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Summary of Proposal LAN1988

TitleMonitoring of Sighisoara UNESCO World Heritage Site Using Space Technologies
Investigator Dana, Iulia Florentina - Romanian Space Agency, Space Applications
Team Member
Prof. Dr. Datcu, Mihai - German Aerospace Centre (DLR), Remote Sensing Technology Institute
SummaryThe monitoring of the sites included on the UNESCO World Heritage List can be performed based on Earth Observation data. Nowadays, remote sensing is an excellent monitoring tool due to the recently launched satellite missions that enable the acquisition of very high resolution optical data (with an information content similar to aerial imagery) together with a wider offer of spectral bands. Moreover, the increased development and launch of very high resolution synthetic aperture radar (SAR) sensors opened the door to a wide range of innovative applications. In this respect, TerraSAR-X and TanDEM-X (both acquiring Earth observation data of unprecedented quality, with a spatial resolution of up to one meter) are considered to bring a "revolution in spaceborne radar". Not only the development of remote sensing sensors is impressive, but also the creation of new interferometric (InSAR) techniques that enable the ground deformation/displacement monitoring up to a few millimeters, based on multitemporal series of SAR data. These techniques are represented by Persistent Scatterers Interferometry (PSI) and Small BAseline Subset (SBAS) and they are currently the most appropriate solution for the monitoring of a World Heritage cultural or natural site that might be subject to ground displacement phenomena. The Romanian World Heritage List consists of the following properties: natural site (Danube Delta) and cultural sites (Historic Centre of Sighisoara, Churches of Moldavia, Monastery of Horezu, Villages with Fortified Churches in Transylvania, Dacian Fortresses of the Orastie Mountains and Wooden Churches of Maramures). Each State Party of the UNESCO "Convention Concerning the Protection of the World Cultural and Natural Heritage" must periodically provide reports regarding the state of conservation and any threats that one World Heritage site might be facing. The goal of the project is to develop a monitoring study with unprecedented accuracy in order to investigate whether the Historic Centre of Sighisoara is affected by ground displacements caused by the landslides produced in the last years using state of the art InSAR techniques (PSI and SBAS) and delivering validated InSAR-based products such as displacement maps by integrating ground-truth data (GNSS for in-situ collection of data). The project aims at complementing the national eGISpat programme (2006-2013) developed in the framework of a partnership between Ministry of Culture and National Heritage, National Institute of Historical Monuments and ESRI Romania (main objective: to develop a GIS for the protection of the fixed cultural heritage) and also enriching the World Heritage Database at the same time. Another key objective consists in improving the information flow and communication among site managers and conservation authorities with the support of the Romanian National Commission for UNESCO and assisting responsible authorities to take better protection and preservation measures. In addition, the project focuses on building the foundation of a pilot monitoring service for the Romanian natural and cultural sites inscribed on the UNESCO World Heritage List. Last but not least, the project aims at strengthening the Romanian expertise within the "Open Initiative". In regard to the data requirements, a number of 20 High Resolution SpotLight images are necessary for building a repeat pass TerraSAR-X series for the PSI and SBAS interferometric applications. The deliverables consist of two displacement maps (derived from PSI and SBAS techniques), as well as progress and final reports. The source of funding is represented by the Postdoctoral Research Project (ID PN-II-RU-PD-2012-3-0653) submitted by the principal investigator to the Executive Agency for Higher Education, Research, Development and Innovation Funding (under the authority of Romanian Ministry of Education, Research, Youth and Sport) within the Human Resources Programme, in the domain of Earth sciences.
