Most DEMs are geoid-based and require a correction before they can be used for terrain correction. The DEM included with an ASF RTC product was converted from the orthometric height of the source DEM 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.
The GeoidEval Utility is a free online tool that computes the height of the geoid above the WGS84 ellipsoid at a specific location and will show the amount of correction that was applied to the source DEM used in creating an RTC product.
About the source DEMs used for ALOS PALSAR RTC processing
The quality of an ALOS PALSAR RTC product is directly related to the quality of the digital elevation model (DEM) used in the radiometric terrain correction (RTC) process. The table below summarizes the various DEM sources, and the map indicates which DEM was used in a given location.
The continental U.S., Hawaii, and parts of Alaska are covered with a 1⁄3 arc-second National Elevation Dataset (NED) at a 10 m resolution. The rest of Alaska above 60 degrees northern latitude was only available at 60 m resolution with 2 arc-second NED data. The best resolution for Canada and Mexico at 30 m was provided by 1 arc-second NED. For the remaining globe, the Shuttle Radar Topography Mission (SRTM) GL1 data at 1 arc-second (30 m) resolution was used. Greenland and Antarctica were mostly covered by ice and glaciers and unsuitable for terrain correction. For areas in Eurasia above 60 degrees northern latitude, no suitable DEMs were available.
DEMs used for ALOS PALSAR RTC processing
| DEM | Datum | Coverage Area | DEM Resolution | Product Resolution | Resampling Approach |
|---|---|---|---|---|---|
| NED13 | NAVD88 | CONUS, Hawaii, parts of Alaska | ~10m (1/3 arc sec) | 12.5 m | No resampling |
| 30 m | Down-sampled DEM to 30-m pixel spacing | ||||
| SRTMGL1 | EGM96 | Latitudes between 60 N and 57 S degrees | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| SRTMUS1 | EGM96 | CONUS, Hawaii, parts of Alaska | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| NED1 | NAVD88 | CONUS, Hawaii, parts of Alaska, Canada, Mexico | ~30 m (1 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | No resampling | ||||
| NED2 | NAVD88 | Alaska | ~60 m (2 arc sec) | 12.5 m | Up-sampled 30-m mapping function to 12.5-m mapping function |
| 30 m | Up-sampled DEM to 30-m pixel spacing |
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 to ellipsoid heights using the ASF MapReady geoid_adjust tool. The pixel reference varied from the center (pixel as a point) to a corner (pixel as area). The GAMMA software, used to generate the terrain-corrected products, uses pixels as area and adjusts DEM coordinates as needed. Where more than one DEM was available, the best-resolution DEM was used for processing. Complete DEM coverage from a single DEM source was required for processing to proceed.
