Georgia Cablea
email: georgia.cablea@gipsa-lab.grenoble-inp.fr

Pierre Granjon
email: pierre.granjon@gipsa-lab.grenoble-inp.fr

Christophe Bérenguer
email: christophe.berenguer@gipsa-lab.grenoble-inp.fr

Method for computing efficient electrical indicators for offshore wind turbine monitoring

Offshore wind turbines availability is an important issue if such wind farms are to be considered a reliable source of renewable energy for the future. Environmental conditions and the low accessibility of such wind farms have contributed to the decrease of the availability of the wind turbines, compared to the onshore ones. In order to improve the reliability, condition monitoring systems and the implementation of scheduled maintenance strategies are a must for offshore power plants. This paper proposes a method of computing efficient electrical indicators using the available three-phase electrical quantities. These indicators are then to be used to obtain fault indicators for fault detection and diagnosis. The electrical indicators are obtained by using the instantaneous symmetrical components decomposition, a well proven method in power networks design and diagnosis. The new quantities are able to fully describe the whole electrical system and provide an effective mean to quantify the balance and unbalance in the system. The method uses the electrical three-phase quantities measured at the output of the generator in a wind turbine to obtain the indicators. The performance of this method is illustrated using both synthetic and experimental data.

Introduction

Wind power plants are a rather new and promising way of providing renewable energy. Moreover, as technology evolves, their reliability improves with time and modern wind turbines achieve a quite high availability [START_REF] Tavner | Offshore wind turbines reliability, availability and maintenance[END_REF] . However, unscheduled wind turbine failures most often result in high maintenance efforts, production losses and economical costs. To further decrease losses and costs, efficient maintenance strategies have to be developed according to each wind farm's specific needs. This need is further emphasized by offshore deployment of wind farms. The offshore turbines have much lower accessibility [START_REF] Tavner | Offshore wind turbines reliability, availability and maintenance[END_REF] so maintenance work has to be planned well in advance. To correctly plan a maintenance strategy, condition monitoring of systems and subsystems is needed. Several condition monitoring techniques have been developed and implemented in wind turbines [START_REF] García Márquez | Condition monitoring of wind turbines: Techniques and methods[END_REF] like vibrations, acoustic emissions, or oil analysis. Most of them are intended to monitor blades or mechanical parts (bearings, gears, shafts…), which are considered as critical wind turbines subassemblies. However, wind turbines electrical generators also have a high failure rate and generate a very high amount of downtime [START_REF] Tavner | Offshore wind turbines reliability, availability and maintenance[END_REF] . For such three-phase electrical systems, new approaches using electric signals have to be developed.

Raw three-phase electric signals contain a great quantity of information on the system they are part of, but are complex and difficult to interpret. In order to extract relevant features, the whole electrical information has to be split into different and specific indicators. One method used to separate such electrical information into meaningful quantities is the symmetrical components decomposition [START_REF] Hobson | Symmetrical components[END_REF] . This method has been first developed by Fortescue in 1918 [START_REF] Fortescue | Method of Symmetrical Co-Ordinates Applied to the Solution of Polyphase Networks[END_REF] and has since been widely used, for example in power networks calculations [START_REF] Paap | Symmetrical components in the time domain and their application to power network calculations[END_REF] (network modeling and control, power quality monitoring, fault detection…). Symmetrical components decomposition approaches have also shown good results in monitoring three-phase electrical motors [START_REF] Granjon | Condition Monitoring of Motor-Operated Valves in Nuclear Power Plants[END_REF] [START_REF] Vaimann | Using Clarke Vector Approach for Stator Current and Voltage Analysis on Induction Motors with Broken Rotor Bars[END_REF] . In this paper, this decomposition is used to analyze electric quantities measured at the output of a threephase generator. The associated estimation algorithm provides several electrical features that can be further processed or directly used as fault indicators dedicated to the generator. The adopted sample per sample approach allows its online implementation, and reaches good estimation and tracking performance in stationary and non-stationary case. The theoretical considerations behind this method are presented in the next section where the electrical features to be estimated are defined. Then, section 3 gives an overview of the algorithm used to estimate these quantities. The proposed algorithm performance is next studied in section 4 thanks to synthetic and experimental data measured on a small-scale wind turbine test bench. Finally, the conclusions and future works are summarised in section 5.

Method of electrical signature analysis for wind turbine monitoring

Electrical sub-assemblies used in wind turbines such as electrical generators are threephase systems. One efficient way to take into account the whole information contained in such systems is to use the symmetrical component decomposition, as it is customary for power networks [START_REF] Paap | Symmetrical components in the time domain and their application to power network calculations[END_REF] . From these symmetrical components, the design of fault indicators dedicated to the monitoring of such subassemblies can be expected. This idea is further develop in this second section, where the definition of symmetrical components is first given, followed by the proposition of some wind turbine electric features that can be used to elaborate fault indicators.

Three-phase electrical system analysis

Balanced three-phase systems

Most often, electrical signals and systems are analysed around their fundamental frequency 0 f . Indeed, the most relevant part of the whole electrical power is located around this particular frequency, where a lot of information is accessible. Following this consideration, electrical signals are often considered as pure sine waves of frequency 0 f with constant amplitude and original phase. A convenient notation for such single-phase electric signal   t x is its phasor notation  x t   defined in Eq. ( 1 ) , where   t x stands for a generic electric signal, either a voltage or a current.

 

  t f j t f j Xe Ae t x 0 0 2 2       . ( 1 )
In that case and for a known frequency 0 f , the complex amplitude

 j Ae X 
is then sufficient to completely characterize the original signal at 0 f , which is linked to its phasor through the simple relation      

t x t x
Re  . This approach can be easily extended to three-phase electrical signals

 x 1 t  , x 2 t  , x 3 t  
that can be represented at their fundamental frequency by three complex amplitudes  X 1 , X 2 , X 3 . In the case of a balanced three-phase system, the three electrical signals have the same amplitude and are shifted from each other by the same angle of 3

2

radians. This leads to the following phasor representation:

      t f j j j t f j e e e X e X X X t x t x t x 0 0 2 3 4 3 2 2 3 2 1 3 2 1 1                                       . ( 2
)
Eq. ( 2), clearly shows that for a known fundamental frequency, the balanced threephase system is fully represented by the single complex amplitude  X , as in the singlephase case of Eq. ( 1 ).

Unbalanced three-phase systems and symmetrical components

However, real electrical three-phase systems are never perfectly balanced. Consequently, Fortescue [START_REF] Fortescue | Method of Symmetrical Co-Ordinates Applied to the Solution of Polyphase Networks[END_REF] proposed in 1918 a method to decompose any n-phase unbalanced system in a set of n balanced systems called symmetrical components. As described in the previous section, the resulting set of balanced systems can be regarded as n single-phase systems, completely characterized by a set of n complex amplitudes. Following this approach, any unbalanced three-phase system can be resolved at its fundamental frequency into a sum of three balanced systems: -the positive-sequence component: three phasors equal in magnitude, shifted by 3 2 radians and in the same phase sequence as the original system, -the negative-sequence component: three phasors equal in magnitude, shifted by 3 2 radians and in the reverse phase sequence of the original system, -the zero-sequence component: three phasors equal in magnitude and with no phase shift between them. In matrix form, the three complex amplitudes  X 1 ,  X 2 and  X 3 of the original system can be related to the ones of the corresponding symmetrical components thanks to the simple linear relations:

                                 0 2 2 3 2 1 1 1 1 1 1 X X X a a a a X X X                                  3 2 1 2 2 0 1 1 1 1 1 3 1 X X X a a a a X X X , ( 3 )
where  X  ,  X  and  X 0 are the complex amplitudes of the positive-, negative-and zero- sequence components respectively and 3 2 j e a  is called the Fortescue operator modelling a phase shift of + 3 2 radians. It can be noticed that in the algorithm presented in Fig. 1, a factor of 2 has to be applied in the right-hand side of Eq. ( 3) to obtain amplitude invariance between the original system and the symmetrical components.

