Monitoring from Space of UNESCO Sites in Danger

Archaeological research from space is becoming more and more usual thanks to the increasingly larger archives of satellite imagery and the progressively higher spatial resolution of the sensors (optical and radar). It is possible to monitor sites in danger and detect ancient features underground using indirect indicators, such us moisture content and vegetation growth detected measured from space. Remote sensing from satellite is a non invasive technique of interest for monitoring archaeological remains in remote areas and/or of difficult access. It constitutes a great support for preservation and conservation plans, even where political events make very difficult the surveys in situ, as in one of the cases presented here.

This study focuses on two ancient cities inscribed in the UNESCO (United Nations Educational Science and Cultural Office) List of Sites in Danger: Samarra (Iraq 43°45’50’’- 43°51’70’’ N 34°25’34’’- 34°05’12’’E) and Chan Chan (Peru, central coordinates 08°06’22’’S 79° 04’26.77’’W). Samarra was the second capital of the Abbasid Caliphate and the Advisory Body of UNESCO withdrew its supervision in 2003 due to the war in Iraq. This city was very well preserved before that date because it suffered from a sudden abandon and in 2007 UNESCO decided to add the Islamic capital in the List of Sites in Danger. In addition, adverse climate condition such as winds and isolated storms are affecting the preservation of the site. Furthermore, since construction laws have not been upheld, also the sprawl of the human settlements occupying archaeological areas is causing the loss of some ancient buildings. Samarra has been studied mostly with optical data. A multitemporal analysis has been done thanks to SPOT-2 and SPOT-4 satellite from 2000 to 2009. For ALOS AVNIR-2 images, spanning from 2007 to 2009, allowed appreciating changes in vegetation growth and / or in settlement expansion, despite the short temporal period. In addition, also two full polarimetric ALOS PALSAR images and seven ENVISAT ASAR data were analyzed for multitemporal analysis. All the data were then overlapped to 20 georeferenced site maps of the city (45 km length) in order to detect anomalies corresponding to archaeological structures.

The archaeological site of Chan Chan is situated in the Trujillo province, in Peru. The archaeological area (about 20 square kilometers) includes citadels and palaces of the Chimu kingdom. The capital, Chan Chan, reached its apogee in the 15th century. The strictly hierarchical urban space in which Chan Chan is organized made it the largest city in pre - Columbian America and reflects a strict political and social strategy. The adobe, or earthen, structures of the site are constantly damaged by natural erosion, exposed to air and torrential rains. For this reason, they require continuous conservation efforts and substantial ancillary measurements. UNESCO decided to add it to the List of Sites in Danger in the 1986. Chan Chan was analyzed predominantly with radar data (Synthetic Aperture Radar, SAR). Three full polarimetric ALOS PALSAR images were processed to distinguish surface and subsurface features as well as land changes in the time frame of two years (2007 - 2009). Moreover, a multitemporal analysis has been carried out thanks to nine ENVISAT ASAR images (2002-2006). All the SAR data were then georeferenced and projected onto the only available map of the site, dating back to 1986. One optical high resolution image from KOMPSAT-2 (2007) was compared to a SPOT-4 image (2000), in order to analyze urban and agricultural changes occurred over seven years.