid-based principle being kept. Many challenges would still need to be resolved in order for separated vehicles to take part into one another perception of the environment, among which localisation or synchronisation inaccuracies, but we believe that autonomous transportation and road safety would benefit a lot from such developments.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Successive camera views

Fig. 3 .

 3 Fig. 3. Occupancy evaluation from our algorithm

Fig. 4 .Fig. 5 .

 45 Fig. 4. Camera capture


INTRODUCTION & BACKgROUND

several sites of the so called "pre-aksumite" civilization have been established in tigrai region between the 8 th and the 5 th century Bc. this civilization had significant contacts with south arabia. script, architecture, and cult of several deities have been brought by sabaean immigrants in the first half of the I st millennium Bc. recent excavations at Maqabər Ga  əwa (near wukro) revealed, for example, the remains of a temple of almaqah, whose cult existed in the kingdom of saba (see I. Gajda et alii 2009). as for the history of the kingdom of aksum, between the end of the 2 nd and the end of the 6 th century aD, it is only known by a few various sources. even if south arabian influence is obvious in economy, trade, and social structures in both "pre-aksumite" and aksumite civilization, the history of ethiopia is still misunderstood. the objectives of our research in the region of tigrai are therefore, both a contribution to the periodisation of the antiquity in ethiopia (no chronological classification of any artefact has been yet established for ancient ethiopia), and a study of the evolution of ancient ethiopian civilisation and its contacts with other countries, from the Mediterranean sea to India.

In this very context, three main axes of research have been defined for the next four years:

1/ a comparative study will be carried out especially on potteries coming from various sites, but also on different types of construction, and artefacts.

2/ a study of local tradition, both written and oral. the aim of this study is to try to localize other antiquities, to identify the toponyms, to trace modern and ancient roads, to search for the historical information in the local tradition, to study the inscriptions in their archaeological context.

3/ a work on the periodisation of the ancient ethiopian history based on the available data and on the results obtained by our mission.

The site of Wakarida

In agreement with ato kebede amare Belay, General Manager of the tigrai culture and tourism agency at Mekelle, a preliminary mission, which took place in March 2010, has been concentrated on documenting several sites first surveyed by the ethiopian team in the region of atsbi-Dera, east of Mekelle (see I. Gajda & F. [START_REF] Dugast | of ethiopia, Ministry of Youth, sport and culture. authority for research and conservation of culture heritage[END_REF]. Further to this first mission, the site of wakarida, in the Tobia / district of sawena, eastern tigrai (fig. 1), has been chosen to start systematic archaeological investigations to achieve our major aims.

the choice of this site is due on one hand to its geographic situation (even if distant of the main axes of circulation already known, it is turned towards the red sea), on the other hand to an important chronological sequence that may be obtained according to the already collected pottery on surface.

Implementation

the main tasks in 2011 were to initiate archaeological excavations and research at wakarida to be proceeded in the following years. In this purpose, our mission made a geophysical survey on the site, as well as test excavations, and initiated a provisional typology of the pottery.

Before an extensive excavation, our priority was in fact to estimate and evaluate the archaeological potential of the site, its area and organization. specific fieldwork objectives were to implement a sampling design which employed geomantic techniques including Gps (Global positioning system) and GIs (Geographic Information system) (see II), to initiate a more systematic survey which employed specific techniques including geophysical methods.

this point has been achieved thanks to an electromagnetic survey which consisted on electrodes resistivity and reflectometry measurements -taking into account the nature of the soil (see III).

Beside, three test excavations consisting on soundings have been opened on the top of the hill as well as in the eastern and northern parts of it, one of which near a cleared out building (see Iv). Besides, a few pedestrian surveys have been carried out around the site, in order to recognize the real expansion of the archaeological area, and even other settlements nearby (see Iv).

as "pre-aksumite" and aksumite artefacts are still not well known, the fieldwork objectives were as well to implement a sampling of each type of ceramics in order to produce a new typology with, as far as possible, dating references. a systematic collection of artefacts with a systematic register and stratigraphic localization, has been initiated and developed into the basis of a preliminary typological study and chronological markers (see v).

Running and team

our project is based on the collaboration of specialists of several domains. at this stage though, only geophysicists, archaeologists and a surveyor were needed. It involved anyway ethiopian scholars: Yohannes Gebre sellassie is part of the mission since the very beginning, and tekle hagos since 2011 even though he could not participate in March 2011. and of course, we were helped by the French centre for ethiopian studies, and the Bureau of culture and tourism of the regional state of tigrai, as well as the Tabia's administrator and officials from sawena, and the local people who were very cooperative and concerned to the investigations. the investigations were directed by Dr Iwona Gajda and Dr Fabienne Dugast, from the centre national de la recherche scientifique (cnrs) in paris, and team members included:

• Dr vittoria Buffa, archaeologist (Italy), working besides in the Italian Mission to oman,

• Dr christian camerlynck, geophysicist from the University pierre & Marie curie in paris (France),

• Quentin vitale, student, geophysicist from the University pierre & Marie curie in paris (France),

• Xavier crapery, surveyor from altea surveyors in lyon (France),

• Yohannes Gebre sellassie, historian of aksumite period, Mekelle (ethiopia),

• haylay teklay, from the Bureau of culture and tourism of the regional state of tigrai in wukro (ethiopia),

• ato tadesse, the Tabia's administrator from sawena (ethiopia),

• amare stetotane, simeneh Bacha and sisay Gethewe, from the French centre for ethiopian studies, in addis ababa (ethiopia),

• abraha hailu, hailu abera, woldu tesfeare, Fitsum Gebru, and lamlam woldu, from wakarida (ethiopia).