One major interest in using symmetrical components is that they provide the means to analyse general three-phase quantities at a given frequency by splitting them into balanced and unbalanced parts. Indeed, in case the original system is balanced, it only consists of a positive-sequence component, while the negative-and zero-sequence components are zero. On the contrary, the existence of negative-and/or zero-sequence components means that there is an unbalance in the three-phase system at the studied frequency. Therefore, the amplitude  X  of the positive-sequence component quantifies the amount of balanced signal, whereas  X  and  X 0 can be used to precisely quantify the unbalance in the original system. Several methods exist to estimate the symmetrical components of a three-phase system from the measured electrical signals, among which the most popular is obviously the discrete Fourier transform [START_REF] Tarasiuk | Comparative Study of Various Methods of DFT Calculation in the Wake of IEC Standard 61000-4-7[END_REF] (DFT). However, this approach is usually implemented on blocks of data, and cannot easily compute instantaneous values of symmetrical components at each new measured sample. One way to overcome this problem is to use the instantaneous symmetrical component transform first introduced by Lyon [START_REF] Lyon | Transient Analysis of Alternating-Current Machinery[END_REF] . This transform simply applies the same matrix appearing in the right-hand side of Eq. ( 3) to the three-phase electrical signals

 x 1 t  , x 2 t
 , x 3 t   instead of the complex amplitudes of Eq. ( 3). As a result, we obtain three complex-valued signals

 x  t  , x  t  , x 0 t   named
the instantaneous positive-, negative-and zero-sequence component respectively. By selecting the content of these signals around the fundamental frequency 0 f , it has been shown [START_REF] Granjon | Condition Monitoring of Motor-Operated Valves in Nuclear Power Plants[END_REF] that we obtain the classical symmetrical components of the original threephase system at 0 f . This last method is used in the algorithm described in part 3.

Wind turbines electrical features

Electrical quantities acquired on real electrical systems are usually voltages and/or currents, measured on a single phase or on all three phases. Such signals are very rich in content and it is difficult to isolate meaningful and clear electrical information using raw data. In this paragraph, the previous ideas and definitions are used to propose simple three-phase electrical features, highlighting particular and precise electrical information such as electrical unbalance, signal distortion or quality, electrical powers, etc. Afterwards, the proposed features can either be used directly or be further processed to obtain efficient fault indicators.

In order to propose well-defined features, several preliminary assumptions must be made. Firstly, the monitored electrical system is supposed to be star connected with a connected neutral wire, and the measured electrical quantities are line to neutral voltages and line currents. Nonetheless for the case of symmetrical components systems, the quantities can be easily converted from line to phase quantities and viceversa [START_REF] Hobson | Symmetrical components[END_REF] . Moreover, measured electrical signals are studied at their fundamental frequency 0 f , even if the same analysis could be made at any frequency. This means that in the following, all phasors and complex amplitudes are computed at 0 f . Finally, due to wind speed and direction fluctuations, wind turbines most often run under nonstationary conditions and generate non-stationary electrical quantities. Therefore, the electrical signals must be considered as non-stationary, and the simple phasor model given in Eq. ( 1) must be adapted to take into account amplitude and frequency variations of the fundamental component [START_REF] Boashash | Estimating and Interpreting the Instantaneous Frequency of a Signal[END_REF] :

        t j t j e t X e t A t x 0 0 ) (     , ( 4 )
where

 j e t A t X ) ( ) ( 
(time varying positive magnitude multiplied by a complex number) and the instantaneous phase

  0 t   is related to the instantaneous fundamental frequency  f 0 t   by:   0 t    2 f 0 u  du 0 t  . ( 5
)
It has also been shown [START_REF] Blodt | Mechanical fault detection in induction motor drives through stator current monitoring -Theory and application examples Fault Detection[END_REF] that in electromechanical systems, most mechanical faults generate amplitude and frequency modulations in electrical quantities. This also justifies the signal model adopted in Eq. ( 4), where

 x t
  stands for voltage or current signals,

either their measured values or their symmetrical components in the three-phase case.

Narrow band electric features

The previous model shows that at a given frequency, electric signals are fully described by their instantaneous amplitude and frequency. Eq. ( 4) and ( 5 ) show that the instantaneous amplitude can be easily obtained as the absolute value of the phasor, and the instantaneous frequency can be extracted from the time derivative of its phase angle. This is summarized by Eq. ( 6).

 A t    x t   ,  f 0 t    1 2 d 0 t   d t where   0 t    x t  . ( 6 )
For the three-phase scenario, the instantaneous frequency is computed as in Eq. ( 6) using only the instantaneous positive sequence component phasor

 x  t  .
The reasoning behind this is that this component usually has the larger magnitude, and that in case of a perfectly balanced symmetrical system, only the positive sequence component would be non-null. The method of symmetrical components provides the means to easily describe electrical unbalance in a three-phase system. Indeed, an obvious unbalance indicator is the ratio between the unbalanced part and the whole part of a three-phase signal at a given frequency. Using the instantaneous symmetrical components phasors, this simply expresses as:

            2 0 2 2 2 0 2 2 0 2 2 2 0 2 ) ( ) ( ) ( ) ( ) ( t X t X t X t X t X t x t x t x t x t x t u x               . ( 7
)
This indicator belongs to the interval   1 , 0 , where the ideal case of a perfectly balanced system would yield 0  x u and the worst case to 1.

Wide band electric features

An electric signal is not distorted or of "perfect quality" if it is constituted by a pure sine wave of frequency 0 f . One way of quantifying this "quality" is then to compute the ratio between the amount of signal contained at the fundamental frequency over the total amount of signal. A simple way to quantify the amount of a signal ) (t x being its mean power

    T T T x dt t x P 2 1
) ( lim , the following feature gives an instantaneous image of the signal quality:

      t P t P t x x x  SQ . ( 8 )
where

 P x t   and  P x t
  are local estimates of the mean power at the fundamental frequency and for the whole signal.

If this indicator equals 1, the signal can be considered a pure sine wave of frequency 0 f . On the contrary, if this feature equals 0, no component is present at 0 f . Finally,

 SQ x t  
can be interpreted as the fraction of signal localized at the fundamental frequency. This feature can be easily extended to the three-phase case noting that the mean power of a three-phase signal is the sum of the signal powers on each phase. Therefore, in terms of symmetrical components, the following relationship exists:

  2 3 2 2 2 1 3 2 1 ) ( ) ( ) ( 2
1 ) ( ) ( ) ( t X t X t X t P t P t P x x x        2 0 2 2 ) ( ) ( ) ( 4 3 t X t X t X      , ( 9 )
and such relations can be used in Eq. ( 8) to measure the quality of three-phase signals.

Electrical powers and phase shift

If both voltages and currents are available (either on one phase or on all three phases), the electrical powers and the phase shift between voltages and currents are interesting features to characterize relationships between these two electrical quantities. Once again, the following features are defined only at the fundamental frequency 0 f . The single-phase instantaneous complex electric power at frequency 0 f is given by:

            t j e t I t V t i t v t p 0 2 1 *    . ( 10 )
In terms of real and imaginary parts this complex power leads to active and reactive powers, while in terms of magnitude and phase angle it leads to apparent power and phase shift between voltage and current:

 P t     p t      1 2 V t  I t  cos  0 t      Q t     p t      1 2 V t  I t  sin  0 t      S t    p t    1 2 V t  I t     0 t    p t   ( 11 )
In practice and in the algorithm described in the next section, the power factor

 cos  0 t     is considered instead of the phase shift   0 t  .
The complex power defined in Eq. ( 10) for single-phase signals can be easily extended to the three-phase case thanks to instantaneous symmetrical components. Indeed, in that case this quantity expresses as:

                t i t v t i t v t i t v t p * 0 0 * * 2 3        , ( 12
)
where phasors at 0 f of the voltages and currents instantaneous symmetrical components are used. Considering that healthy electrical systems should only develop positive sequence power, the proposed feature for the complex three-phase power only considers the positive sequence components [START_REF] Granjon | Condition Monitoring of Motor-Operated Valves in Nuclear Power Plants[END_REF] :

      t i t v t p * 2 3     . ( 13
)
In healthy systems, this would give an almost ideal approximation of the total threephase power. Once again, real and imaginary parts or magnitude and phase angle can be used to obtain active, reactive, apparent powers and power factor in the three-phase case.

