Gravity terrain corrections calculated using digital elevation models

Abstract

Corrections for terrain effects are required for virtually all gravity measurements acquired in mountainous areas, as well as for high‐precision surveys, even in areas of low relief. Terrain corrections are normally divided into two parts, one part being the correction for terrain relatively close to the gravity station (the “inner‐zone” correction) and the other part being the correction for more distant, say, >2 km, terrain. The latter correction is normally calculated using a machine procedure that accesses a digital‐terrain data set. The corrections for terrain very close to the gravity station are done manually using Hammer’s (1939) procedures or a similar method, are guessed in the field, or simply are neglected. Occasionally, special correction procedures are used for the inner‐zone terrain corrections (e.g., LaFehr et al., 1988); but such instances are uncommon.

Revising gravity terrain corrections in Tasmania

“…Less than a dozen stations of the 253 exhibited a difference of more than 0.25 mGal between the 25 metre and 1 metre DTMderived TCs. The slightly positive nature of the average value is indicative of terrain (or at least terrain model) roughness at wavelengths less than resolvable by the 25 metre DTM, compared to the smooth surface interpolated by the RasterTC© code (Renka, 1984;Cogbill, 1990). Nevertheless, the error introduced by using the coarser DTM is less than would be the case if the inner zone component were omitted altogether inside 75, 50 or even 25 metres.…”

“…Two different automatic terrain correction algorithms, TERRAIN (Roach, 1994; based on the method of St John and Green, 1967) and RasterTC© (Cogbill, 1990) were run for the gravity data set, initially for distances from 75 metres to 22 km from station locations. These gave very similar results (Yu, 2014), giving rise to confidence that both codes are valid.…”

Gravity Terrain Effect of the Seafloor Topography in Taiwan

“…Historically, terrain corrections were computed using Hammer (1939) charts at each station. However, terrain corrections can now be computed efficiently from the regular grid of a DEM (Kane 1962;Cogbill 1990). Nowadays, there have been considerable enhancements in the capabilities of laptop computers; with digital terrain data and computers, terrain corrections can be calculated in a matter of minutes.…”

Field estimates of gravity terrain corrections and Y2K-compatible method to convert from gravity readings with multiple base stations to tide- and long-term drift-corrected observations

“…Lyman and others (1997) described a method to rapidly "shoot" a reflectorless laser range finding gun at key points such as slope changes, maxima, and minima. The input data are automatically recorded, and TCs, for example, to 50 m can be computed in the field by using apparently proprietary software partly based on algorithms referenced by Cogbill (1990). …”

“…Inasmuch as TCs are determined to farther distances by overlaying topographic maps with "templates" or by utilizing digital map elevations (Plouff, 1977;Cogbill, 1990), field TCs commonly are determined to distances of 53.3 m (175 ft) from the gravity station location to include the C ring of the Hammer (1939) TC system or 68 m (223 ft) to include the B ring of the Hayford TC system (Hayford and Bowie, 1912, p. 18;Swick, 1942). If a point with a known elevation is located at a site where the local terrain effect is high and a description of topography is complicated, the gravity station preferably should be located nearby where the effect of topography is minimized, and the elevation is adjusted to the station location by leveling.…”