During this very first campaign, we have got in touch with two scholars from the University of Mekelle -Yohannes aytenew and hiruy Daniel -, who paid a short visit to the site and showed an interest in helping the mission. It is dependent on sa  ese tse  da emba administrative Woreda / region, in the Tabia / district of sawena.

I. ThE sITE OF WAKARIDA

the site has been established on a rocky outcrop, which overlooks a large valley surrounded by chain of mountains, respectively from north to east and south to west: Daima, afedadae, Dagaraebe and arebata (fig. 2). From now on, the place is made up of cultivated lands and farm houses, which belong to four heads of family: ato haile Maryam, ato hagos, ato hailu and ato woldu.

Preliminary survey

the site has been surveyed in 2004 by tekle hagos, from addis ababa University, and habtamu Mekonnen, from the tourism and culture commission of tigrai regional state at Mekelle (report, 2004: 16-18).

a complex has been found on the north-eastern side of the hill, consisting of 4 adjoining rooms, somewhat 3 x 3,50 m square each, arranged 2 by 2. In the middle of one of the rooms was a pillar like wall, actually destroyed by the inhabitants. More structures arose on both sides of the complex, consisting as well of regular groups of adjoining rooms. actually, local people use the stones as building material for their own houses, which are erected mostly on ancient structures.

More structures were visible nearby, from top to mid-slope of the hill. the area seemed to cover some 5 ha, looking north and east, on the valley. trails open north and east in the red sea direction.

Ethno-historical investigations

During our stay at wakarida, with the help of hailay teklay, representative of the Bureau, Yohannes Gebre sellassie has been able to record oral history and legend relating to the site. he discussed with three informants: ato Gebre selassie tesfay, ato hailu abera, and ato Qashe aberha.

-According to the legend, the earliest settlers of wakarida were the people of hamädä and Dob  a. these people, says the legend, came from south arabia, and settled in the highland part of tigrai. when they came to wakarida, they were tigrigna speakers.

During the epoch of Dob  a, there was a recurrent famine, due to lack of rain for straight 7 years, and continuous attacks waged by a powerful king called tobas. this tobas, says the legend, was reigning in south arabia and some parts of tigrai. Because of the recurrent famine and war, the local historians say that the settlers of sawena were forced to leave their homeland. Before leaving, the elders say that the inhabitants buried their goods.

-According to Ato gebre selassie Tesfay, the first migrants from south arabia settled at ra  elä, not far from sawena. the evidence of their settlement, he says, is the existence of dam structures visible to this day. From ra  elä, they settled at sawena, after sawena they moved to aksum. ato Gebre selassie says that agriculture and irrigation started in ra  elä and sawena. the structures we see in sawena belong to these early settlers.

-According to Ato hailu Abera, long ago there were two tribes who settled at sawena, and the surrounding areas. these two early settlers were Dob  a and hamedo. when hamedo was reigning (they call him Zemene hamedo), there were innumerable settlers in the area. Hamed in tigrigna means "earth" or "soil". During this time, because of overpopulation, there was scarcity of arable land and fertile soil. ato hailu told us that even dowry (endowment provided during marriage ceremony) was with soil:

Just before the marriage ceremony, the family of the bride-groom builds a platform, and the family of the bride endows them with soil. the newly married couple cultivates sorghum (mashila), and subsists the whole year with the cereal.

the structure we see at sawena, according to ato hailu abera, belongs to the ancient urban settlement of the early settlers, hamedo and Dob  a.

-after the introduction of christianity into aksum, ato hailu claims that Maryam sawena was constructed at sawena. emperor Zera Yakob (1434-1468), who learnt about the greatness of Maryam sawena, is said to have donated a guild land to the church. Ato Qashe Aberha adds that when the cathedral of aksum tseyon was pillaged and burnt by the army of Gragn ahemed (1527-1543) and Ga  ewa, the tabot of tseyon Maryam took refuge at sawena.

II. TOPOgRAPhY

X. Craperi

Before beginning any geophysical and archaeological work on the site of wakarida, it was necessary to set up geo-referenced points and grid, as well as a topographical mapping of the site.

Implementation

wakarida's area is located on a rocky outcrop. considering the constructions and the ground evolution, it was necessary to make a polygonal (many-sided land survey).

Because of the kind of soil -compact and stony, rocky outcrop -, the stations have been first located with plastic stakes (bought in France), which suited to the ground, and were sufficient. Four perennial geodesic markers (bronze nails into concrete blocks) have been set up on the area, placed in line in order to make each of them visible from the others.

a reference grid was necessary before making any geophysical measurement. In agreement with the geophysicists, we chose 50 meters squares turned towards north (see III).

on the first two days, we made a rough land-survey, marked the boundary of area, and placed the existing constructions.