Eq. ( 6), ( 7), ( 8), ( 10), ( 11) and ( 13) form a set of electric features that can be used to describe in detail the electrical content of one-phase or three-phase electrical signals measured for example on wind turbines.

Estimation algorithm

The goal of this section is to briefly describe the general structure of the algorithm used to estimate the electric features previously proposed. Figure 1 depicts the part of the algorithm dedicated to estimate features defined for one electrical quantity (voltage or current). The input is either a single-or a three-phase electrical quantity, and the outputs consist in the different per-quantity features defined in the previous section. Both single-phase and three-phase indicators are presented in this figure in order to emphasise the similarities in computing the indicators for the different scenarios. Operations depicted in purple are specific to the three-phase scenario, while blue parts are applicable in both cases.

Figure 1. Estimation algorithm for instantaneous electrical features dedicated to one electrical quantity (LPF stands for lowpass filter)

This structure has been chosen to obtain a "sample per sample" algorithm, able to update the features at each new input sample. It is organized around a complex bandpass filter [START_REF] Reilly | Analytic signal generation-tips and traps[END_REF] used to select the content of its input around the fundamental frequency. Thanks to its complex-valued coefficients, this filter avoids the symmetry around the zero frequency, thus only selecting the content around 0 f  and not the one located at 0 f  . Therefore, the output of such a filter consists in complex phasors such as the ones defined in Eq. ( 1) or ( 4 ). As in [START_REF] Granjon | Condition Monitoring of Motor-Operated Valves in Nuclear Power Plants[END_REF] , this filter is implemented in the time domain with a finite impulse response structure. The down-sampling operation located in front of the complex filter is used to minimise the number of filter coefficients. By using a time-invarint filter for selecting the frequency content, computational time is kept low at the cost of limitation on the variations of the fundamental frequency. This limitation is given by the bandwidth of the filter. Simple and instantaneous operations are then applied to the obtained phasors in order to estimate the desired electrical features. Figure 2 depicts the part of the algorithm dedicated to the estimation of features defined when voltages and currents are available: electric powers and power factor. Here also, these features can be computed for both single-and three-phase cases. The inputs are phasors obtained thanks to the complex bandpass filter, on which simple and instantaneous operations are applied to obtain instantaneous values of active, reactive, apparent power and power factor at the fundamental frequency.

Figure 2. Estimation algorithm for instantaneous electrical features dedicated to two electrical quantities

It is worth noting that all the operations described in the two previous figures (the filters and the different instantaneous operations) can be real-time implemented, leading to a real-time sample per sample estimation algorithm. The performance of this algorithm is illustrated in the next section on synthetic and experimental electric signals.

Results

In this section two sets of results will be presented. The first set is obtained using synthetic data in order to compare the estimated features with their theoretical values and measure the performance of the proposed estimation algorithm. The second set is obtained using experimental data obtained from a test-bench running under nonstationary conditions, highlighting the tracking capabilities of the algorithm.

Synthetic results

A set of synthetic signals is generated in order to illustrate the behaviour and performance of the proposed electrical features and estimation algorithm. These threephase electrical signals are considered to be line-to-line voltages (their zero-sequence component is then null) and line currents. The three-phase currents are perfectly balanced with a positive sequence of 10 Amps. The three-phase voltages are perfectly balanced with a positive sequence of 100 Volts, except between 10 and 17 seconds where a 1% increase in magnitude has been generated on phase 1 only, leading to a local voltage unbalance of 0.5%. The voltages also contain non-stationary high frequency components between 20 and 45 seconds to illustrate the signal quality feature. At 20s, a high frequency component was added to the voltages with the same magnitude as the fundamental component, and was divided by two at 33s and cancelled at 45s. For each quantity, the same 2% random variations are introduced on the fundamental frequency of 50 Hz. Finally, the phase shift between the positive-sequence components of voltages and currents is kept constant throughout the simulation.

The results obtained by the proposed method are presented in Fig. 3. Theoretical values are in light blue while estimated ones are in dark blue, and the delay of 0.38 seconds (775 samples) due to the bandpass filter used in the algorithm has been cancelled. Features associated to the voltages are represented in Fig. 3(a) to 3(e) (positive-and negative-sequence magnitudes, unbalance, instantaneous frequency and quality). The magnitude of the current positive sequence component is plotted in Fig. 3(f), followed by active and reactive powers (Fig. 3(g) and 3(h)). Fig. 3 clearly shows that the proposed algorithm globally reaches very good performance. Estimation errors are small for stationary and non-stationary parts of each feature. The influence of the increase of phase 1 voltage is perfectly highlighted and quantified, either thanks to voltage features (see for example the voltage unbalance feature in Fig. 3(c)) or thanks to powers (see active and reactive powers in Fig. 3(g) and 3(h)). Even the random fluctuations of the fundamental frequency are correctly estimated and followed. This means that whatever the feature, this sample per sample algorithm presents at the same time good estimation and tracking performance.

Experimental results

A test bench located in the CETIM laboratory (Senlis, France) has been used to generate a set of three-phase electric signals under stationary and non-stationary operating conditions. This test bench, depicted in Fig. 4, has been elaborated in the context of the KAStrion European project in order to simulate the operation of a real wind turbine. At the input of the structure, a geared induction motor drives a low-speed shaft connected to a gearbox used as a multiplier. The high-speed shaft at the output of the multiplier drives an induction three-phase generator connected to the power network through a PWM power converter. The user can specify the desired speed profile thanks to an inverter supplying the input geared motor. Electric quantities are measured at the output of the generator using 3 current probes and 3 voltage probes. Measured quantities are the three line currents and the three line-to-line voltages. Sampling frequency is 40 kHz.

Figure 4. Experimental test bench

The results presented in Fig. 5 have been obtained by applying the proposed algorithm on the electric quantities measured for two different operating states. In each state, the speed of the low-speed shaft (LSS) is considered as the input of the whole system. State A represents steady state operating conditions with a constant LSS speed of 20 rpm. On the contrary, State B represents non-stationary operating conditions with a LSS speed of 20 rpm added with some small linear variations. This speed profile was used in order to simulate typical wind turbine behaviour. Considering that there was no electrical unbalance at the generator stator, the value of this indicator is very close to zero, whatever the state. Nonetheless, it is worth mentioning that this indicator is independent on the operating conditions of the system. Indeed, the speed variations of the shaft, depicted in Fig. 5(a), do not induce any correlated variations in the evolution of this particular indicator. The voltage signal quality indicator is presented in Fig. 5(h). The spectrum analysis of the measured signals has shown that the PWM converter connected at the output of the generator is responsible for high harmonic distortion, thus confirming the poor signal quality obtained by the algorithm. From all these results, it can be concluded that the proposed electrical features successfully describe the electric quantities generated at the output of the test-bench, and that the proposed algorithm successfully estimates their values and follows their variations.

Conclusions

In this paper, a method generating meaningful electrical features from electric measurements is presented. In the beginning of this paper the definition of electrical features was given. These features are able to describe the whole electrical information contained in both single-and three-phase electrical systems. The second part of the paper proposed an algorithm to efficiently estimate such quantities. The algorithm is able to process signals sample per sample and is suited for online implementation. The results were validated using both synthetic and experimental signals. This approach gives good results in terms of features estimation and tracking.

Figure 3 .

 3 Figure 3. Results obtained on synthetic data

Fig. 5 (

 5 a) depicts the two speed profiles of the low speed shaft (constant for state A and time-varying for state B). Fig. 5(b) depicts the estimated instantaneous fundamental frequency of the electric signals computed from the positive sequence component of the measured voltages. The strong correlation between the speed profiles and the estimated frequency observed in these two first figures highlights the good estimation and tracking performance of this algorithm. Fig. 5(c) depicts the magnitudes of the positive sequence component for voltages, while Fig. 5(d) shows the magnitudes of the positive sequence component for currents. These magnitudes are computed around the fundamental frequency of the signals and show, as expected, variations correlated with the speed profiles. The estimated values of active and reactive powers are depicted in Fig. 5(e) and 5(f), and are also closely linked to the speed variations. The voltage unbalance indicator is represented in Fig. 5(g).