Final ReportIntroduction Persistent Scatterer Interferometry enables the detection, measurement and monitoring of ground displacements. This differential synthetic aperture radar interferometry technique was applied to investigate the ground stability of the Historic Centre of Sighisoara, a cultural UNESCO World Heritage Site. In line with the objectives of the "Open Initiative on the Use of Space Technologies to Support the World Heritage Convention", the monitoring of Sighisoara World Heritage Site was performed based on Earth Observation data. The multi-temporal series of very high resolution TerraSAR-X data received with LAN 1988 project was analyzed and processed and the results provide the local authorities in-charge of the site protection and conservation a basis for informed corrective measures and strategies. Test area description Located in the heart of Romania (N 46 13 04, E 24 47 32), in the Mures County, the Historic Centre of Sighisoara was inscribed on the World Heritage List in 1999 as a cultural site due to its architectural and urban monuments that were built by the German craftsmen and merchants starting with the 13th century. According to the UNESCO description, Sighisoara "is a fine example of a small, fortified medieval town which played an important strategic and commercial role on the fringes of central Europe for several centuries". A considerable number of significant monuments and buildings are still present today in the Sighisoara Citadel, i.e. the Church on the Hill, the Joseph Haltrich High School, the City Hall, the Monastery Church, the Venetian House, the Scholars' Stairs, the Stag House, the St. Joseph Roman-Catholic Church, the Georgius Krauss House, and the towers that were used in the past to guard the fortified settlement. 14 towers were originally built by the guilds of craftsmen, out of which 9 are still standing, namely the Clock Tower, the Tanners' Tower, the Tailors' Tower, the Furriers' Tower, the Ropemakers' Tower, the Butchers' Tower, the Blacksmith Tower, the Shoemakers' Tower, and the Tinkers' Tower. In the last years, the city of Sighisoara has been affected by floods that led to landslides that damaged some parts of the Historic Centre external wall, thus threatening the integrity of the World Heritage site. Such events, caused by heavy rainfalls, have been reported in the local press and there was a general concern that the Sighisoara Citadel might be in danger. Moreover, the UNESCO monitoring reports corresponding to 2005-2012 mention the "deterioration of monuments in general and fortifications in particular" and the "lack of protection and maintenance measures, local responsibility and funding strategies". In line with the objectives of the "Convention Concerning the Protection of the World Cultural and Natural Heritage" adopted by UNESCO in 1972, each State Party has the obligation to evaluate the state of conservation and identify the threats that might endanger the national World Heritage Sites and the outcomes are periodically reported to the UNESCO World Heritage Committee. Methodology At the moment, EO satellite imagery can be favorably used for the monitoring of the UNESCO World Heritage Sites. Both cultural and natural heritage can strongly benefit from the usage of satellite imagery, as their spatial, spectral and temporal resolutions are constantly improving with every new mission. In addition, the spatial resolution of the optical and SAR imagery (especially the data acquired by the X-band sensors) is presently adequate for the monitoring of cultural heritage sites, considering the required level of detail in comparison with the natural heritage sites. Generally, the monitoring of a World Heritage Site can be performed using several remote sensing techniques. A classical approach is change detection based on at least two optical satellite images acquired at a certain time interval over the same area of interest. In this case, the analysis of the land cover (LC) and land cover change (LCC) in time and space is a convenient technique, especially in the case of the natural sites. Depending on the spatial and spectral resolution, it can detect coarse and fine scale status and change that might be validated using ground truth data. For a thorough analysis, a multi-temporal series of images acquired by optical space-borne sensors can be used. This technique enables the monitoring of the conservation state over a specific time frame and identifies the major changes in the site itself and its surroundings. But even in this case, the results show only changes in the landscape and not proofs of ground deformation that might endanger the World Heritage Site. For a highly detailed and accurate assessment of ground displacement, subsidence, landslides or building stability, the new differential SAR interferometry (DInSAR) techniques are currently the most appropriate solution for cultural heritage monitoring. These techniques require large series of multi-temporal SAR data. An example of a DInSAR method applicable to urban areas is represented by Persistent Scatterers Interferometry (PS-InSAR). This technique is very sensitive to small displacements (millimeter-level motions) and it has the great advantage of monitoring wide areas while offering the possibility to measure independent features such as buildings. Another example of a DInSAR technique is Small Baseline Subset Interferometry (SB-InSAR) that also monitors the evolution of ground displacement with a high degree of temporal and spatial coverage. This technique is mainly recommended for the monitoring of the cultural heritage sites located in non-urban area. In some cases, the monitoring of the World Heritage Sites additionally requires information related to the elevation. Optical stereoscopy based on aerial or satellite imagery offers acceptable results when if certain criteria are met at the same time, namely an optimal base-to-height ratio, a minimum