Related Software and Terrain Data
InnerTC and LaserTC are designed to calculate terrain corrections due to terrain very close to a gravity station. At greater distances, more traditional terrain correction programs, such as that of Plouff (1966), are typically utilized. A version of Plouff's program called OuterTC is available for download. Terrain data for the lower 48 states of the United States are also available for download (12.9 Mb file!), though such data will have to be reformatted somewhat in order to be used by Plouff's code. A code called MAPFILE that will perform a particular reformatting is provided here, as well. Additionally, the 30-m DEMs that are most commonly used in conjunction with InnerTC are now available -- for a limited time -- for FREE from the U. S. Geological Survey. The catch is that these DEMs have been reformatted into the new Spatial Data Transfer Standard (SDTS), which is utterly incompatible with the previous format. However, a conversion code is available here, which will convert the SDTS-formatted DEMs into the original format, thus permitting the SDTS-formatted DEMs to be used with InnerTC. More details of the code are available here, as well.
National Elevation Dataset
The U. S. Geological Survey now makes available continuous elevation data in the U.S. and many of the territories of the U.S. The data set is called the National Elevation Dataset (NED), and it has several advantages over the older Digital Elevation Models. These advantages can be summarized as follows:
- The elevations are continuous, unlike the 7½-min DEMs.
- Users can order (typically by simple download) arbitrary quadrangles of data.
- The data are distributed in geographic coordinates, rather than UTM coordinates.
- The horizontal datum used by the NED is the 1983 North American Datum; the vertical datum is the 1988 North American Vertical Datum. Note that most of the 7½-min-DEMs distributed by the USGS used the 1927 North American Datum for horizontal coordinates and the 1929 North American Vertical Datum for elevations.
The USGS distributes the NED through its seamless distribution website. At this site, not only U.S. data but also nearly worldwide SRTM data (albeit at resolutions limited to about 90m) can be downloaded. The elevation data are almost always available at 1 arc-sec spatial resolution, but are often also available at 1/3 arc-sec or, sometimes, 1/9 arc-sec. The available download formats are BIL, GridFloat, and ArcGrid.
The NED elevation data can be used by RasterTC, but must first be converted rectangular coordinates, rather than geographic coordinates. We recommend Geosoft's Oasis-Montaj or Global Mapper for such conversions.