Measures & calculations

Measures and calculations have been made thanks to a closed polygonal of 8 stations (1000 to 1007). each of them measures two other stations, which were taken with a total station leica Flexline ts06 (fig. 3). the polygonal has been calculated on covadis calcul software, in an independent system.

we chose the station number 1004 as the starting point, as it was the lowest (so that it makes altitudes easy to understand), considering X=1000, Y=2000, Z=100. station number 1004's horizontal angle was established at 0, pointing magnetic north (quite close to the geographical north there) in order to get a plan already positioned towards north, to simplify the reference grid.

the reference grid has been superimposed on the drawing on autocad, to make the stake out easier. those points have been then converted into a GIs file for the leica total station. stake out & drawing stake out has been set up thanks to the two usual methods: orientation and triangulation.

thanks to the referenced grid, the geophysicist's magnetic cards and the topographic plan could be superimposed, and completed. Finally, we completed the land survey, noting down each detail in the area, about every 10 meters, in order to draw the level lines (fig. 4).

all these information enable to draw geodesic markers index card with description and pictures of the references, in order to have an orientation for each marker even if it is not possible to see another st (destruction, new building between both of them, vegetation...).

Landscape

the outcrop shows up a kind of round or even oval-shaped, stony ground, looking south-east to some 250 m long.

the hill reaches 2,360 m at its highest point, on the north-western part, and 2,340 m at its lower point, next to the valley. It slopes down steeply on the north, on the way to sawena, but far more gently on the south to the valley, on a quite fertile soil.

actually, two modern houses have been set up on the western side of the hill (ato woldu) and one on the eastern side (ato hagos). the south-eastern slope is divided into few parcels of land, delimited with drystone walls. several other houses have been built up in the valley -and even on top of the surrounding hills -, to farm the land nearby.

III. gEOPhYsICAL sURvEY

Chr. Camerlynck & Q. Vitale the aim of the geophysical survey was to estimate and evaluate the expansion and the organization of the site of wakarida before any extensive excavation.

geological context

Based on the Geological Map of ethiopia and observations on the field, it seems that the site of wakarida is formed of the tsaliet Group formation (fig. 5). It is a late precambrian unit (upper proterozoic) that is mainly composed of low-grade metamorphic and metavolcanic rocks. chlorite schist outcrops are visible down on the north-western slope of the site, and metamorphic sandstones outcrops are located on the top of wakarida hill. the whole site is covered with decametric rock blocs composed of the surroundings outcrops, and almost all the tsaliet Group rocks are hence represented on the site.

Choice of the methods

In an archaeological context, the most common geophysical survey methods are the electric, the magnetic, the Ground penetrating radar (Gpr) and the electromagnetic (eM) ones. each of them has of course advantages and drawbacks. the choice of a particular method depends on the type of archaeological structures we expect to find (size, material, etc.), and the local geological background. the available time on the field is also a constraint that has to be taken into account in the choice of the method. Based on the previous 2010 survey performed at wakarida, the site was expected to be quite rocky and was assumed to be electrically very resistant. It has hence be decided to use preferentially a method which does not need direct contact with the ground to take measurements (electrical survey for instance, needs metal electrodes to be pushed into the ground, therefore if the ground is too rocky or too resistant, not enough current can be sent into it, and the results might not be significant).

considering the very short time available to perform this first geophysical survey, we decided to use a method which is easy and fast to set up on the field, and allows us to cover as much ground as possible. we choose the magnetic method, as it requires less equipment, and can be set up more easily than Gpr. It also generally allows covering more ground.

Because of the natural geomagnetic field orientation at those latitudes (close to the magnetic equator, the inclination of the geomagnetic field is only 14°), and the potential presence of volcanic rocks on the site, we were not sure that the magnetic contrast between geologic and archaeological structures would be at the maximum. we therefore brought a resistivity meter in case the ground would be soft enough to perform an electrical survey.

Equipment and survey

• Magnetic survey the earth natural magnetic field (geomagnetic field) can be approximate by a magnetic dipole with its magnetic south pole close to the geographic north pole and its magnetic north pole close to the geographic south pole. this field mainly originates from the earth liquid outer core movements (internal magnetic field), but also in a smaller part from the interferences between solar winds and the upper layer of the atmosphere (external magnetic field).

the geomagnetic field intensity varies with the geographical position at the surface (internal field), and also with time (the external field varies on a day scale for instance). the geomagnetic field intensity is ranging from 20 000 nt (nano tesla) in south america to over 60 000 nt near the magnetic poles.

More locally, on an archaeological survey scale, variations of the magnetic field can also be caused by the magnetic properties of soils and rocks. those local variations (referred to as "anomalies") can either originates from natural heterogeneities or from human activities (fire place, brick walls, etc.).

to get rid of the natural daily variations we are using two sensors one on top of the other. this technique called gradiometer also allows us to get rid of the deep geological low frequency variations and enhances the weak shallow variations that might originate from archaeological structures. since the signal of both sensors is record we can still study each signal independently. Mapping the magnetic gradient on the field reveals magnetic anomalies that can then be interpreted as humanmade or natural structures depending on their organization, their geometry and other clues.

the magnetometer used on the field is a G-858 MagMapper from Geometrics. It is composed of two magnetic sensors, one on top of the other, and an acquisition consol, all fixed to an aluminium carrying structure. the power supply is ensured by a simple battery belt (fig. 6). the operator carries the whole equipment on his shoulders and belt, and walks across the prospecting areas to survey.

the survey was originally cut up in 50 x 50 m squares areas, spread over the site. the survey is performed along 50 m long profiles, separated from each other by 1 m (fig. 7). In order to maximize the magnetic signal received by the sensors at these latitudes, they must be tilted at an angle of roughly 45° with the ground, and the profiles must be oriented in the north-south direction. In the field we actually performed our profiles with a slightly shifted direction (n 166°).