time interval between the acquisition of the two images and the maximum percent of overlap between the two images. Another approach is conventional radar interferometry (InSAR) that consists in the use of the phase of a radar signal by comparing two SAR images acquired simultaneously or in a certain time interval. Likewise, the results are influenced by the quality of some parameters such as the angle and direction of acquisition, the geometric and the temporal baseline, the moment of acquisition, the coherence and the atmospheric conditions. The potential of the remote sensing techniques was demonstrated in a large number of studies focused on different World Heritage Sites around the world. Examples include the Simen Mountains National Park in Ethiopia, the Historic Centre of Rome, Italy, Southern and Northern Tuscany, Italy, the Ming Great Wall of China, the cultural heritage in Cyprus, the Sundarbans in Bangladesh, and the Lines and Geoglyphs of Nasca in Peru. Further, within the "Open Initiative" several projects were developed for the monitoring of the World Heritage Sites located in Rwanda, Uganda, Democratic Republic of Congo, Irak, Russia, Brazil, Argentina, Guatemala, Peru, South Africa and Mexico. In the present study, PS-InSAR was applied to investigate whether the architectural and urban monuments located in the Historic Centre of Sighisoara are affected by displacements. For the monitoring of the Historic Centre of Sighisoara, a multi-temporal series of TerraSAR-X (TSX) High Resolution SpotLight (HS) images was programmed and acquired between March and October 2014. The HS images cover an area of approximately 5 km x 10 km and have 2.1 m ground range resolution and 1.1 m azimuth resolution. Simple VV polarization and a bandwidth of 300 MHz define the HS data. The images were acquired from a descending orbit (orbit 47) with a view angle of 350 (beam spot 038R) at a revisiting time of 11 or 22 days. The processing of the TSH HS data have been performed based on ENVI SARScape, the Interferogram Stacking module (purchased within the PN-II-RU-PD-2012-3-0653 postdoctoral research project). The method uses a single-master image mode. Results The PS-InSAR displacement map corresponding to the Historic Centre of Sighisoara contains a very high density of PSs. Only in the area of the Citadel Hill the number of PSs is lower due to the presence of vegetation. There are approximately 15,000 PSs that have coherence higher than the threshold of 0.85. The elevation information was extracted from SRTM. Using the mean displacement velocity of each PS, global statics have been computed. The results show that the average overall mean displacement velocity evens a value of -0.73 mm/year (standard deviation of 2.97 mm/year) with the investigated time interval (March October 2014). Conclusion The PS-InSAR technique allows the estimation of the displacement velocity at millimeter level. Although the technique has yet some limitations, it has the definite benefit of providing accurate and useful displacement information both over very large areas and at the level of individual features, as extensively demonstrated by previous scientific studies. By providing detailed displacement information, the study may improve the measures that have to be adopted by the responsible authorities for the protection and preservation of the Historic Centre of Sighisoara. The millimeter-level displacement information provided by the space-borne PS-InSAR technique represents a preventive diagnosis and should be validated based on ground-truth data (i.e. leveling, GNSS or GB-InSAR/GB-InRAR data). Future work The interpretation of remote sensing satellite-derived information might lead to erroneous results if validation based on ground truth data is not performed. DInSAR techniques, and in particular PS-InSAR, require the execution of terrestrial survey actions and the integration of ancillary and background data. As it was not possible to conduct ground truth campaigns at the same time with satellite data acquisition, the data provided by the Sighisoara City Hall (i.e. the condition of the buildings) will serve as reference for the validation of the results. The rigorous application of SB-InSAR based on the same TSX HS dataset is foreseen for the cross-validation of the results considering the utterly different processing workflows and error sources. The initial results showed an apparently good agreement, but the analysis must be further investigated. As future work, the development of a pilot monitoring service for all the Romanian cultural and natural World Heritage Sites using the most appropriate Earth Observation technology has already started. The monitoring service will be designed to meet the different requirements of each site. A group of potential users (containing representatives of the local, regional and national authorities) was established during the implementation of the project. The group offered critical information for each site (i.e. the factors that affect the UNESCO cultural site, the current monitoring and assessment method, the willingness to provide in-situ data in support to the validation of the satellite-derived results, the interest in becoming a potential user of the service, etc.). Also, the group welcomed the idea of such a service taking into account the size of some sites that make the periodic field survey very problematic. The monitoring of both natural and cultural sites using advanced space technologies will offer very accurate results that can help the local authorities to better understand the characteristics of each site in order to take the best protection and preservation measures. Furthermore, future work includes the securement of funding for the development of the pilot monitoring system and its operational phase. The monitoring system will cover all the UNESCO World Heritage Sites in Romania.

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