Figure 5 .

 5 Figure 5. Experimental results

Acknowledgements

The authors would like to thank CETIM (Centre Technique des Industries Mécaniques) for providing the test-bench and experimental data.

Future work implies the evaluation of the proposed algorithm using real wind turbine data. This will also enable the definition and validation of fault indicators specific to wind turbines. Nonetheless, the algorithm to obtain the faults indicators could be extended to other electrical systems. Furthermore, the algorithm can be improved by implementing adaptive filtering for the fundamental frequency selection step.


Emma Maglio
email: emaglio@mmsh.univ-aix.fr

A Venetian rural villa in the island of Crete. Traditional and digital strategies for a heritage at risk

Keywords: Rural architecture, Crete, survey, 3D modeling, GIS map, digital humanities I. SCIENTIFIC

The Trevisan villa, an example of rural built heritage in Crete dating back to the Venetian period, was the object of an architectural and archaeological survey in order to study its typology and plan transformations. Considering its ruined conditions and the difficulties in ensuring its protection, a first reflection was started on possible strategies of digitalization, in the perspective of a digital Humanities policy: a 3D modeling and simulation and a digital archive on a GIS framework to make information accessible.

rather they were regarded with indifference or even hostility» [START_REF] Candia E | Cipro: le due isole maggiori di Venezia[END_REF] . These ones were abandoned or demolished and only recently, especially before Greece entered the EU, remains of Venetian heritage were recognized in their value: but academic research and conservation practices slowly develop. House architecture, in particular, is almost completely absent from the literature, preventing a full understanding of Venetian building policy and historic phases of the buildings. The lack of researches joins up to that of conservation: touristic development and urbanization works are degrading their environment and the Greek authorities cannot ensure their protection because of economic crisis. As a result, several buildings are poorly known or likely to disappear, especially far from the cities: the mansions, inhabited or abandoned, are often ruined, when they are not a quarry for new buildings [START_REF] Curuni | Creta veneziana: l'Istituto veneto e la missione cretese di Giuseppe Gerola[END_REF] .

Studies on built heritage are growing under the patronage of the EU, supported by a legislative body as UNESCO World Heritage Convention (Paris, 1972) and the Convention for the Protection of the Architectural Heritage of Europe (Granada, 1985). These documents emphasized the value of recognizing heritage (having an outstanding universal worth from the point of view of history, art or science) and the role of European countries in its inventory and documentation.

II. PURPOSES AND METHOD

The Trevisan villa is a representative case of a hidden built heritage. Venetian rural houses are an evidence of past ways of life and building techniques, which underwent the Ottoman period until today: they are a mirror of reinterpretation of Venetian architectural models in the case of a noble house. The evaluation of architectural, historical, environmental and aesthetic characteristics of the building is the first step towards its conservation or revitalization: however, the present ruined conditions of the building hardly limit restoration possibilities, also taking into account economic and practical difficulties.

During a mission on the field in March 2013, we carried out a first architectural and archaeological survey of the villa using traditional and digital instruments, in order to provide graphic data and envisage possible digitalization strategies, with the final goal to support operations of heritage knowledge and protection [START_REF] Kolodny | La Crète: mutations et évolution d'une population insulaire grecque[END_REF] .

An archaeological study was carried out dividing the villa into zones, sectors and stratigraphic units (SUs), in order to facilitate its comprehension. The SUs are the smallest analysis units of a monument (such as a door, a window etc.). This analysis, based on a "stone by stone" observation and derived from geological stratigraphy, allowed creating a repertory of SUs (detailed with metric and geometric data), in order to register them in Syslat software, used for archaeological site database [START_REF]The Ottoman Empire, the Balkans, the Greek lands: toward a social and economic history: studies in honor of John C. Alexander[END_REF] . We will get a relative chronology of the building by means of a diagram (Harris matrices), grouping the SUs and the sequence of building actions [START_REF] Gerola | Monumenti Veneti dell'isola di Creta[END_REF] . This is a preliminary work in the absence of any prospecting operation, which could help to establish an absolute chronology of the monument.

In meantime, a survey of its accessible parts was achieved using a total station (an electronic theodolite) connected to AutoCAD through TachyCAD application 8 , to generate 3D results in real time. With PhoToPlan application, then, the elevations survey was completed by a true-to-scale digital image rectification, allowing recording the most remarkable elements of the facades (such as decorations, cracks, plastered surfaces and alterations) 9 . The result will be a complete 3D survey of the mansion in its today conditions, giving us information about the original plan and its transformations over the time.

We are collaborating to extend an existing GIS web-based platform, Digital Crete, created by the researchers of the IMS-Forth (Institute for Mediterranean Studies) of Rethymno 10 : it contains heterogeneous information (photos, chronological data and bibliography) of the major urban monuments of Crete since the ancient period. Our role is to produce data on rural mansions, starting from the Trevisan villa, also including the survey results and a 3D model of the sample buildings.

III. THE CASE OF STUDY

A. Land occupation

In the 12th century, on the eve of Venetian conquest, there were a few installations on Crete, mostly towns and fortresses dating back to the Byzantine or more ancient period. While harbors were more active starting from the 14th century, other settlements remained rather rural, despite urbanization works: it was about small villages or multiple gatherings of properties (metochia), which inherited the role of the Byzantine choria. Located away from the coast, at the foot of the slopes or at the boarders of cultivable lands, they were typical countryside [START_REF] Kolodny | La Crète: mutations et évolution d'une population insulaire grecque[END_REF] The building is part of a sample of mansions detected in the western part of in the context of a Post-Ph.D. project. [START_REF]The Ottoman Empire, the Balkans, the Greek lands: toward a social and economic history: studies in honor of John C. Alexander[END_REF] M. Py, Lattara 10. Syslat 3.1 Manual de Référence. Lattes: Mélanges d'histoire et d'archéologie de Lattes, 1997, pp. 26-30. The SUs are connected each other by temporal relations: a SU is "above" another one if it is more recent, or "below" another one if it is older; a SU is "equivalent" to another one if they belong to a same building action (such as two parts of a wall). 7 E.C. Harris, Principi di stratigrafia archeologica. Roma: Nuova Italia Scientifica, 1987, pp. 134-140, 165-170. 8 TachyCAD software is useful for on-site surveying and data capture through a network of reference points shot inside and outside the building. 9 PhoToPlan similarly works on AutoCAD platform, adding functionalities to the classical photogrammetric dual image evaluation. 10 http://digitalcrete.ims.forth.gr/index.php?l=1. settlements and the medieval economic unit 11 . Turkish attacks began in 1645: Rethymno, Chania and several fortresses were conquered in 1647, while Candia was taken in 1669. In the early years, characterized by Cretan revolts, the Ottomans emphasized military control 12 : many smaller settlements disappeared and a few new villages were founded, but the 16th-century rural fabric survived until today, as well as most of place names of villages 13 .

The Trevisan villa is in the locality of Travasianà 14 , a few distance from the small village of Drapaniàs, east of Kissamos (Fig. 1). Unfortunately, Venetian written sources about Chania region (from which Kissamos area depended) were lost, and the only source to learn the architectural typology and the building techniques is the building itself.

