SRTM Water Body Dataset

The SRTM Water Body Data files are a by-product of the data editing performed by the National Geospatial-Intelligence Agency (NGA) to produce the finished SRTM Digital Terrain Elevation Data Level 2 (DTED 2). In accordance with the DTED 2 specification, the terrain elevation data have been edited to portray water bodies that meet minimum capture criteria. Ocean, lake and river shorelines were identified and delineated. Lake elevations were set to a constant value. Ocean elevations were set to zero. Rivers were stepped down monotonically to maintain proper flow. After this processing was done, the shorelines from the one arc second (approx. 30-meter) DTED 2 were saved as vectors in ESRI 3-D Shapefile format. In most cases, two orthorectified image mosaics (one for ascending passes and one for descending passes) at a one arc second resolution were available for identifying water bodies and delineating shorelines in each 1 x1 cell. These were used as the primary source for water body editing. The guiding principle for this editing was that water must be depicted as it was in February 2000 at the time of the shuttle flight. A Landcover water layer and medium-scale maps and charts were used as supplemental data sources, generally as supporting evidence for water identified in the image mosaics. Since the Landcover water layer was derived mostly from Landsat 5 data collected a decade earlier than the Shuttle mission and the map sources had similar currency problems, there were significant seasonal and temporal differences between the depiction of water in the ancillary sources and the actual extent of water bodies in February 2000 in many instances. In rare cases, where the SRTM image mosaics were missing or unusable, Landcover was used to delineate the water in the SRTM cells. The DTED header records for those cells are documented accordingly.

SRTM水体数据文件是美国国家地理空间情报局（National Geospatial-Intelligence Agency, NGA）为制作最终版SRTM数字地形高程数据二级产品（Digital Terrain Elevation Data Level 2, DTED 2）所开展的数据编辑工作的附带产物。依据DTED 2规范，已对地形高程数据进行编辑，以呈现符合最低采集标准的水体：已识别并勾勒海洋、湖泊与河流的岸线，将湖泊高程设为恒定值，海洋高程设为0，河流高程则按单调递减规则处理以维持合理的水流流向。完成上述处理后，将1弧秒（约30米）分辨率DTED 2数据中的岸线以矢量形式保存为ESRI三维Shapefile格式文件。在多数情况下，每个1×1单元格均可使用两幅1弧秒分辨率的正射影像镶嵌图（分别对应上行轨道与下行轨道采集的数据），用于识别水体并勾勒岸线，这些影像将作为水体编辑的主要数据源。本次编辑的指导原则为，水体的呈现需与2000年2月航天飞机采集数据时的实际状态一致。研究团队同时将地表覆盖水体图层（Landcover water layer）及中比例尺地图海图作为补充数据源，主要用于佐证影像镶嵌图中识别出的水体。由于地表覆盖水体图层主要源自比航天飞机任务早十年采集的Landsat 5数据，而地图数据源同样存在现势性不足的问题，因此在多数场景中，辅助数据源中的水体呈现与2000年2月的实际水体范围存在显著的季节与时间差异。仅在极少数情况下，当SRTM影像镶嵌图缺失或无法使用时，才会使用地表覆盖水体图层勾勒SRTM单元格内的水体范围，对应单元格的DTED头文件将据此进行标注。