monitoring of unesco world heritage sites in danger ...useful for the creation of an action plan for...

Remote Sensing for Science, Education, Rainer Reuter (Editor) and Natural and Cultural Heritage EARSeL, 2010

Monitoring of UNESCO World Heritage Sites in Danger, Including Samarra (Iraq) and Chan Chan (Peru),

Using Optical and Radar Data Nicole DOREa,1, Jolanda PATRUNOa,1, Francesco SARTIb,

Ana B. RUESCASb, and Mario HERNANDEZc aUniversity of Rome, Sapienza

b European Space Agency, ESA-ESRIN

cUNESCO World Heritage

Abstract. The purpose of this research is monitoring two archaeological UNESCO sites in danger: Samarra (Iraq) and Chan Chan (Peru) declared by UNESCO as sites in danger in 2002 and 1986 respectively. Climate change, wars, urban and agricultural expansion are affecting the integrity of some of the most important ancient cities in the World. This is why satellite imagery can be useful for the detection of changes elapsed over the years. In this research we used optical and SAR data in order to find a complementary non-invasive technique for archaeological investigations. The limited availability of imagery for both the sites prevented us from disposing of a complete dataset. For this reason, Samarra was studied primarily thanks to a multitemporal optical analysis; Chan Chan thanks to a radar multitemporal sequence. In spite of that, it has been possible to detect changes elapsed over years and to check the constant threats to which these cities are exposed.

Keywords. Archaeology, optical data, Synthetic Aperture Radar, polarimetry, Vegetation Indices.

Introduction

Archaeological features detected on aerial photographs have long been used for archaeological pur-poses, due to the archived historical series of photographs that allow the observation and detection of underground features not visible anymore, and to their very high spatial resolution. However high-resolution optical sensors from satellite (such as IKONOS, KOMPSAT-2 and QuickBird) have demonstrated to be useful for archaeological applications when monitoring archaeological sites. In particular, the most important characteristic linked to satellite images is the temporal resolution, that is, the frequency of images acquired by the satellites.

The difficulty of seeing through ground and clouds and the acquisition times of both aerial pho-tographs and satellite images prompted archaeologists to look beyond traditional approaches, like using only the channels in the visible range of the spectrum. Their interest moved towards radar sensors acquisitions. However, at the moment, the available space borne radar sensors have still a low spatial resolution for archaeological purposes. This is why the joint use of radar and high-resolution optical imagery is recommended, analysis that should always be complemented with ar-chaeological maps when available. The selected test sites for this work are located in Iraq and Peru. They are two well known archaeo-logical sites, two cultural heritage sites threatened by climate and human activities (expansion of urban settlements and agricultural fields).

We used optical data sets of the Advanced Land Observing Satellite (ALOS) Panchromatic Re-mote-sensing Instrument for Stereo Mapping (PRISM), Advanced Visible and Near Infrared Radi-

1 [email protected], [email protected].

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Nicole Dore, et al.: Monitoring of UNESCO World Heritage Sites in Danger

ometer type-2 (AVNIR-2), KOMPSAT-2, and radar sets with quad-polarisation images from ALOS Phased Array type L-band Synthetic Aperture Radar (PALSAR) [1].

The processing of polarimetric radar (PALSAR) data was carried out with the European Space Agency (ESA) [2] package PolSARpro [3]. A validation process is done using ancillary informa-tion such as archaeological maps. The results show the changes that occurred over years and are useful for the creation of an action plan for the preservation of this invaluable historical heritage.

1. The study areas and the data

The Archaeological city of Samarra (Figure 1) is located in Iraq, Salah al-Din Governorate. The city is 130 km north of Baghdad, and it was the second capital of the Abbasid Caliphate (Baghdad was the first). This ancient city, located on both the sides of the River Tigre, was the Capital of the country for 58 years (836-892 A.D.). Mysteriously the city was suddenly abandoned. This rapid abandonment is the reason for which Samarra represents today one of the best preserved plan of an ancient large city, attestation of architectural and artistic innovations of that period. The city (45.472 ha) is bounded by the coordinates longitude 43°45’50’’– 43°51’70’’ and latitude 34°25’34’’– 34°05’12’’ [4].