B. The architectural typology

The villa is a sample of "Italian Renaissance" style house: mansions of this type were probably built from the 15th to the 17th century, mostly keeping their appearance in the Ottoman period. Gerola struggled to distinguish between Veneto-Cretan and Turkish features: such an interpretation remains difficult even today, because of the lack of sources and the continuity of use of the sites 15 . In particular, he described the Trevisan villa as «one of the best models of countryside mansions of a clear 16th-century Venetian style», most likely in coherence with other similar houses 16 . The whole building became a defensive place against the local rebels during the Turkish occupation; it was enlarged with annexed buildings and was 11 M. Gallina, Una società coloniale del Trecento. Creta fra Venezia e Bisanzio. Venezia: Deputazione Editrice, 1989, pp. 26-28. 12 Greece, the hidden centuries. Turkish rule from the fall of Crete to Greek independence, ed. by David Brewer. London: I.B. Tauris, 2010, pp. 121-140. 13 E.-Y. Kolodny, "La Crète : mutations et évolution d'une population insulaire grecque", Revue de géographie de Lyon, n. 43-3, 1968, pp. 227-290, pp. 229-230. 14 G. Gerola, Monumenti Veneti dell'isola di Creta, vol. 3. Venezia: Istituto Veneto di Scienze Lettere ed Arti, 1905-32, p. 258. 15 Gerola, vol. 3, pp. 250-252. He distinguished two general types of houses, the towers and the villas. These last assumed special characters in different regions: in the area of Kissamos, in particular, they had a ground floor used as a warehouse and a first floor with a central room and symmetrical side smaller rooms. 16 Gerola, vol. 3, p. 258. inhabited until the beginning of the 20th century (Fig. 2). It is yet a private property, but two decrees declared it as historical monument in 1965. Thirty years ago, the 28th Ephorate for Byzantine and Post-Byzantine Antiquities carried out an architectural survey, but the current building is very different from that measured (Fig. 3). For this reason, our first aim was to update available information, correcting the existing survey on the field.

C. The mansion in its today conditions

Surrounded by recent buildings, the villa is accessible by one of the streets going towards the coast from the old Chania-Kissamos road (receded to a role of provincial road after the construction of the highway in the 1980's further north). Supposing Venetian topography compared to the current aerial image, land use is mostly unchanged, with agricultural lands and olive groves: we can assumed for the villa a privileged location for noble holidays and/or for land control (perhaps through appropriate buildings standing at a short distance). The mansion belonged to a Venetian family, which probably patronized its foundation and lived there between the end of the 16th and the first half of 17th century: it is the approximate date given by the scholars.

It is a large two-storey building with a rectangular plan (13.30 x 15.30 m), with an east oriented main front. In the external staircase with two orthogonal flights, we recognized two series of steps. In the flight parallel to the main front, a first series of steps is in tuff stone, whose molded profiles stand out; a second series of higher steps in white limestone was put on the older steps but was shorter in size, because of a low wall (railing) built on the previous steps. In the other flight, we can see the previous steps recomposed and cut next to the more recent ones. A barrel vault on two pillars, giving a beautiful effect on the façade, supports the balcony.

The high-quality limestone structure is in heavy ruin: the intermediate floor and the wooden roof structure are missing; a part of the southern front partially fell down, while the northern front almost entirely collapsed after an earthquake in 2006; several vertical cracks probably refer to failures of foundations, which compromised the static equilibrium and caused localized collapses. Recent earthquakes aggravated the situation. However, despite alterations over the centuries and the current state of abandonment we documented in the survey, several decorated openings and corbels for the roof beams are still in place, revealing that the owners could afford a rich mansion.

We firstly studied the most remarkable built elements, distinguishing those belonging to a same series of building actions. We noticed that the whole staircase structure was just leaned against the façade, so it was subsequently added. We do not know if there was to be a previous staircase structure and if, like many Venetian villas of the motherland, another flight was on the other side of the central arch. We also identified at least two types of stone, a white limestone and a yellow tuff, which degraded in different ways. We analyzed all decorated openings still in place, creating a repertory in order to study their typologies and decay conditions (Fig. 4). The study of relative chronology of the main built elements is yet in progress: we identified the four perimeter walls, the staircase and the fireplace, basing on the SUs subdivision.

The building has got a ground floor, eventually used as a warehouse, and a residence at the upper floor (piano nobile): both of them are organized according to three spans (ab, cd and ef). At the ground floor (Fig. 5), spatial distribution of the We did not measure all the wall panels because of serious dangers of collapse and access difficulties. In particular, the perimeter walls lost their vertical arrangement and vertical cracks clearly show they are "opening up": we measured the whole crack pattern through image rectification (Fig. 6).

IV. CONCLUSIONS AND PERSPECTIVES

This survey inaugurates the inventory of a greater sample of buildings: the start of a mission on the field to document the Trevisan villa is the first step to know a vast built heritage at risk on the island. The work is in progress: we will enter the exposed data and those yet to develop on the GIS web-based platform of Digital Crete, to create a database for a multi-level documentation. In particular, we will provide each building with a 3D digital model (of its present and earlier appearances) as a restitution of the real object and a VR experience of simulated fly-through for researchers and public.

In view of the continuation of the project with surveys of other houses, our aim is to involve advanced digital strategies, in particular terrestrial laser scanning and photogrammetry, together with traditional human sciences. This will optimize the results, allowing managing quantitative and qualitative information about the monuments in an accessible cultural information system18 . The perspective of this collaboration fits into the broader context of digital Humanities and looks essential to encourage knowledge and protection of minor architectural heritage: Venetian houses and villas in Crete are mostly private and abandoned, so a survey on the field may be long and dangerous. Moreover, in the current Greek context, digital strategies may be successful to inventory a built heritage at risk of disappearing.

Fig. 1 .

 1 Fig. 1. Aerial photograph with indication of Drapaniàs and Trevisan villa.

Fig. 2 .

 2 Fig. 2. The eastern façade of the villa in a photograph of 1900-1902 from [11], photographical collection n. 424.

Fig. 3 .

 3 Fig. 3. The eastern façade in a photograph of 2013.

Fig. 4 .

 4 Fig. 4. The tympanum of the main door at the first floor in a photograph of 2013. original plan was on a T-shaped main room (b, c, d and f) with lateral rooms: in Ottoman times, the arches connecting b, d and f rooms were closed and a wall divided the central room in

Fig. 5 .

 5 Fig. 5. Architectural survey of the ground floor.

Fig. 6 .

 6 Fig. 6. The eastern front after digital image rectification with PhoToPlan.Even if the development of a 3D model of the building is yet in progress, the first results are encouraging: we completed the survey of one of the richest examples of Venetian rural villas in Crete, significant as for architectural and building strategies of modern age. Venetian models, adapted to local materials and skills, were generally reinterpreted or rethought. In spite of that, rural mansions as the Trevisan villa are almost

O. Gratziou, "Venetian monuments in Crete: a controversial heritage", in A singular antiquity. Archaeology and Hellenic identity in twentieth-century Greece, ed. by D. Damaskos and D. Plantzos. Athens, 2008, pp. 209-222, pp. 209-2012.

An emblematic case is a villa in Ano Drapaniàs, a village west of Chania. At the beginning of the 20th century it was intact and inhabited, but today only a few ruins survive: some blocks were reused to build a new house next to it.

J. Dimakopoulos, "Italian Renaissance in Crete", The Architectural Review, n. 960 (1977), pp. 129-132.

L. De Luca et al., "A semantic-based platform for the digital analysis of architectural heritage", Computers&Graphics, n. 35 (2011), pp. 227-241. A reference of digital application for cultural heritage is Nubes, a web-based interface allowing comparative studies starting from heterogeneous data.

ACKNOWLEDGMENT

The author would like to thank Véronique Rinalducci and Heike Hansen for technical collaboration and the 28th Ephorate for Byzantine and Post-Byzantine Antiquities for official permission. The present work benefited from funding and equipment support given by LabexMed and LA3M.


Arch Emma Maglio

Knowledge and preservation of ancient Rhodes. From a typological analysis of urban fabric to a practical project for urban heritage

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Introduction

The town of Rhodes, as its whole island, shares with the Aegean lands a Mediterranean koinè, the result of the passage of some of the ancient world's greatest civilizations like Greeks, Romans and Byzantines, but it retains its own cultural identity strictly related to its territorial and urban morphology. Archaeologists confirm that the island has been inhabited since the New Minoan age (1700-1400 BC) and that the area south of the present Rhodes has been inhabited since the Mycenaean period (1600-1200 BC), according to various obsidian and flint fragments found west of the town (sites of Ialyssos, Trianda and Asomatos) and south of it (near Koskinou). Such proofs tell that the choice of Rhodes site was not casual: the area has been civilized since the most ancient historical phases of the island, by reason of its privileged position, its bays and its natural clay soil for ceramic production I In 1200 BC the Dorians settled in the island and founded the three main cities of Ialyssos, Lindos and Kamiros, establishing commercial relations with Greece, Phoenicia, Crete and thereafter Corinth, Syria, Egypt and Cyprus. The island began a strategic port of call in Mediterranean trade, basing its supremacy on ships manufacture, transportation of valuable goods (such as oil and wine) and transit trade of grain (from Greece and Asia Minor to Egypt and Magna Graecia) . II . According to the tradition, the city of Rhodes was founded in 408 BC through an act of synechism among the three cities and began the new capital of the island III .