the measurements are recorded continuously (every 0.1 s) along each profile. the measurements positioning is done automatically, assuming that the operator walks regularly along a profile, to reduce positioning errors.

the survey area is materialized on the ground by pulling 50 m-tapes on each side of the square. all the profiles are not materialized on the ground but a mark is placed on the tape on the other side of the square at the corresponding position, and then displaced of 1 m to the next position… this way of operating allows us, in good conditions, to cover over 2 ha a day. at wakarida, because of the presence of lots of walls (fig. 7), the 50 x 50 m areas were actually cut up in smaller pieces, slowing down the survey efficiency.

the map of the local magnetic field is then obtained by interpolating the field between each profile. Because of the survey gridsize resolution (1 m between each profile), we could not detect small structures with this procedure, as but their magnetic response may be very strong (fire place, etc.). on the other hand, large enough structures such as houses' walls are generally well detected.

• Electric survey the electrical survey is based on the measure of the soil's electrical resistivity. this resistivity is defined as the property of a rock (or soil) to oppose the flow of an electrical current passing through it. the resistivity naturally varies from 1 to 10 6 Ω.m, and depends on the soil's constitution, porosity and water content. Basically, when a soil is porous and well wet, it will be less resistive than a dry and dense soil. Using four electrodes pushed into the ground, two injecting an electrical current (called a and B), we can measure the resistivity of the volume of ground contained in between the two others electrodes (called M and n).

collecting the value of resistivity of the whole prospected area, we obtain a map of the soil electrical resistivity. If there is a buried archaeological structure, such as a wall, and that a significant resistivity contrast exists between the structure surroundings, the archaeological structure will hence be enlightened in the obtained map. For instance, a wall is generally denser than the surrounding soil and less water can therefore penetrate in the wall porosity. this will make the buried wall appears as a resistive "anomaly" compare to the rest of the soil.

a small electric survey was performed in a plowed plot which appeared to be soft enough to push the electrodes into the ground. the resistivitymeter used was a 4point light 10W from lippman. It has to be connected to four electrodes: two are injecting an electrical current into the ground, and the other two are measuring the induced tension.

we used a pole-pole configuration, that is to say that two electrodes are placed far away from measured point. this configuration allows us to move only two electrodes (called probe electrodes) instead of four during the survey (fig. 8), which increases the area covered by the survey. the position of the measured point is defined as the middle of the two probe electrodes, and we consider that the depth of investigation is roughly equal to the electrode separation. In our case that corresponds approximately to one meter.

the survey is basically performed likewise the magnetic one, but with this method, there is no need for a special orientation of the profiles. we did continue to use north-south profiles anyway.

the measures are not recorded continuously, but one point is taken every meter (the measured gridsize is therefore 1 x 1 m). the position is then read on a tape which basically has to be displaced for each profile. the way of proceeding is hence slower that the magnetic survey, but the electrical contrast do not suffer from the metavolcanic nature of the geological background.

Interpretation

• Interpretation basics [Magnetic survey] (fig. 9)

to read the map properly, one has to keep in mind that magnetic anomalies are generally composed of two visible parts: a negative and a positive one. we generally consider that the real corresponding buried structure is basically centred on the negative-positive transition. the boundaries of the real structure are not those of the visible magnetic anomaly (which extension depends on the depth). Due to the low inclination of the geomagnetic field, magnetic anomalies may look spread, in comparison with similar anomaly acquired at mid latitudes with a more vertical geomagnetic field. therefore anomalies may look broader, little clear and thus difficult to understand, making the map difficult to "read". the magnetic field intensity has to be considered algebraically, that is to say high values of magnetic field, either negative or positive, correspond to highly magnetic bodies. likewise, small magnetic intensities, either negative or positive, will correspond to poorly magnetic bodies.

-the first remarkable magnetic anomaly (1) goes from the north-western corner to the south-eastern corner of the map. considering its size (over 300 m long and 10 m width) and its magnetic intensity, it corresponds to a deep geologic perturbation. the direction of the anomaly (roughly n 128°) can be correlated with an andesitic-sill outcrop (6), with vertical dip, pinpointed on the map. logically, we suppose that the visible magnetic anomaly corresponds to the underground extension of that sill that goes across the site. events highlighted in (5) represent off-wall topographic effect.

Unfortunately, this huge geological anomaly hides every archaeological magnetic signature that might overlay it.

-the global magnetic signature of the rest of the map is way lower. some punctual high-intensity anomalies are visible (see for instance 3) and might correspond to fireplace associated to blacksmith activities. this last hypothesis is supported by slag found on the surface of one of these anomalies. Unfortunately, interpretation only based on the magnetic map is not enough to assess the age of the corresponding structures.

-at the bottom of the top north-western corner we can see an alignment of low-intensity anomalies in the n 315 direction (circled in 2 and detail in fig. 17). this anomaly could correspond to several walls alignments. the top corner of this anomaly was dug and perpendicular walls were found as expected (see the archaeological description in Iv). this shows that the expansion of the habitation area in top of the hill is bigger than the houses visible at first time.

this also shows that the magnetic answer of walls on the site is weak. this is explained by the fact that houses are built with the same materials as the geological surroundings, which does not provide a strong magnetic contrast. anyway, the presence of andesitic rocks in the foundation makes them visible on the magnetic map. -the anomaly circled in 4 (north to the excavated house) has the same type of magnetic answer as (2), and might also be due to houses foundations.