The Chan Chan archaeological city (Figure 2) is bounded by the coordinates 08°06’22.18’’S

and 79° 04’25.26’’W. It was the capital of the Chimu kingdom, located in Peru, in the District of Huanchaco, 4 km west of Trujillo. Chan Chan represents one of America’s largest and most impor-tant pre-Hispanic mud-brick architecture [5]. Its complex urban layout reflects the well-structured economic, politic, social and ideological civilization that was alive between 1000 and 1470 A.D. It was conquered by the Incas. The archaeological area has an extension of about 20 km2. Because of climate change and human urban expansion, the intensity of threat to the site reached its peak in 2009 [6].

Figure 1. Samarra geographic location

Figure 2. Chan Chan geographic location 30


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Samarra and Chan Chan satellite imagery are provided by ESA through the online catalogues Earth Observation Link (EOLi) [7]. Samarra ancient city was analysed using a multitemporal ap-proach with optical sensors (PRISM and AVNIR-2 sensors). The PALSAR image was used in order to analyse what SAR reveals in the ancient city, the remains of which are still well visible in the area. The ancient city of Chan Chan, on the contrary, was analysed observing the changes through time using three SAR polarimetric products (PALSAR). Also one optical KOMPSAT-2 hi-res im-age acquired in the area was used, and it was the base for a correct interpretation of the results using SAR.

In addition, archaeological maps were provided for both the sites, which are fundamental for the proper location of the ancient features in the ground.

2. Methodology

As mentioned before, the site of Samarra was studied using optical multi-temporal analysis, in order to observe changes in the landscape and to detect vegetation changes over the archaeological struc-tures in a time frame spanning from May 2007 to April 2009. For this reason the archaeological maps were georeferenced and overlain onto a mosaic of two PRISM images. All the maps were fi-nally overlapped to different regions of interest (ROIs) corresponding to each archaeological map and changes over years were analysed. In order to test capabilities of radar polarimetry in such de-sert area of Samarra, one PALSAR image full polarised was processed as well.

The area of Chan Chan was analysed using a multi-temporal approach by means of SAR po-larimetric data. The KOMPSAT-2 image was used as a basis in the georeferecing process for both SAR data and the archaeological map of the site. The georeferenced products were overlain onto the KOMPSAT-2 image in order to detect changes in the radar response over the time period of March 2007 to March 2009.

3.1. Optical processing

The processing applied to optical data had the purpose of detecting changes arisen over years. As a first step, we realized a mosaic (used as master image) between two PRISM images because of the width of the city (about 40 x 15 km) upon which we georeferenced all the archaeological maps (UTM WGS 84). The archaeological city is divided into several sheets related to the 20 sheets available in the UNESCO website. We then co-registered the four AVNIR-2 images and created a stack with them.

When all the data were exactly overlapped, the NDVI index was calculated over the 20 ROIs corresponding to the archaeological sheets. The NDVI statistics were extracted for each sheet on the four AVNIR-2 images. In order to have an easier visualization of the differences, several ranges with different colours were created by means of the density slice technique. The analysis of the condition of the vegetation in the different areas showed that there is a change in the cropping tech-niques since 2007, specially in the irrigation methods, with a visible increase of the use of the centre pivot irrigation mode. These changes can put archaeological remains in danger, causing a more rapid erosion due to the increase in the humidity content of the soil, as the structures are made of fired bricks, mud-bricks and adobe.

The lack of optical data for Chan Chan city led us to consider the KOMPSAT-2 imagery as the most recent source of information about the vegetation extent and the urban expansion in the area. The Panchromatic band (1 m x 1 m pixel resolution) is used as a reference for the co-registration of the products. The last updated acquisition regarding the changes occurred in time observed in SAR data. The Infrared band has been observed and compared to SAR data to better analyze vegetation extension in the close vicinity of the site.