The Greek planned city

Rhodes had five harbours protected by ramparts and fortifications consisting in a double wall (Figure 1). The oldest walls, which were high and thin, dated back to the late 5th-early 4th century BC: few remains of them are still located I Benzi, M. 1996. Gli scavi preistorici nelle isole del Dodecaneso. In M. Livadiotti and G. Rocco (eds.), La presenza italiana nel Dodecaneso tra il 1911 e il 1948. La ricerca archeologica, la conservazione, le scelte progettuali, 3-10. Catania, Edizioni del Prisma. Δρελιώση-Ηρακλείδου, A. 1999. Παλαιά και νεα ευρήματα προ του συνοικισμου από την πόλη της Ρόδου. In Ρόδος 2400 χρόνια. Η πόλη της Ρόδου από την ίδρυσή της μέχρι την κατάληψη από τους Τούρκους (1532), A, 21-28. Αθήνα, 4η Εφορεια Βυζαντινων Αρχαιοτητων. II Arnaud, P. 2005. Les routes de la navigation antique. Itinéraires en Méditerranée. Paris, Errance and Rice, E.E. 1999. Relations between Rhodes and the Rhodian Peraia In V. Gabrielsen, Hellenistic Rhodes: politics, culture and society, 45-54. Proceedings of the Congress (Fuglsang Manor, Denmark, August 29-September 1, 1994). Cambridge, Aarhus University Press. The three cities, which were gradually overshadowed by Rhodes, were inhabited until the early Byzantine period. III Hoepfner W. and Schwandner E. L. (1986). Haus und Stadt im Klassischen Griechenland. München, Deutscher Kunstverlag, 21-26. Before the end of the 5th century BC there was probably a sympoliteia among the three cities, a 'double citizenship' which included common rights and a system of alternation in the public offices. The sinechism probably led to a political unification and to the displacement of population to the new capital from the Ialysia region, from Athens and the rest of Greece: Rhodes probably had over 50,000 inhabitants. near the commercial harbour. These walls were reinforced in the Hellenistic age by thicker external walls with towers, enclosing a large urbanized area. Southward, torrent Rhodini was another element of natural defense, while several cemeteries occupied a large territory south and west walls. Excavations in 1989 in the southern part of the city revealed wall remains dating before the 3rd century BC, confirming Amedeo Maiuri's hypothesis that in the 4th century BC Rhodes was less extended than the Hellenistic city and it did not include the southern and western neighbourhood: urban development was probably not yet accomplished in the mid-4th century BC, when the Greeks began to endow the city with the first great public buildings IV The Greek city of Rhodes covered, it is said, an area of more than 120 hectares and had a regular urban structure inspired hippodamian consisting of a road network. The secondary roads had a section ranging from 5.30m to 6.30m, what was unusually wide if compared to other poleis of the same period. Among the main roads, that had a width from 9.00m to 16.00m, there were some more important streets, such as those leading west from the port structures up to the Acropolis (P6, P10, P14, P15 and P18 streets) and those connecting the temples in the northern part of the city with the southern urban gates (P25, P27, P29, P30, P32 and P34 streets). However there were not paramount monuments at the crossroads of the paths identifying a main urban polarity or a most important street among the others. The wider roads created a 'grid' with large squared blocks of about 200m x 200m: each of them was divided by the secondary road system in four blocks (in sense north-south) and a number from five to nine blocks (in sense east-west), with dimensions of 26.00m x 47.00m. Each block probably consisted of three plots that mostly housed small stores and homes of merchants .

V IV Pimouguet-Pédarros, I. 2004. Rhodes à la fin du IVe siècle : fortifications urbaines et pratiques défensives. Studia anatolica et varia II, 212-239. [START_REF] Maiuri | Haus und Stadt im Klassischen Griechenland[END_REF]. Rodi. Roma, Alfieri e Lacroix. The inner town walls protected the ports and the highest urbanized area and coincided with good possibility with the perimeter of medieval castrum (14th-16th century). Remains of Hellenistic wall foundations were found along the current medieval walls, on the eastern and southern sides of castrum, below the Grand Master's Palace at its western side, along the ramparts on the northern side of the town and finally near the old bridge at torrent Rhodini. The southern walls were provided with three doors at least, a wide moat and advanced defensive systems (proteichismata). At the end of the 4th century BC at least one third of the city was still not built or unused: it is possible that the city was endowed with large public buildings only after 357 BC, when Ecatomnide dynasty used considerable resources in it.

. The non-residential blocks, instead, housed several urban polarities of religious and public use.

The Acropolis was located in the western area of the walled city and was structured with terraces, according to a scheme also repeated in Hellenistic Lindos and Kamiros. In the heart of the city there was to be a religious building, perhaps dedicated to Helios, near the probably site for agora at the crossroads of P27 and P13 streets. In the eastern part of the city it was said to be a great Doric peripteral temple dating from the classical age, probably a 'market temple': perhaps the remains of late-classical columns found nearby belonged to it. To the north of the agora, in the area of V Hoepfner, W. 1999. Zur Gründung und zur Architektur von Rhodos. In Ρόδος 2400 χρόνια. Η πόλη της Ρόδου από την ίδρυσή της μέχρι την κατάληψη από τους Τούρκους (1532), A, 51-58. Αθήνα, 4η Εφορεια Βυζαντινων Αρχαιοτητων. The planning of the city, traditionally attributed to Hippodamus of Miletus, had probably to be conceived and carried out by one of his pupils or a member of his school, as confirmed by scholars. The width of the main paths refers to the streets of Piraeus, as well as the size of some of the blocks (205m of Piraeus against 206m of Rhodes), which housed in both cases rather small plots. medieval castrum, there are traces of the towing ramps of the Hellenistic arsenal; little further east of the arsenal there was a temple of Aphrodite and west, finally, a small sanctuary of Helios, which probably was on the site of subsequent Roman citadel and Byzantine kastron VI .

The decline during Roman age

When Rome began its ascent in the eastern Mediterranean, Rhodes supported it but their alliance soon became rivalry:

Rome feared Rhodes maritime supremacy and engaged to weaken its power through the establishment of taxes and territorial dispossessions. In 164 BC the Rhodians were declared 'soci atque amici imperii Romani', a status requiring them to have the same enemies and allies than Rome. These events altered the political and economic balance: the emperor Diocletian put Rhodes at the head of Provincia insularum (AD 297), but the city was reduced to a provincial role that was only a shadow of its former power. Between the 2nd and the 3rd century AD Rhodes was only a secondary port of call in Mediterranean trade going east-west and north-south VII There is little information about the Roman town of Rhodes and studies on this subject are very few if compared to the large bibliography about the Greek city. The researches testify an appreciable impoverishment and a phase of urban contraction from the 2nd to the 3rd century AD, compounded by several earthquakes, epidemics and lootings which caused the ruin of most buildings: in particular, the looting of Mithridates (88-85 BC) caused the destruction of several monuments in the classical city and the siege of Cassius (42 BC) led to the collapse of the agora. The Romans had inherited a large built city and they widely reused houses, public buildings, the Acropolis, the necropolis and the sewage system of the previous Greek city . VIII VI Μιχαλάκη-Κόλλια, M. 1999. Μνημειώδες στωικό οικοδόμημα στις υπώρειες της ροδιακής ακρόπολης. To τέμενος του Ηλίου ή δημόσιο κτίριο. In Ρόδος 2400 χρόνια. Η πόλη της Ρόδου από την ίδρυσή της μέχρι την κατάληψη από τους Τούρκους (1532), A, 73-74. Αθήνα, 4η Εφορεια Βυζαντινων Αρχαιοτητων. Gabrielsen, V. 1997. The naval aristocracy of Hellenistic Rhodes. Cambridge, Aarhus University Press. Among the arsenal buildings (neōria) the archaeological remains revealed that there were covered sheds (neōsoikoi) to hold larger vessels. These structures made up the inner part of a vast complex, so the military port was large about twice the current. West and south harbors had to be similar facilities for the storage of boats and materials, but only systematic excavations will advance a still partial knowledge about the city's ports.