• Electrical survey (fig. 10)

Just a small area (40 x 20 m), in the very south of the site (see 7 in fig. 9), has been electrically surveyed in a plowed plot. contrarily to what we first expect, the soil turns out to be reasonably resistive and even surprisingly conductive in some places.

-we can see three resistive zones in yellow (above 150 Ω.m) surrounded by conductive ground (roughly 50 Ω.m). this organization corresponds to a visible topographic step.

-another interesting feature is the presence of two very resistive (< 200 Ω.m) area in the north-eastern corner of the middle platform and the north-western corner of the prospected areas. this resistive area corresponds to visible sandstone outcrops, and thus to removal of soil material. It seems that the electrical survey in that plot does not yet provide archaeological information but gives clues to the geological substratum vicinity.

Conclusion

the short magnetic surveys performed show that the settlement previously studied in 2010 might be bigger than just the already visible remains. First of all because the expansion of the habitation zone in the northwestern area (2, fig. 9) is bigger than initially visible. then because there are other anomalies of the same type on the site (4, fig. 9, etc.) which might mean other buried buildings. Moreover, punctual anomalies (such as 3, fig. 9) are visible and might be associated with the presence of ancient fireplaces.

there are lots of other anomalies which are visible, but it is very difficult to assess their origin (either anthropic or geologic) be of the nature of the magnetic signal (weakness of the contrast between structures and geological surroundings).

Furthermore, a part of the prospected areas is under the influence of a massive geological anomaly (1, fig. 9) hiding every magnetic structures that might overlay it.

the use of the electrical resistivity method on a small area shows that instead of what was expected, the ground resistivity is not too high to prevent prospecting with this technique. Furthermore, the electric contrast between archaeological structure and the surrounding soil seems to be bigger than the magnetic contrast (soil more conductive than expected), which lets assume that mapping the electrical conductivity of the whole site might allow us to detect more structures.

...and Prospective

Based on this previous survey of the site of wakarida, we would like to improve our geophysical knowledge of the site by using other geophysical methods.

we have seen that the electrical properties of the soil and archaeological structure might be convenient to map the conductivity of the site with good results. the electromagnetic technique will allow mapping the conductivity of the site without any need of pushing electrodes into the ground. this will permit the survey even in the rockiest part of the site. nevertheless a systematic electrical survey should be considered over the area. we will also be able to investigate the part of the site under the influence of the big geologic magnetic anomaly without suffering from it. the use of Ground penetrating radar is also to take into consideration for precise localization of potential structures and direct information about depth.

Iv. sOUNDINg & sURvEY

V. Buffa & F. Dugast

together with the geophysical survey, three soundings have been opened, still on the outcrop, in three opposite places -north-west / B2, north-east / D2, and south / c4 (fig. 11) -, to help insuring both magnetic and resistive "anomalies", and the presence of ancient structures. one of them (the last one), next to the complex unearthed in 2008 (D2), aimed at establishing a first stratigraphic sequence, taking into account the important elevation owing to the accumulated sediments.

sounding in square B2

this area, on the upper part of the site, was chosen because of few anomalies showed by the geomagnetic survey. a square 2 x 2 m wide was opened to check if one of the anomalies corresponded to a structure (fig. 11).

the top soil, Us2, was removed. It consisted of reddish brown loam, fine, loose. It incorporated some stones and some animal bones. Underneath a layer of stones was present. the stones have been removed in half of the trench. another layer of brown loam was present; in it an area of ashes. sounding in square C4

another small sounding was performed around a visible structure in square c4 on the eastern slope of the site (fig. 11).

Us1 was removed. It consisted of fine, reddish brown loam, hard packed on surface, softer underneath. It incorporated some medium size stones. very few pottery shards were recovered, and also some animal bones. Under about 10 cm, some yellowish and whitish stones of small and medium size begin to appear. In order to check the depth of the anthropic deposit on the site, and hopefully of a long stratigraphic sequence, a sounding has been performed the last two days along the southern wall of the structure. the sounding is located in square D2 (fig. 11).

a trench 2,60 x 6,50 m was opened. here it was necessary to remove the soil recently put to level the area (used now as a barnyard). no finds were recovered from the modern surface. Under the top soil a collapse of large stone blocks was unearthed. It was probably in relation with the modern digging of the structure. no finds were recovered in the collapse.

the layer underneath was named Us3. It consisted in loose brown loam of fine structure, incorporating medium size stones. also Us3 can be put into relation with modern activities (fig. 16).

Us4 was the layer underneath and the first ancient anthropic deposit. It consisted in fine brown loam with some medium size stones (fig. 17). It yielded some vessels, mainly jars, in large fragments. Us5 was excavated for a small stretch under Us4. It consisted of the same loam, but did not contain any stones.

a wall is running west-east parallel to the wall of the first excavated structure, roughly 2 m from it. another wall, forming a corner with the previous one is closing the space to the north-east (fig. 16). the walls unearthed during this campaign, like the ones already exposed, show the typical aksumite technique in the alternation of blocks dressed only on the external side and horizontal long slabs (fig. 18), which can be compared with the so called "palace of Dongur" near aksum.

the top soil (Us6) was then cleared out to the west of the trench to join the wall unearthed in past years. here a threshold going into the room previously excavated was cleared out. It consisted of a well arranged line of stone slabs (fig. 19).

survey in surrounding

In other parts of the site, some tidying of the area has been performed in order to clear out parts of walls visible on the surface. the 2010's survey had located a few of them on the eastern side of the hill (I. Gajda & F. Dugast 2010: 13), consisting on a regular suite of adjoining rooms, somewhat 3 m square, along the slope (sT D3, fig. 11).

apart from the northern and eastern structures -that is on the top of the hill, next to B2 sounding (st B2), down on the northeastern side of the slope (sT D1), along the slope on the eastern part of the site (sT D2 and sT D3), and on the southern side of the slope (sT C4) -, a few others, very similar, have been detected on the very southern side of the hill (sT B5, fig. 11 &20). Moreover, the survey has shown more structures and artefacts down in the valley, expecting now a larger expansion of the site.

another survey has been set up on a small hill south-east of wakarida to find out ancient settlements the inhabitants mentioned to the team -with no real results. But extensive survey may be more successful.

