This section briefly describes the main data sources that have been used in the generation of HydroSHEDS. The actual processing steps are addressed in section 3. Please also refer to the flowchart of Appendix A in the technical documentation.
2.1 Shuttle Radar Topography Mission
The primary data source of HydroSHEDS is the digital elevation model (DEM) of the Shuttle Radar Topography Mission. SRTM elevation data were obtained by a specially modified radar system that flew onboard the Space Shuttle Endeavor during an 11-day mission in February of 2000. The SRTM project is a collaborative effort by the National Aeronautics and Space Administration (NASA), the National Geospatial-Intelligence Agency of the U.S. Department of Defense (NGA), as well as the German Aerospace Center (DLR) and the Italian Space Agency (ASI). NASA’s Jet Propulsion Laboratory (JPL) managed the mission and the Earth Resources Observation and Science Center of the U.S. Geological Survey (USGS EROS Data Center) has the responsibility of hosting, distributing and archiving the final SRTM data products. A general description of the SRTM mission can be found in Farr and Kobrick (2000).
2.2 SRTM elevation data, Version 1 (SRTM-1 and SRTM-3 unfinished data)
The raw SRTM data have been processed into an initial research quality DEM by JPL. No further editing has been performed, resulting in a dataset that may contain numerous voids and other spurious points such as anomalously high (spike) or low (well) values. Since water surfaces produce very low radar backscatter, water bodies are generally not well defined and appear quite “noisy”. Coastlines, as well, are not clearly defined. For areas outside of the conterminous United States (CONUS), the original 1 arc-second data (SRTM-1; cell size approximately 30 meters at the equator) were aggregated into 3 arc-second data (SRTM-3) by averaging, i.e. each 3 arc-second data point is generated by averaging the corresponding 3x3 kernel of the 1 arc-second data. For more details see NASA/JPL (2005).
2.3 SRTM elevation data, Version 2 (DTED-2 and DTED-1 finished data)
After JPL completed the raw processing of the SRTM-1 and SRTM-3 data, NGA performed quality assurance checks and then carried out several additional finishing steps to comply with the required data standards of the Digital Terrain Elevation Data (DTED®) format (NASA 2003). Spikes and wells in the data were detected and voided out. Small voids were filled by interpolation of surrounding elevations. Large voids, however, were left in the data. The ocean was set to an elevation of 0 meters. Lakes of 600 meters or more in length were flattened and set to a constant height. Rivers of more than 183 meters in width were delineated and monotonically stepped down in height. Islands were depicted if they had a major axis exceeding 300 meters or the relief was greater than 15 meters. All finishing steps were performed at the original 1 arc-second resolution, resulting in DTED Level 2 data products. DTED-2 was then aggregated into 3 arc-second DTED-1 data. Unlike SRTM-3, however, DTED-1 has been generated by subsampling, i.e. each 3 arc-second data point is generated by assigning the value of the center pixel of the corresponding 3x3 kernel of the 1 arc-second data. For more details see NASA/JPL (2005).
2.4 SRTM tiling format and data availability
SRTM elevation data have been processed in a systematic fashion and mosaicked into approximately 15,000 one-degree by one-degree tiles. Following the DTED convention, the names of the individual data tiles reflect the longitude and latitude of the lower-left (southwest) corner of the tile. For example, the coordinates of the center of the lower-left pixel of tile N40W118 are 40 degrees north latitude and 118 degrees west longitude. In the case of DTED-1 and SRTM-3 data, a single tile consists of 1201 data rows and 1201 data columns. Due to the definition via pixel centers, the four edges of a tile each exceed the assigned coordinates by half a pixel and the outermost rows and columns of adjacent tiles are overlapping. For more details see NASA/JPL (2005).
Outside of the CONUS, the 1 arc-second products (SRTM-1 and DTED-2) are only available upon request for scientific purposes. The 3 arc-second products (SRTM-3 and DTED-1) are public domain and may be obtained from NASA via anonymous ftp at ftp://e0srp01u.ecs.nasa.gov/srtm/ or from the USGS EROS Data Center via the The National Map Seamless Server at http://seamless.usgs.gov/.
2.5 SRTM Water Body Data (SWBD)
SRTM Water Body Data files are a by-product of the data editing performed by NGA to produce the finished SRTM DTED-2 data. Ocean, lake and river shorelines were identified and delineated from the 1 arc-second DTED-2 data (for details see NASA 2003) and were saved as vectors in ESRI 3-D Shapefile format (ESRI 1998). There are approximately 12,000 SWBD files since only those SRTM tiles that contain water have a corresponding SWBD shapefile.
The guiding principle for the development of SWBD was that water must be depicted as it was in February 2000 at the time of the Shuttle flight. In most cases, two orthorectified SRTM image mosaics were used as the primary source for water body editing. A landcover water layer and medium-scale maps and charts were used as supplemental data sources. 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 SRTM data and the ancillary sources. For more details see NASA/NGA (2003) and NASA (2003)
2.6 Digital Chart of the World (DCW) global vectorized river network
The Digital Chart of the World (ESRI 1993) is a global vector map at a resolution of 1:1 million that includes a layer of hydrographic features such as rivers and lakes. DCW (also known as VMAP-0) is generally considered to provide the most comprehensive and consistent global river network data currently available. It is based on the US DMA (now NGA) Operational Navigation Charts (ONC) whose information dates from the 1970s to the 1990s (Birkett and Mason 1995). The positional accuracy of DCW varies considerably between regions, and there is no distinction between natural rivers and artificial canals.
2.7 ArcWorld global vectorized river network
The ArcWorld data set (ESRI 1992) includes a global vector map of surface water bodies at a resolution of 1:3 million. As part of its classification scheme, it distinguishes linear rivers into natural (perennial and intermittent) or artificial (canals) and provides approximately 7000 polygons of large open water bodies (including rivers and lakes). Although digitized at a coarser scale, ArcWorld seems to include some corrections and updates as compared to DCW and provides a consistent focus on major rivers and lakes of the world.
2.8 Global Lakes and Wetlands Database (GLWD)
The Global Lakes and Wetlands Database (Lehner and Döll 2004) combines a variety of existing global lake and wetland maps (at 1:1 to 1:3 million resolution) into one consistent coverage. It provides shoreline polygons of approximately 250,000 lakes and reservoirs worldwide, including their surface areas and other attributes. As for lakes and reservoirs, GLWD is largely based on DCW and ArcWorld but also includes various updates and data corrections.