. We know that between the end of the 2nd and the early 3rd century AD the Romans built a domed passage on pillars (tetrapylon) near the towing ramps of the Hellenistic arsenal (which was incorporated in a general raising of the ground level), at the crossroads of Greek P31 and P6 streets which had become VII Kollias, E. 1998. The medieval city of Rhodes and the palace of the grand master. 2. ed., Athens, Archaeological Receipts Fund. The Provincia Insularum included Rhodes and other Aegean islands, about 19 in all. From the 1st century BC Rhodes played the role of intermediate port of call from Anatolia to Greece, from Constantinople to the south and south-east Mediterranean, from Greece and Rome to Syria and Egypt. VIII Ibidem. The historiography about the Roman city mostly includes works on Rhodes in the wider landscape of antiquity: among the more recent texts that systematized the previous studies see M. Livadiotti and G. Rocco (eds.), La presenza italiana nel Dodecaneso tra il 1911 e il 1948. La ricerca archeologica, la conservazione, le scelte progettuali. Catania, Edizioni del Prisma.

Roman cardo and decumanus. Tetrapylon was a gate leading from the arsenal to the agora through a paved street with shops and Corinthian columns; this street had to have an important economic and symbolic role IX (Figures 2,3).

The growing of the town until Middle Age

After the division of Roman Empire, Rhodes, the Aegean islands and the eastern provinces became part of the Eastern Roman Empire. Rhodes had an organized Church with a bishopric as early as the 3rd century AD. In the late 4th century AD the island became Metropoli of Provincia insularum, head of the Cyclades eparchy, and endowed itself of numerous places of worship such as churches and basilicas. According to the sources, the ancient town was destroyed by an earthquake in AD 515 and was rebuilt, it is said, by the Emperor Anastasius I. From the 7th to the 11th century AD the island was disputed between Byzantines and Muslims and was subject to repeated invasions from the Sassanid, the Umayyad, the Seljuk, the Caliph of Baghdad and the Turkish pirates. Subsequently, from the 12th to the 14th century, the island was at the center of the confrontation between Byzantine governors, Venetians, Genoese and Turks.

At last the Knights of St. John of Jerusalem seized Rhodes, they took the island after a long siege (1306-9) and controlled it until 1522, when Rhodes was conquered by the Ottomans X After the reconstruction following the earthquake of AD 515, the town was reorganized with a smaller fortified perimeter and developed through successive expansions, accomplishing the current urban form in the 15th-16th century AD. Associating the study of archaeological and written sources to the typo-morphological analysis of the urban 'organism', according to the method introduced by the School of Architecture in Rome, five formative urban phases were depicted, identifying the growing of the walled perimeter, the urban gates and polarities, the street pattern and In this contribution are identified and drawn on a town map the predominant alignments in the streets and in the urban fabric (5 degrees and its orthogonal direction) and the typologies of urban paths, in a diachronic and synchronic analysis. The method of typo-morphological analysis was introduced by S. Muratori in the School of Architecture in Rome since the second half of the 20th century. parts of apse churches and fragments of mosaics from antiquity and late antiquity were located north and west of the medieval walls; the ruins of a basilica appearing to have been brutally destroyed (perhaps after the earthquake of 515 AD) were spotted south of the ramparts; graves dating the 3rd century AD were discovered near the Hellenistic necropolis south and north-east of the town; remains of a large basilica with graves dating the mid-6th century AD (perhaps the center of a larger religious complex) were found south-west of the medieval town. The cemeteries, according to the Byzantine religious tradition, refer to extra-moenia buildings and necropolis and allow to assume an urban perimeter which excluded them. In the 7th century AD Byzantines built a rectangular kastron with a moat, double walls and four doors: in particular in the mid-southern wall, matching the old P31 street, a double door was open, according to sources, until the beginning of the Ottoman age; before the mid-4th century AD a northern door flanked by semicircular towers and overlooking the harbour was standing, in axis with the first door, before being closed when the sea level was lowered. In the 8th century also the village south of kastron had to have walls, defining an almost square (300m long) which remained the same until 1306. The enclosure could have included only a part of the ancient town, especially extra-moenia churches and basilicas: other Byzantine ruins and cemeteries remained out of it and were destroyed after 1309. At the end of the 11th century AD new fortifications were built and the previous ones were repaired and restored. At some point in the north-west corner Byzantines set the akropolis, a fortified foyer, so the town took its form divided into akropolis, kastron and merchant village. The kastron was crossed east-west by a processional street, linking the Greek cathedral and the eastern gate to the citadel and western suburban routes: this street developed as a direct path between two polarities and it really substituted its parallel P6 street, while P31 street retained its connective function between kastron and the village.

In the 13th century AD, in conjunction with a phase of prosperity, new walls were built in order to optimize the urban shape and the town perimeter was possibly also extended towards north and east. However, in the absence of archaeological remains, we still wonder if before 1306 this perimeter was ruined and weakened to the point of not preventing the attacks moved from the Turks and the Knights. After the conquest by the Hospitallers, the last urban expansion of the walled town was consolidated during the 15th century AD and then in the Ottoman period : new fortifications were built to enclose the existent town. Rhodes kept the tripartite Byzantine urban form with a citadel (occupied by the Grand Master Palace), a castrum and a village, and was endowed with new public and religious buildings.

Roman contribution: a first urban hierarchization

Such an historical and archaeological survey shows that Rhodes urban fabric keeps a rich architectural inheritance and the method of typo-morphological analysis of urban phases is a contribution to the knowledge of this heritage, allowing to outline the importance of some ancient elements in the urban planning. If the remarkable Greek finds allowed archaeologists to bring back the original city plan with its paths and monuments, Roman traces are scanter and it is not possible to say what the extent of the town in decline was.

The new settlement overlapped the Greek urban grid, but the Romans didn't simply reuse streets and buildings. They created a first urban hierarchy and their contribution was relevant for the subsequent urban development, remaining clear for longtime in Byzantine Rhodes before partially disappearing in medieval period: the dimensional check and the typological analysis show it very well, as I am going to explain. Moreover, the placement of tetrapylon at the crossroads of P31 and P6 streets had important effects on the structuring of the whole Roman town.

At first, concerning the urban plan, P31 street was located between P30 (today almost entirely disappeared in the urban fabric and partially surviving in current odos Homerou) and P32 streets (current odos Apellou). It divided the original block, which was approximately 216m wide if measured between the street axes, into two modules of 108m, each of them being about 3 Roman actus. The measurement on Rhodes cadastral map and on the reconstructed Greek plan confirm that the urban grid was reorganized and re-planned by the Romans, overlapping a new quadrangular grid based on land division and standing in a relation of about 1:2 with the Greek urban module XII Compared to the classical city, P6 street (decumanus) retained its importance and was confirmed as privileged connection between the main urban polarities, that is the harbour and the Acropolis, through two places of worship belonging to the previous Greek city: the temple of Aphrodite (probably reused as a place of pagan worship) and the temple of Helios (perhaps converted in a sort of Roman citadel). The old P31 street (cardo), that was only one of the main streets in the Greek planned city, took a double function: a new urban axis, reaching the center of the ancient agora and a public square to its south (the Roman forum according to the scholars), and a limes, a monumental sign of boundary indicating the ending of the town, in direction of the harbour, and its beginning, towards the hinterland and the citadel on the site of the temple of Helios. As shown by architectural measure drawings and archaeological plan reconstructions, the street was re-designed and enlarged till 13m wide (that was the distance between the axes of two specular columns): the porch and the space of the shops disposed on both sides of this colonnaded street had to pass the depth of 6.50m. The paved street reached, as stated before, the heart of Roman public town, the forum and several monumental buildings: their surviving ruins are today located in the southern part of the walled town, in current odos Ippodamou (nearby St. Mary's Tower, in the south-western corner of the walls). Significant remains of foundations of (Figure 4). XII Conventi, M. 2004. Città romane di fondazione. Roma, L'Erma di Bretschneider. The author provides an overview of some of Roman cities in the Italian peninsula and exposes their typical characteristics: the cities had a regular urban fabric based on land division and having square or rectangular blocks (the latter with ratios between the sides from 1.4 to 1.5, in relation with the examined examples). An actus amounted to 120 pedus, which is about 35.52m and corresponded to the stretch of arable land by two oxen in a single dash. The metric comparison for Rhodes is the following: 108m/35.52m = 3.04 actus and helps to explain the planning intentionality of the new settlement. the colonnaded street are located along the P31 direction in the area of medieval castrum, under the ground floor of medieval buildings of the French Tongue and of Villaragut Mansion.