Conclusion

several structures have been detected all along the hill, only a few unearthed. If no chronological connection can be absolutely insured today between these structures, one has to notice the same direction of the buildings -roughly south-west to north-east -, bordering the slope.

the amount and closeness of structures induce one to think about a grouped settlement rather than a scattered one, set up by an organized community whose leader or head may have lived in the D2 building. the latter reminds in fact a kind of if not public at least monumental building because of its technical construction.

the expansion of this maybe urban or at least rural community seems to be broader than expected in 2010. considering the presence of artefacts and structures down to the valley, it may reach almost 10 ha, even more.

v. POTTERY

V. Buffa a systematic collection of artefacts, with a systematic register and stratigraphic localization, has been developed into the basis of a preliminary typological study and chronological markers. a digital database will help sorting them by type (either ware, slip, decorations, and forms restored from type of rims, bottoms or/and handles), and survey area in order to establish a first classification reference and chronometry.

In fact, in the present state of research, the identification of pottery is still conjectural. Its study is based on an typology which is even now imprecise due to the lack of stratigraphic and technical data. no distinction of production has thus been established, either at the regional or territorial scale, or even at the import one. no question has been asked on the methods of production that seem to continue until today.

Implementation

the ceramic assemblage from the site of wakarida comprises:

1/ WAKA11, s -349 pottery shards collected randomly on the surface all over the site.

2/ WAKA11, Us1 -11 pottery shards from the cleaning of some structures visible on the surface in square c4.

3/ WAKA11, Us2 -21 pottery shards from the cleaning of some structures visible on the surface in square B2.

4/ WAKA11, Us4 -16 large fragments of vessels from the sounding in square D2.

5/ WAKA11, Us5 and Us6 -several pottery shards from the last sounding, in square D2, not studied yet.

all the shards have been numbered with subsequent number, Us by Us (Us3 in D2 sounding has not yielded any pottery shards). at the end of the mission the collecting has been stored in plastic wrap and plastic boxes with mention of the origin. the samples have been left at the tourism and culture commission of tigrai regional state at wukro.

During the short campaign it has been possible to make a first assessment of the assemblage. this has been done on the site itself. no drawings have been made, but the majority of shards have been photographed, except the one yielded in the last sounding D2.

the shards have been studied Us by Us, but no significant difference could be appreciated, in fabric or shape in the different Us. the observations that will be made below are therefore valid for all the layers, except from the very last one, that is Us5 and Us6.

general characteristics

First of all the pottery has been divided according to the fabrics. In doing so it could be observed that the assemblage of wakarida shows a broad division in orange, brown and grey/black fabrics. It seems possible that the division corresponds to the one mentioned by Munro-hay in his account of the research done by n. chittick in aksum in 1972-1974: "red axumite ware", "Brown axumite ware" and "Grey and Black axumite ware" (Munrohay 1989). this assumption needs further verification also because the assemblage gathered in wakarida shows a more complex variety of fabrics that at first noticed.

Fabric

all the shards under study are handmade. six type of fabric can be noticed: a. Fine or compact bright orange paste with a quantity of mica and some tiny mineral grits, usually white; surfaces always carefully burnished. In few cases black core. this fabric is probably the most common on the surface of the site.

b. compact orange paste, coarser that the previous one, with some mica and mineral grits; surfaces burnished; sometimes black core.

c. compact paste, black thick core and orange surfaces. the orange surfaces do not seem to have been obtained by a slip, and the black inside does not seem to be the result of firing in a reduced atmosphere. surfaces burnished. Further investigation is needed.

d. rather compact brown paste with steatite temper; surfaces burnished.

e. compact black paste with steatite temper; surfaces burnished.

f. compact reddish yellow paste with mica (possibly a variation of fabric b); surfaces burnished.

Decoration

Incised and impress decoration with geometric patterns are the most common (fig. 21). some painted decoration on fine vessels are present (fig. 22).

Function

as to the function of vessels, it has been noticed the predominance of table vessels, and the almost absence of storage vessels. It has been observed that in modern time containers other than ceramics are used for storing goods; it could have happened the same thing in the aksumite times. It is most probable that the steatite tempered vessels were used for cooking.

