The user has a number of choices to make prior to calculating the inner-zone terrain corrections. The use of the default radii for the terrain correction calcuations is often adequate, but the user may want to experiment with the radii used. If a DEM having a resolution different from 30 m is used, the user will almost surely wish to use different values for the correction radii than the default values. Another important factor is to determine if a bias exists between the DEM elevations and the elevations provided with the gravity measurement.
The default values for Rmin, Rmed, and Rmax are 45, 250, and 2000 m. These values are appropriate for USGS DEM's that are normally distributed with a resolution of 30 m. However, the correction program is quite general and can accept DEM's having resolutions as small as 1 foot. In such cases, the value Rmin, especially, should be changed to a much smaller value I suggest a value 1.5-3 times the spatial resolution of the DEM, depending upon the vertical accuracy of the DEM. The value of Rmed depends in part on the amount of storage and time required for construction and interpolation of the terrain surface near the gravity station. If very high-resolution DEM's are utilized, the number of samples within 250 m of a gravity station could become prohibitively large; in such cases, the user will probably have to decrease Rmed.
It is very important for the user to attempt to determine what bias, if any, exists between the elevations in the DEM and the elevations accompanying the gravity measurements. The author's experience has been that such biases are relatively common. To assist in the identification and estimation of such biases, InnerTC notes the actual and the interpolated value for all elevations in the gravity data set. Additionally, any gravity measurement that is "close" (within 2.5 m) to a DEM elevation is noted and the two elevations are compared. Such information appears in the listing file generated by the program. If a systematic elevation bias exists between the gravity data elevations (as well as the auxiliary elevation data, if used) and the DEM elevations, the user should either (a) always use interpolated elevations rather than actual elevations in the terrain correction calculation or (b) apply a constant shift to the gravity data elevations in order to minimize errors associated with the elevation bias. Note that if auxiliary elevation data are used, it is probably best to apply an elevation shift (which is always applied to both the gravity station data as well as the auxiliary elevation data) rather than simply use interpolated elevations, as the interpolated elevation data could vary rapidly horizontally if the DEM data and the auxiliary elevation data, which collectively form the basis for the interpolation, are used when a bias exists. Note that one can always apply a bias correction and use interpolated elevations.