Development of an Integrated Geospatial Information Network for South America (GeoSur)

Generation of a High-Resolution Water Map of South America

The U.S. Geological Survey (USGS) has developed a small subset of SRTM Level 2 (30m) DEM derivative products for the Geospatial Network for South American Integration (GeoSur) Program. GeoSur facilitates planning and development of national and regional infrastructure in South America by providing geospatial information to participants in an accurate, detailed, and consistent format for both regions and the continent. An elevation data set and its derivative products are critical for most analyses. Currently in South America there are varying levels of geospatial capacity; some countries have sophisticated spatial databases with high levels of detail and other are at the opposite end of the spectrum. GeoSur and USGS seek to meet the needs of users with these diverse capabilities.

On February 11, 2000, the U.S. National Aeronautics and Space Administration (NASA) launched the Space Shuttle Endeavour with the Shuttle Radar Topography Mission (SRTM) payload onboard (Ramirez et al., 2005). During its eleven days in space it collected the most complete near-global database of the Earth's land surface elevation (Ramirez et al., 2005). These elevation data postings were collected at every 1 arc-second (approximately 30 meters) between 60 degrees north and 56 degrees south latitude covering 80 percent of the Earth's surface (Shuttle 2006). The National Geospatial-Intelligence Agency (NGA) made the digital elevation model (DEM) publicly available at a reduced resolution of 3 arc-seconds (approximately 90 meters). This DEM provides the most detailed, accurate, and consistent data set for the continent of South America and can fulfill core spatial data requirement for GeoSur countries. Some participants may have higher resolution DEMs, but the SRTM 3 arc-second DEM can fulfill the requirement of providing consistent information throughout a region and the continent. To further enhance this DEM the NGA has allowed the USGS to secure, process, and release a set of derivative products at 1 arc-second (30m) resolution shown in table 1.

The USGS developed the set of derivatives products for a pilot area located in Ecuador at the headwaters of the Nabo River (Figure 1). The study area has contrasting topography from west to east, and demonstrates the full range of this high resolution data set.

The processing sequence for creating derivative products from the SRTM 1 arc-second DEM has two phases, surface preparation and derivative development. First data gaps or voids are filled. Some of the voids in the data are caused by the complex nature of IFSAR technology (Dowding, 2004), and others are caused by topographic shadowing (Grohman, 2006). There are two methods for filling these voids, interpolation or inserting other DEM data. Both methods perform well in smaller voids, but for the larger voids filling with existing data is the preferred method. In the pilot study, voids were filled with NGA's Delta Surface Fill method. Figure 2 shows how an adjustment of an existing DEM (the “fill surface”) is adjusted to match the SRTM elevation surface (Grohman, 2006). A seamless transition is created between the SRTM DEM and the fill surface (Futrue 2). Second a technique called stream "burn in" is used in areas of low slope on the elevation data set (Maidment, 200X). Existing vector drainage networks are intersected with the DEM. Elevation values along the stream network are adjusted so that the subsequent derivative products will generate realistic drainage networks. Although this is a very important step in the process, it was omitted in the pilot project because the study area did not have areas of low slope.

Primary products were derived directly from the DEM, such as slope, aspect and hillshade. Secondary products are created from a primary product, as shown in table 1. Most of the primary derivatives were developed using the ArcGIS Spatial Analyst functions shown table 1. The secondary products were also developed using Spatial Analyst functions, but these were included in a model designed in ArcGIS's Model Builder application. This hydrologic model applied standard hydrologic functions along with custom functions to ensure consistent output. Figure 3 shows the flow diagram of the hydrologic model. The pilot project did not include validation of the output, but typically data sets were run through the model several times. After each run the output was reviewed and changes were made to the respective input data to correct for undesired anomalies. The model is run on the data until there are no undesired anomalies or a minimum threshold is met. Typically corrections are to the stream "burn in" data set, adjusting the stream network to better match realistic conditions.

By completing these data sets for the entire continent, GeoSur will provide topographic and hydrological geospatial information at an accurate, detailed, and consistent format. It will also narrow the disparity of geospatial capacity among South American countries.

References
Dowding, S., T. Kuuskivi and X. Li, 2004. Void Fill of SRTM Elevation Data – Principles, Processes and Performance, Images to Decisions: Remote Sensing Foundations for GIS Applications, ASPRS 2004 Fall Conference, Kansas City, MO, USA.

Grohman, G., G. Kroenung, and J. Strebeck, 2006: Filling SRTM Voids: The Delta Surface Fill Method, Photogrammetric Engineering & Remote Sensing 73 n 30, p 216, March 2006.

Ramirez, Eric, editor. 2005: SRTM Mission Overview [Internet]. NASA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena (CA); August 2005. Available from: http://www2.jpl.nasa.gov/srtm/missionoverview.html. English.

Shuttle Radar Topographic Mission [Internet]: NASA, Jet Propulsion Laboratory, California Institute of Technology, Pasadena (CA); August 2005. JPL 400-713, Rev. 1 7/98; [cited 2006 Feb 2]. 2 p. Available from: http://www.nlm.nih.gov/pubs/formats/internet.pdf. System Requirements: Adobe Acrobat. English. Maidment