Provisional typology

• Type 1. shallow ledge-rim bowl on foot. reddish compact or fine paste with white mineral grits and some mica, sometimes black core; surfaces slipped or plain; both surfaces burnished; incised or impress decoration on rim. [s295,261,5,267,321,278,332,145,71,249,291,292,259,70,11,154;Us4,15.] • Type 2. some ring bases probably belonging to type 1see Munro-hay 1989: 253, fig. 16, 137, 119. [s180, 181, 287, 328, 269, 283, 188, 43, 310, 264, 265.] • Type 3. shallow bowl with everted rim. reddish compact paste with white grits, some black and mica, grey core;

both surfaces burnishedsee Munro-hay 1989: 260, fig. 16, 170. [s19, 34, 153, 94, 35, 285, 286, 33, 283, 158.] • Type 4. large plate with everted, rounded or thinned rim, slightly convex profile. reddish compact paste with white grits and mica; both surfaces burnishedsee Munro-hay 1989: 270, fig. 16,232. [s326, 177, 184, 41, 158, 59, 13, 174, 173, 161, 336, 198, 46, 172, 157, 159, 187, 197, 149, 280, 255, 156, 165, 178, 188, 44, 272, 290, 292, 294, 33, 186, 185, 10, 47, 170, 171, 142, 169, 164, 143, 175, 4, 337.] • Type 6. cylindrical beaker, incised and painted decoration. Fine orange paste with mica, both surfaces burnish. [s335, 330, 133, 284, 91, 301.] • Type 13. small decorated jar with vertical handle. compact reddish paste with white grits and mica; surfaces smoothened. Decoration of bands of incised oblique lines and row of impress dots. [s276, 69, 6, 36 belong to one vessel. ] [s245, 236, 274, 141, 251, 243, 121, 237, 289, 254, 324, 242, 257, 312.] • Type 14. small convex profile bowl, everted rim, painted decoration on rim. orange fine paste with no visible temper, ext. burnished. Bands of purple paint on inner rim and on body.

[s316, 129, 128.]

• Type 15. large convex profile bowl with thickened and rounded rim. compact orange paste with white grits and mica; one fragment reddish brown; surfaces burnished. For the purple paint see 18,14,137,16,263,266,240,62.] one fragment of a ridged amphora has been shown to the members of the mission by a local inhabitant. It seems that it was collected while digging an aksumite room on the northern slope of the site. a few shards of the same type have been collected by the team on the surface of the site. the presence of amphores cotelees is already known from the aksumite layers in Matara (anfray 1966: 16, fig. 9) and adulis (paribeni 1908, fig. 58). anfray has pointed out that the presence of amphores cotelees is the proof of contacts with the Mediterranean (anfray 1974: 745-765).

Besides, very few fragments of fine red polished ware seem to be imported terra sigillata, Italian or eastern sigillata, though this identification is not sure. anfray has also mentioned the presence of terra sigillata at Matara (anfray 1974: 17).

even fragments of what anfray had called "braseros" at Matara could be identified in some fragments at wakarida (anfray 1966: 8, pl. Xv).

vI. CONCLUsION

I. Gajda & F. Dugast

the second season of the French-ethiopian project of archaeological and epigraphic investigations in tigrai region has been successful. the different investigations performed on the site of wakarida have permitted to estimate its archaeological potential.

-the geophysical survey proved to be really helpful in spite of particular, difficult conditions of the site: stony, hard soil, presence of geological formations. several structures have been detected, more may be suspected.

-three soundings have been undertaken, the choice of which was determined by the presence of visible structures (some of them unearthed by the inhabitants) and by the results of the geophysical survey.

-the structures unearthed in square D2 show a clear resemblance with aksumite construction techniques. are to be noticed in particular the walls built in unhewn stones separated in regular intervals by thin layer of large stone slabs which bring to mind the construction techniques used at the so called "Dongur palace" near aksum.

-the pottery collected on the surface reflects the intense occupation of the site in the aksumite period. a preliminary typological study of the ceramics has been undertaken. a detailed study including drawings will be done during the next season. -the quick survey of the surroundings of the site has shown its expansion to the eastern slope of the hill. hence, the site proves to be larger than we have expected.

-the local oral tradition, several versions of which have been recorded by the team members, mentions settlers coming in ancient time from the southern arabia. some vestiges of a dam, that should subsist near wakarida, but which are not visible today, are attributed by the inhabitants to these early settlers.

all the methods of investigation employed during this season were complementary and facilitated the team to establish a strategy of investigations. all bring us to the same conclusion: the site was a small town or village during the aksumite period.

Perspectives for the next season

as the main objective of our research is the contribution to the periodisation of the ancient ethiopian history, our investigation should be undertaken, in our opinion, both on the so-called "pre-aksumite" sites in the I st Millennium Bc and the aksumite sites in order to try to understand the transition from the "pre-aksumite" to the aksumite civilisation.

Investigations of the Aksumite site of Wakarida

our first aim during the next season is to start a regular excavation of the site of wakarida. the sounding in the square D2 will be continued and extended to the west. new soundings will be opened in the area c2-D2 and c4 (and around) where both the geophysical investigation and the numerous pottery shards on the surface let us suppose the presence of important structures.

a systematic survey around the site will be undertaken, especially at ra  elä and sawena, to endorse the legend.

the new collected pottery will be studied together with the one left in 2011 at wukro, in order to fulfill the provisionnal typology.

