Validating Global Digital Elevation Models with Degree Confluence Project Information and Aster-dem on Geo Grid

There are two major freely available global digital elevation datasets, known as GTOPO30 and SRTM. To know the accuracies of those data in global scale, relative evaluation of these data and validation with global scale are performed. We compared elevation values of GTOPO30 and SRTM 30 arc second datasets globally and found there are places in the world with more than kilometres differences in elevation especially in mountainous areas. Not only in mountainous areas, in planes also differed much. The values of SRTM are not always higher than that of GTOPO30. These differs exist randomly in global scale. Not only elevation, but also slope inclination and aspect of those data also very differ. To evaluate these differs, we develop a system, namely “Science DCP”, which enables to validate these global DEMs at degree confluences, latitude and longitude integer degree intersections, in the world. ASTER DEM elevation data, field visit GPS elevation information from Degree Confluence Project, where in DCP were used as validation data. By using this system, we found that there exist distortions in the two datasets especially near 60-70 west longitudes. So we focused on 13°S 73°W where the values of these two datasets elevation differs about 600 m and where the values of these datasets much well. By using this system, user can easily validate global DEMs and the accuracy of global DEM will be improved. * Corresponding author. Koki Iwao [email protected] 1. BACKGOUND 1.1 Importance of global DEMs GTOPO30 (Gesch et al., 1996) and SRTM (Farr et al., 2000), which are known as Global Digital Elevation Model data, are used in various applications such as visualization of geology, hydrologic modelling, remote sensing data processing. For example, HYDRO1k (USGS, 2000) is a geographic database developed to provide comprehensive and consistent global coverage of topographically derived data sets, including streams, drainage basins and ancillary layers derived from the GTOPO30. Whereas, HydroSHEDS (Lehner et al., 2006), Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales, is a mapping product that provides hydrographical information for regional and globalscale applications in a consistent format. It offers a suite of georeferenced data sets (vector and raster) at various scales, including river networks, watershed boundaries, drainage directions, and flow accumulations based on SRTM. Those data are widely used for global earth simulation such as global water cycle modelling (Alcamo, et al., 2000). 1.2 Validation of global DEMs GTOPO30, completed in late 1996, was developed over a three year period through a collaborative effort led by staff at the U.S. Geological Survey's EROS Data Center (EDC), is a global digital elevation model (DEM) with a horizontal grid spacing of 30-arc seconds (approximately 1 kilometer) and was derived from several raster and vector sources of topographic information. The Shuttle Radar Topography Mission (SRTM) obtained elevation data on a near-global scale to generate the most complete high-resolution digital topographic database of Earth. SRTM consisted of a specially modified radar system that flew onboard the Space Shuttle Endeavour during an 11-day mission in February of 2000. SRTM data was used to update the older USGS GTOPO30 global DEM, by averaging to 30-arc second resolution and replacing GTOPO30 between the latitudes of 60° north and 56° south. As for these data, verification is performed, respectively. For example, to validate GTOPO30, SLA: Shuttle Laser Altimeter data was used (Harding et al., 1999). Sun et al also used SLA for validation of SRTM (Sun et al., 2003).ERS-1 and ENVISAT satellite altimeter data also used for near-global validation of the SRTM DEM (Berry et al., 2007). Also two datasets are compared each other at volcano (Gerstenecker, et al., 2005). According to those reports, accuracies of GTOPO30 and SRTM30 are less than several hundred meters generally. However there is very limited to compare these DEMs globally. GTOPO30 has a horizontal grid spacing of 30 arc seconds which corresponds to 21,600 rows by 43,200 columns in Geographic projection. Usually, elevation data are provided as 16-bit data, total amount of the size run into about 1.74 gigabytes, which might be one of the reasons why global scale comparison were not well performed till now. 1.3 Proposed methods for global DEMs validation The validation methods proposed in the past focused on statistical evaluation using a small amount of validation information in global scale. Some researcher used satellite altimetry data for global validation of DEMs. Those are not a “ground” based actual information.