Derived DEM Information
What is a Digital Elevation Model?
A digital elevation model (DEM) is the digital representation of the land surface elevation with respect to any reference datum. DEM is frequently used to refer to any digital representation of a topographic surface and is the simplest form of digital representation of topography. DEMs are used to determine terrain attributes such as elevation at any point, slope and aspect.
The SRTM DEM and ASF RTC DEM show different elevations over the same area. Why?
Most DEMs are geoid-based and require a correction before they can be used for terrain correction. The DEM included in an ASF radiometrically terrain corrected (RTC) product file was converted from source DEM orthometric height to ellipsoid height using the ASF MapReady geoid_adjust tool. This tool applies a geoid correction so that the resulting DEM relates to the ellipsoid. An online tool is available that computes the height of the geoid above the WGS84 ellipsoid, and will show the amount of correction that was applied to the source DEM used in creating an RTC product:
Video tutorial: How to download DEMs. Cómo descargar DEMs
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Video created by Géo Tech
Source DEMs Used for ALOS PALSAR RTC for Data Processing
The quality of the terrain correction results of ALOS PALSAR RTC products is directly related to the quality of the digital elevation models (DEMs) used in the process of geometrically and radiometrically correcting the SAR imagery. Table 1 summarizes the various DEM sources and the way they are used in the radiometric terrain correction (RTC).
DEMs used and their priority in RTC processing.
The continental U.S., Hawaii, and parts of Alaska are covered with 1⁄3 arc sec National Elevation Dataset (NED) at a 10 m resolution. The rest of Alaska above 60 degrees northern latitude is only available at 60 m resolution with 2 arc sec NED data. The best resolution for Canada and Mexico at 30 m is given by 1 arc sec NED. For the remaining globe, Shuttle Radar Topography Mission (SRTM) GL1 data at 30 m resolution is used. Greenland and Antarctica are mostly covered by ice and glaciers and not suitable for terrain correction. For areas in Eurasia above 60 degrees northern latitude, no suitable DEMs are available.
Coverage of the various DEM sources used for terrain correction.
The DEMs were pre-processed by ASF to a consistent raster format (GeoTIFF) from the original source formats: height (*.hgt), ESRI ArcGrid (*.adf), etc. Many of the NASA-provided DEMs were provided as orthometric heights with EGM96 vertical datum. These were converted by ASF to ellipsoid heights using the ASF MapReady tool named geoid_adjust. The pixel reference varied from the center (pixel as point) to a corner (pixel as area). The GAMMA software, used to generate the terrain corrected products, uses pixel as area and adjusts DEM coordinates as needed. Where more than one DEM is available, the best-resolution DEM is used for processing. Complete DEM coverage from a single DEM source is required for processing to proceed.