## 4 Preconditions and Current Limitations of Multi-Temporal SAR Methods
However, at discontinuous urban fabric with its higher percentage of green spaces (covered with vegetation), the applicability of the aforementioned methods is less suited, as vegetated areas lose correlation within several days. * Flat areas are best suited for the application of the interferometric damage assessment methodologies as rough topographic relief has strong negative influences or even prohibits the use of the SAR images for InSAR applications (keywords: spatial de-correlation (_cf._ Section 2.1) and layover and shadow effects [27, 74]). * According to [PERSON] [74], a useful damage assessment requires pre-defined regions to assess the damage level. * For each important area of interest exposed to natural hazards, a geodatabase containing useful GIS data (e.g., boundaries of city parcels, water bodies, parks, and important infrastructure such as hospitals or shelters) should be set up and continuously updated. According to [PERSON] and [PERSON] [2], possible sources for the required GIS data to support the damage assessment procedure (points VII and VIII) are (a) local GIS, (b) internet maps or (c) interpretation and classification of pre-disaster optical imagery. The availability, topicality and quality of the GIS data strongly differ from country to country. In general, in industrial countries much more detailed and up-to-date GIS data is available than in Third World countries. Also the coverage and topicality of open GIS data such as OpenStreetMap (OSM [133]) data strongly varies from place to place. Additional geodata is provided by Web Mapping Services (WMS) such as the ESRI base maps [134] and Internet maps (e.g., Bing Maps [135], Google Maps [136]/Earth [137]). At several gazetteer web sites (e.g., the United States Census Bureau [138], or GeoScience Australia [139]) additional reference data is available. Currently, the Global Earthquake Model [140] works on the set-up of a Global Exposure Database (GED), which will contain information on buildings and people form the country-level down to individual buildings. The first version of the GED will contain aggregated information on population, number of buildings and percentage of built area at a spatial resolution of 1 km. The level of detail as well as the information content will be increased in future. Regarding the points I to IV: Nowadays useful archive SAR imagery shortly recorded before the crisis is in most cases not available. Two pre-disaster images recorded with the same imaging geometry as the post-disaster image are very rarely available. Modern very high spatial resolution SAR missions such as TerraSAR-X, COSMO-SkyMed or Radarsat-2 are not able to provide worldwide coverage within a short time period--_i.e._, no archive image recorded shortly before the event (especially not with the same imaging geometry within a certain AoI, which is required for interferometric applications and hereon based damage assessment procedures). Regarding these interferometric applications, these satellites are only useful to investigate and monitor a limited number of selected AoIs, as these very high spatial resolution SAR sensors are only able to image small parts of the Earth. Consequently, these SAR missions usually record the crisis area only after the event happened [24]. Therefore, in most cases the described multi-temporal data based damage assessment methods cannot be applied so far.