Table 1. Optical data and ancillary information

Kompsat-2 ALOS AVNIR-2 ALOS PRISM Achaeological maps

Samarra

-

2007/05/22

2007/11/22

2008/05/24

2009/04/11

2009/04/11

2009/11/22 20 Archaeological maps

Chan Chan

2010/02/12 - - 1 Archaeological map

Sites Time Processing Level

Meteorological conditions

Chan Chan

2007/03/25

2007/ 05/10

2009/03/30

1.1 T max/min 26°/16° Hum.78%

T max/min 24°/14° Hum.78%

T max/min 25°/19° Hum.83%

Samarra 2008/11/15 1.1 Not available

Table 2. Polarimetric data (ALOS PALSAR)

3.2. SAR processing

Three ALOS PALSAR data, spanning from March 2007 to March 2009, have been used to analyse the state of conservation of the Chan Chan archaeological area. As a first step, all the SAR images were processed with PolSARpro software, mostly for the extraction of the T matrix, the first indica-tor of all the interactions between the target and the radar beam. The second step was the identifica-tion of the Entropy and Alpha angle parameters, values which contain the principal scattering mechanism and its relation with the secondary ones. The processing products obtained for each of the three scenes have been subsequently georeferenced, having as master image the panchromatic band (1 m) of the KOMPSAT-2 imagery. The archaeological map was georeferenced as well.

The second step was the analysis of the Entropy and Alpha angle behaviour all over the ar-chaeological area, overlapping them with the archaeological map and with the KOMPSAT-2 im-agery. The meteorological condition in the days of the images acquisition is a very important ele-ment to be kept in consideration, as a high percentage of humidity reduces the regular path of the radar beam. Thanks to the archives of the WeatherOnLine website [8], the meteorological condi-tions for each date are known.

Samarra polarimetric processing was the same as the previous site. Unfortunately, there is no available information about meteorological conditions of the acquisition day in the area. This made an exhaustive valuation of the Entropy and Alpha angle values difficult.

3. Results

3.1. Results using optical data

AVNIR-2 images were processed in order to better highlight archaeological structures and features. It is possible to recognize almost the whole ancient topography of the city.

Four NDVI products were extracted from the AVNIR-2 images of 2007-2009. The date of the 22 November 2007 was excluded from interpretation because its almost total absence of vegetation. The ROIs of the three images were compared between them. In order to highlight differences in vegetation intensity, density slices were created. Some archaeological sheets show a change in the

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irrigation technique, as visible in Figure 3. This figure shows an area belonging to al-Zanqur site. In the upper left corner, the change in the traditional agricultural techniques for the central pivot irriga-tion is visible. There are situations in which archaeological remains have been affected for longer time, like in the case of Tell al-Wazir site, where archaeological structures are totally invaded by vegetation (Figure 4).

Figure 3 al-Zanqur (left images). The five images show changes in irrigation techniques. Two images with archaeological maps and density slices on the top; three images with near infrared above.

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We can observe a progressive invasion of the ancient structures during the years due to the ex-

tent of the fields.

Figure 4 Tell al-Wazir

In other archaeological sites, the situation seems to be the same between May 2007 and April 2009. This means that Samarra ancient city is constantly threatened by men activities.

In the case of Chan Chan archaeological city, the optical analysis is limited to the observation of the KOMPSAT-2 imagery (2010/02/12). Displaying the Near-Infrared band in the Red channel and comparing it to the SAR product acquired three years before (2007/03/25), it is possible to notice the intense increase of the agricultural fields around the archaeological remains. In some cases, the crops occupy the structures of the ancient location and contribute to the loss of the still existing ar-


chaeological features, heavily damaged by the construction of modern animal food plants, farm-houses and urban expansion.

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Figura 5. Left: Infrared band of KOMPSAT-2. Agricultural fields invading archaeological structures. Right: SAR imagery ALOS PALSAR. The brightest pixels correspond to urban areas.

