| 1. | What is the difference between INNERTC and LASERTC? |
| Both INNERTC and LASERTC are designed to
calculate terrain corrections for land gravity stations with extremely
high accuracy. LASERTC is designed to calculate corrections for
terrain very close to a gravity station using irregularly-distributed
elevation samples, whereas INNERTC is designed to calculate terrain
corrections using Digital Elevation Models of the type distributed
by the U. S. Geological Survey. |
| 2. | How accurate are the calculated terrain corrections? |
| The terrain corrections calculated by either program are
calculated using extremely accurate numerical procedures designed to keep
the integration error below 0.1 µGal. However, one should realize that
the usual limtation in the achievable accuracy depends not upon the
numerical procedure used, but upon the accuracy of the terrain data that
is used for the calculation, as well as upon how well the gravity
station's coordinates are known. |
| 3. | Do either LASERTC, INNERTC, or RASTERTC provide standard gravity reductions? |
| No, these programs only calculate terrain corrections, they do not provide standard gravity reductions. However, a
public-domain software package called OUTERTC is distributed along
with the LASERTC, INNERTC, and RASTERTC. This
package, originally written by Donald Plouff of the U. S. Geological
Survey, not only provides standard gravity reductions, but can also
correct for the effect of distant terrain. Being public-domain software,
it is provided without charge. |
| 4. | What are EMX and RSX? I never heard of them. |
| EMX and RSX are run-time libraries developed by Eberhard Mattes (EMX) and Rainer Schnitker (RSX). These libraries provide the basic code needed for the routines in the TC package. EMX is designed to support the OS/2 operating system as well as DOS (if the DOS in use is using a VCPI memory server). RSX is designed to support Windows 95, Windows NT, and DOS (if the DOS in use is using a DPMI memory server). These run-time libraries, as well as a C development system, are freely available for personal use. |
| 5. | Why cannot LASERTC correct for the effect of terrain at large distances? |
| LASERTC is inherently designed to correct for terrain
very close to a gravity station, using irregularly-distributed elevation
data supplied by the user. To achieve such accuracy, a smooth surface is
fitted to the elevation data supplied by the user; the surface is then
numerically integrated using an adaptive numerical integration scheme. The
main reason that LASERTC cannot calculate corrections for distant
terrain is that the number of terrain samples that the surface-fitting
procedure can accommodate is limited. For example, to correct for terrain
at distances > 1000 m, a supplementary terrain correction
procedure should be used (INNERTC or RASTERTC). |
| 6. | Why do I have to preprocess my DEMs with DEMREAD? |
| The main reason is simply for efficiency. In many instances the DEMs must be read several times by INNERTC; the binary file produced by DEMREAD is much more efficient to read, and is typically about 70% smaller in size than the original DEM files. In addition, the DEMs used by INNERTC are required to be in a single UTM zone; DEMREAD checks for this. |
| 7. | How far out from a station can INNERTC be used for terrain corrections?
| In principle, there is no limit to how far out INNERTC can be used. Typically, though, INNERTC is used to a distance of 3-5 km from a gravity station. Terrain at greater distance is corrected using a terrain correction program designed for distant terrain; such programs typically use terrain data having spatial resolutions much coaser than the 30-m DEM data that is commonly used by INNERTC. However, there are users who regularly use INNERTC to correct for terrain as far as 20 km from gravity stations. The main disadvantage to using INNERTC to correct for terrain at distances greater than 10 km is that the calculation takes significantly longer (the time required is proportional to the area of the calculation, or approximately 3.14×R², where R is the maximum distance to which the calculation is carried out.) |
| 8. | I have digitized terrain data in addition to DEMs. Can I make use of these data? |
| In a word, yes. INNERTC can use auxiliary terrain data directly; the user must simply supply a file containing the auxiliary terrain data. The auxiliary terrain data file must be in the same format as the user's gravity data file; that is, the terrain data must be supplied in geographic coordinates (latitude & longitude), plus an elevation. However, sometimes a user's auxiliary terrain data are inconsistent with the elevation data provided on the DEM. Because INNERTC combines such auxiliary elevation data with the elevation data from the DEM, inconsistent data sets can lead to wildly inaccurate results. In such cases, LASERTC can be used to correct for the terrain closest to the gravity station, using the user-supplied auxiliary terrain data. The, INNERTC is used to calculate the corrections for more distant terrain, and the results are combined. |
| 9. | USGS distributes 3" terrain data free of charge. Can I use such data? |
| Basically, no, although such data can be reformatted and used in Don Plouff's terrain correction program for distant terrain, OUTERTC. The 3" data distributed by USGS were derived from 1:250,000-scale maps, and are simply not accurate enough to be used to correct gravity data for near-in terrain. At distances greater than about 3-4 km, such data could be used. Note that the data format used for the 3" terrain data is completely different from the format used for the 30-m DEMs distributed by USGS. |
| 10. | Can I use DEMs generated other than those distrbuted by USGS? |
| Yes, any Digital Elevation Model that conforms to the format specified by USGS can be used, down to DEMs having a spatial resolution as small as 1 foot. |
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