In this sense, as for the architectural contribution itself, Roman tetrapylon represented an important urban gate leading from the harbour to the public center of the new town; it had to have stairs and a pitched roof supported by pillars and semi-Corinthian columns. From the point of view of urban fabric, the tetrapylon structure justified the hierarchization of P31 street among the other main Greek streets and gave a new order to Roman town. The remains of tetrapylon and of the colonnaded street were found at the beginning of the 20th century, in conjunction with the first excavations in the area between the Street of the Knights and the northern town walls, in order to launch drainage operations. In a residential garden archaeologists found the medieval chapel of St. Demetrius, which was built in 1499 on the foundations of a more ancient church which was built in its turn on the foundations of tetrapylon: the chapel was then literally disassembled and reassembled further west to release the area of excavation XIII .

An heritage not to be neglected

Archaeological traces of urban and architectural interventions of Roman age still exist today within the medieval walled town, but they have low visibility or, worse, they seem to be neglected. Some of these are probably in phase of study, some others consist of very few traces, insufficient to the study of the former buildings, but in both case one can observe that they are little or no enhanced. Undoubtedly an important problem concerning regions rich in historical traces such as Greece is the lack of sufficient resources, but archaeological finds are essential to understand a city and they must be considered as the center of preservation projects: a correct archaeological site management is very important, at first for the preservation of this inheritance and after to return them to the city and to the visitors. In the case of Rhodes, Roman remains are relevant to deeply understand the urban history and deserve to be taken into greater consideration. In fact the tetrapylon and the area around it are currently inaccessible as they are closed in a wide garden property of the Dodecanese Eforia (Figures 5,6). Similarly, the ruins of Roman forum and monumental buildings in odos Ippodamou appear just fenced and reported, but unfortunately totally neglected: nothing prevents new construction works and the degradation of adjacent buildings, so the whole site is not protected (Figures 7,8). Moreover, not a better fate hit the ruins of the Doric peripteral 'temple of the market', whose drums of columns lie down perfectly unknown to the observer, just next to the ruins of the twelfth-century Byzantine walls which are simply fenced but totally abandoned. XIII Gabriel, A. 1923. La cité de Rhodes 1310-1522. vol. 1, Paris, Boccard. Cante, M. 1996. Il tetrapylon. In M. Livadiotti and G. Rocco (eds.), La presenza italiana nel Dodecaneso tra il 1911 e il 1948. La ricerca archeologica, la conservazione, le scelte progettuali, 26-30. Catania, Edizioni del Prisma.

The improvement of the visibility of these underestimated remains imposes itself as a necessary step for the preservation of the ancient town of Rhodes: the policy management of these archaeological sites could be and should be revised, in order to make more accessible the patrimony of urban archaeology without impair its safety.

The Roman remains found in the southern area of the town, for example, could be easily enhanced. The entire area should be cleaned up, since today it's a small dump occupied by vegetation. Then the ruins could be made more visible through information platforms including a plan reconstruction of the Roman town, in order to make understand its role within the settlement and what the original appearance of the buildings may have been. With greater attention to the extent of the area, the same project could be carried out around tetrapylon: the archaeological remains are in an Eforia's property but they could be made visible by turning the area into a small open-air museum with a visit route on wooden footbridges. Information panels could explain to the visitors the importance of tetrapylon and of the colonnaded street in the urban fabric of Roman town through special graphic reconstructions, without interventions on remains and without producing superstructures using other materials: in this way it could be possible to establish a direct relationship with the remains of the street foundations below the present museum in Villaragut Mansion.

Conclusions

Today archaeological traces of Roman age are not kept in a great consideration, if compared to other monuments and ruins (of Byzantine and Hospitaller period, for example, which are often so well preserved to be closed to the public).

When they are carefully protected, as the area around tetrapylon, they are completely hidden and remain unknown. It would be possible and quite easy to improve their visibility, since at moment the ruins have not the possibility to express in a 'choral' way their history and meaning in relation with the original extent of the town. It should be necessary to work for this purpose, with non-invasive interventions on the archaeological records and creating an intelligent organizing and visiting system: it would improve the visibility of the ruins inside the town, filling a real 'void' in urban and historical knowledge of the ancient town of Rhodes.

Arch. Emma Maglio Post-Ph.D. Researcher Aix-Marseille Université -LA3M (Laboratoire d'Archéologie Médiévale et Moderne en Méditerranée) -LabExMed

Maison Méditerranéenne des Sciences de l'Homme 5, rue du Château de l'Horloge, BP 647 13094 Aix-en-Provence (France) emaglio@mmsh.univ-aix.fr

  century the early Christian town had to have a smaller perimeter, following the model of the Roman castrum structured on the old Greek P6 and P31 streets. Various remains and fragments show an unmistakable urban vitality:.IX Gabrielsen, V. 1997. The naval aristocracy of Hellenistic Rhodes. Cambridge, Aarhus University Press. Cante, M. 1986-7. Rodi: l'arco quadrifronte sul decumano massimo. ASAIA LXIV-LXV, 175-266 and Pugliese Carratelli, G. 1986-7. Epigrafi dal tetrapilo di Rodi. ASAIA LXIV-LXV, 267-293. X For an analysis of political and economic events involving Rhodes from the second Byzantine period (9th century AD), especially during the Genoese governorship (12th-13th century AD) the references are: Malamut, E. 1988. Les îles de l'Empire Byzantin, VIIIe-XIIe siècles. vol. 1, Paris, Publications de la Sorbonne; Savvides, A. G. C. 1988. Rhodes from the end of the Gavalas rule to the conquest by the Hospitallers. 1250-1309. Bυζαντινοσ Δομοσ 2, 199-232. As for the political context during the Knights period see: Luttrell, A. 2007. Studies on the Hospitallers after 1306. Rhodes and the West. Aldershot, Ashgate Variorum; Luttrell, A. 1999. The Hospitaller state on Rhodes and its Western provinces, 1306-1462. Aldershot, Ashgate Variorum; Vatin, N. 2001. Rhodes et l'Ordre de St-Jean de Jérusalem. Paris, CNRS. XI Maglio, E. 2011. Città e storia: il caso di Rodi. In The Architectural Project between Education and Research -The Research. Proceedings of the First International Congress of ReteVitruvio (Bari, Italy, May 2-May 6, 2011), 967-976.

List of captations

Abstract

The Romans inherited by the Greeks a large planned city, but Rhodes went through a phase of decline and urban contraction because of new political conditions. After an earthquake it was rebuilt with a smaller perimeter and developed through successive expansions. The typo-morphological analysis of the urban 'organism', depicting the formative urban phases of Rhodes, shows that Roman interventions had important effects on the urban growing until Middle Age.

The Romans didn't simply reuse streets and buildings. They created a first urban hierarchy, overlapping a new 'grid' structured on cardo and decumanus (Greek ancient roads). The first, that was one of the main streets in the Greek city, was re-designed as a colonnaded street and became an urban axis, also by the placement of a tetrapylon, a gate leading from the arsenal to the town center. The surviving Roman ruins of tetrapylon, the street and public buildings survive today in conditions of low visibility or abandonment. The policy management of these sites should be revised in order to make accessible this heritage and to make clear the role of these elements within the Roman settlement: the purpose is to fill a 'void' in historical knowledge of ancient Rhodes.

Keywords

Polarity -Tetrapylon -Urban contraction -Reuse -Urban phases -Heritage -Urban fabric -Site management