3.2. Results using SAR data

As a first step for SAR analysis of the archaeological city of Chan Chan, the extraction of the T ma-trix for each of the three ALOS PALSAR acquisitions was performed. An initial operation of multi-look (window: row 1, col 5) was applied, resulting in an image with approximately equal ground range pixel spacing. All the polarizations have been then visualized in the RGB channels (R: HH, G: HV, B: VV), in order to highlight in the green channel the vegetation response. All the SAR proc-essed products were then overlapped to the archaeological map of the city. It has been possible to dis-tinguish sporadic vegetation evidence spread all over the site, corresponding to the pre-existent walk-in-wells present in each citadel. The surrounding areas are strongly affected by intensive agricultural activities, which takes up part of the ancient structures and, in some cases, obliterate them.

The archaeological area still unoccupied by human activities, in red circles in Figure 6, is mostly made up of mud-brick structures very much exposed to wind and erosion.

Figure 6. Left: ALOS PALSAR, 2007/03/25. Right: ALOS PALSAR, 2009/03/30. Agricultural fields and urban areas seems to occupy the same por-tion of archaeological structures over the years 2007-2009.


Because of their superficial irregular shape and due to the low resolution of ALOS PALSAR we cannot obtain a clear response for each archaeological structure. What we can observe, is that agri-cultural and urban settlements expansion, corresponding to brighter pixels outside of the red circles, did not increase considerably in these last two years, thanks to the recent preservation measures taken by local authorities.

Observing the Alpha angle/ Entropy values for all the ALOS PALSAR products, it has been possible to confirm a constant behavior of these values in time in such distributed target as the ar-chaeological area is. In fact, in some cases a high response of a single ancient structure (point tar-get) blends with other scattering mechanism due to vegetation or different surface features, as can be seen in α/H details in yellow circle in Figure 7, where the ancient walk-in-wells of the “Labirin-tum” citadel lose their own backscatter mixing it with the surrounding features.

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α

We in fact observed, for the Alpha angle and Entropy values registered ( : 30° H: 0.80), a scat-

tering mechanism of surface roughness (Zone 8) combined with vegetation response (Zone 5), as we can see in Figure 7 (red circle).

Regarding Samarra SAR processing, we realized ROIs in Entropy and Alpha angle products, corresponding to the ancient remains in each archaeological sheet. We then overlapped these prod-ucts with the archaeological maps in order to distinguish anomalies in scattering behaviour over ar-eas with antropic remains. As visible in Figure 8, we can observe similar values over the sites both in H and in Alpha angle products. However it is possible to notice an exception for two areas in which values are lower than the other archaeological zones (red circles). The reason of the differ-ence might be linked to land cover or archaeological pattern homogeneously distributed in the stud-ied areas, but surveys in situ are necessary to confirm or discard this hypothesis.

a

b

Figure 7. Alpha angle (a) and Entropy (b) values on the left. The higher values correspond to walk-in-wells and surrounding features scattering mechanism H/α plane on the right.

Figure 8 Graphs with Alpha angle (left) and Entropy (right) values over archaeological areas.


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4. Conclusions

An exhaustive analysis of satellite images for archeological purposes is possible when both optical and radar data are available.

Samarra case study shows how important is the observation of agricultural techniques changes occurred in time, especially for this site in which surveys in situ are not allowed because of the war since 2003.

Chan Chan case study is the example of usefulness of SAR data where the data set lack in opti-cal images. However, due to the not-immediate comprehension of SAR images, the period of time elapsing within the imagery should be ideally wide, in order to better appreciate changes of natural and human activities on the site.

However, surveys in situ are always necessary to confirm or discard observations made by means of satellite imagery, which constitute a valid complementary technique for archaeological investigation.

Acknowledgements

We thank ESA-ESRIN for providing us software and images and M. Lavalle for SAR data interpretations.

References

[1] http://www.eorc.jaxa.jp/ALOS/en/about/palsar.htm [2] http://www.esa.int/esaCP/Italy.html [3] http://earth.esa.int/polsarpro/ [4] http://whc.unesco.org/en/list/276/documents/ [5] Alan L. Kolata (1990) The Urban Concept of Chan Chan, In: The Northern Dynasties kingship and statecraft in

Chimor,.Edited by M E Mosely and A Cordy-Collins, pp.107-144 [6] http://whc.unesco.org/en/list/366/documents/ [7] http://earth.esa.int/EOLi/EOLi.html [8] http://it.allmetsat.com/clima/peru

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