Figure 1 :

 1 Figure 1: Localisation of the site of Wakarida, Eastern Tigrai (FD 2011).

Figure 2 :

 2 Figure 2: The site of Wakarida, from South (FD 2011).

Figure 3 :

 3 Figure 3: Total station Leica Flexline TS06 (IG 2011).

Figure 4 :

 4 Figure 4: Topography of the site of Wakarida (XC 2011).

Figure 5 :

 5 Figure 5: Main geological formations of the site of Wakarida and its surroundings.[Extract from the Geological Map of ethiopia, 1/2 000000, 1972]

Figure 6 :

 6 Figure 6: Magnetometre G-858 MagMapper from Geometrics.

Figure 7 :

 7 Figure 7: Prospection grid on the field.

Figure 8 :

 8 Figure 8: Pole-Pole electrical configuration used on field.

Figure 9 :

 9 Figure 9: Magnetic survey map (CC & QV 2011).

Figure 10 :

 10 Figure 10: Comparison of Magnetic and Electrical survey on a small parcel (CC & QV 2011).

  Figure 11: Localisation of the three soundings 2011 and the visible structures (XC 2011).

  two walls have been cleared out, at right angles (fig.15). they are about 0,50 m thick, and made of rubble-stone. they may be in chronological connection with other walls immediately next to them, on the eastern part of the same square (st C4, fig.11).

Figure 12 :

 12 Figure 12: Sounding in square B2 -US3 (FD 2011).

Figure 13 :

 13 Figure 13: Next to the sounding in square B2, 2 walls in right angles cleared out by the inhabitants (FD 2011).

Figure 14 :

 14 Figure 14: Next to the sounding in square B2, several structures at right angles visible on the surface (FD 2011).

Figure 15 :

 15 Figure 15: Sounding in square C4, two walls at right angles (FD 2011).

Figure 16 :

 16 Figure 16: Sounding in square D2 -US3 (FD 2011).

Figure 17 :

 17 Figure 17: Sounding in square D2 -US4 (VB 2011).

Figure 18 :

 18 Figure 18: Sounding in square D2 -The aksumite wall (VB 2011).Figure19: Sounding in square D2 -US6 (VB 2011).

Figure 20 :

 20 Figure 20: Structures detected in square B5 (FD 2011).

Figure 21 :

 21 Figure 21: Incised and impress decoration (IG 2011).Figure22: Painted decoration (IG 2011).

Figure 21 :

 21 Figure 21: Fragment of amphores coteless (Ig 2011).

  survey in the region of Feresmaitogether with the excavation on the aksumite site of wakarida, we would like to start a survey in the region of Feresmai where "pre-aksumite" sites were reported, namely addi Be'akel (see I. Gajda & F. Dugast 2010: 15-16) in order to find a connection between "pre-aksumite" and aksumite civilisations.

  Decoration of curved triangular incised lines, filled with black paintsee Munro-hay 1989: 242, fig. 16.

	[s1, 320, 319, 313. ]

• Type 7. cylindrical beaker, painted decoration. Fine or compact orange paste with mica or some white grits; surfaces burnished. purple paint, horizontal band on rim and vertical lines on bodysee

Munro-hay 1989: 311, fig. 16, 455, 458. [s248, 308, 102, 125.]

• Type 8. convex profile bowl, painted decoration. Buff or orange , fine paste with no visible temper, surfaces burnished, horizontal lines with inscribed triangles in brown paint.

[s239, s120.]

• Type 9. cylindrical beaker, incised and painted decoration. Fine orange paste with white grits and mica; inner surface smoothed, external surface burnished. purple painted lines pending from two horizontal incised lines.

[s2.]

• Type 10. cylindrical bowl? very compact orange paste with white grits and mica; surfaces burnished. ovoid knob below the thinned rim; horizontal painted band on rim.

[s293.]

• Type 11. convex profile bowl, thin walls, everted rim. very compact orange paste with some white grits and mica. one fragment has incised lines on the lip.

[s343, 271, 252, 8, 96. ]

• Type 12. convex profile bowl, as type 11, but coarser paste and thicker walls. compact orange paste with white grits and mica. one fragment burnished and painted with a purple line under rim.

  Deep globular bowl, thin walls. light reddish brown compact paste with white small grits and mica; surfaces burnished. carenated deep bowl with horizontal handle. compact grey paste with white grits and mica; surfaces smoothed. Incised triangles on upper wallseeMunrohay 1989: 303, fig. 16, 427.

	[Us4, 6.]
	• Type 20. Globular bowl with painted decoration. Fine light
	beige paste with mica, light orange slip on exterior;
	surfaces burnished. two incised lines and painted below
	the rim and incised lines forming triangular patterns,
	black paint filling the space between them.
	[Us4, 4.]

• Type 16. [Us4, 2.]

• Type 17. necked jar with incised decoration. compact grey paste with steatite temper; incised oblique lines.see

Munro-hay 1989: 307, fig. 16, 439, 440. [Us4, 9.]

• Type 18. Footed jar. ovoid jar with everted rim; a fragment of a ring base should belong to this vessel. Medium compact grey paste with white, black grits, some mica and some steatite temper. outer surface orange.

[Us4, 3.]

• Type 19. necked jar with incised decoration. high tronconical neck with everted rim, rounded bottom; vertical strap handle. Geometric incised and impress decoration on neck and shoulder. two fragments compact orange paste with some white grits and mica (possibly some steatite temper), black rim, surfaces burnished, vertical knob on shoulder [Us4, 5, 7.] one fragment fine grey paste with mica, grey slip on both surfaces; incised oblique lines and horizontal row of impress small crosses on neck.

• Type 21. shallow decorated large plate. compact orange paste with white grits, grey core. oblique incised lines on exterior, excised triangles on interior.

[Us4, 1.]

• Type 22.

• Type 23. ovoid cooking jar with vertical handle. compact black paste with steatite temper; surfaces burnished.