GRAVITY ANOMALY COMPUTATIONS
Terrestrial Gravity Anomalies
The terrestrial gravity infonnation being brought to this new solution is being processed
by DMA. The goal is to estimate 30' x 30' mean surface free-air anomalies based on the
best data sets available to DMA. The new terrestrial data brings to the solution a
considerable improvement in the data used in previous models such as OSU91A and new
data not previously available for combination model estimates. The areas with improved
data coverage include South America, the former Soviet Union, Greenland, Southeast
Asia, and Antarctica. Improved anomalies have been computed for North America,
Europe, and Australia by incorporating new data. All coordinates for the gravity data are
converted, when feasible, from a local datum to a single geocentric-based reference system
to reduce systematic errors. Figure 1 shows the land areas over which terrestrial gravity
data are available for the project Figure 1 should not be considered the [mal data coverage
since additions are possible before the final solution is carried out. In areas where no
terrestrial gravity data are available, predicted 30' x 30' anomalies are determined using a
topographic/isostatic model as described by Pavlis and Rapp (1990). New harmonic
expansions have been made using updated and improved elevation models.
The DMA Point Gravity Anomaly (PGA) file contains over 30 million values that were
available to be used for the project and is considered to be the most extensive gravity
collection in the world. The procedure to create the terrestrial anomalies consisted of the
following:
a. All gravity anomalies have been placed on WGS-84 before referencing them to ITRF91.
b. Point gravity data are adjusted to IGSN71 (International Gravity Standardization Net).
c. Station coordinates are checked for elevation and position discrepancies.
d. Point data are contoured and analyzed for validity and detailed accuracies are assigned.
e. Molodensky free-air anomalies on the Earth's surface were created.
In addition to the PGA file, a 5' worldwide mean elevation file was created mainly from
DMA's Digital Terrain Elevation Data (OTED) file with additions from map sources, the
National Geophysical Data Center (NGDC) 5' worldwide elevation file, and NASA
altimetry sources. This file was necessary for computations of:
a. Terrain corrections.
b. Spherical harmonic models of the topography.
c. Restoration of free-air from Bouguer anomaly computations.
d. g 1 analytical reduction terms.
e. Topographic/isostatic anomaly computations.
The computational steps for each 1 0 terrestrial area consisted of the following:
1. The PGA data were selected at a 2' x 2' interval for most areas of the world. If the PGA
data were sparse or insufficient, then the interval of selection was increased.
2. A 360 x 360 spherical harmonic representation of the free-air or Bouguer anomalies was
then removed from the PGA data using the JGM-2/0SU91A geopotential harmonic
coefficient models. The Bouguer anomalies required the creation of a spherical harmonic
model of the topography from the 5' mean elevation file to degree and order 360.
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Terrestrial Gravity Anomalies
The terrestrial gravity infonnation being brought to this new solution is being processed
by DMA. The goal is to estimate 30' x 30' mean surface free-air anomalies based on the
best data sets available to DMA. The new terrestrial data brings to the solution a
considerable improvement in the data used in previous models such as OSU91A and new
data not previously available for combination model estimates. The areas with improved
data coverage include South America, the former Soviet Union, Greenland, Southeast
Asia, and Antarctica. Improved anomalies have been computed for North America,
Europe, and Australia by incorporating new data. All coordinates for the gravity data are
converted, when feasible, from a local datum to a single geocentric-based reference system
to reduce systematic errors. Figure 1 shows the land areas over which terrestrial gravity
data are available for the project Figure 1 should not be considered the [mal data coverage
since additions are possible before the final solution is carried out. In areas where no
terrestrial gravity data are available, predicted 30' x 30' anomalies are determined using a
topographic/isostatic model as described by Pavlis and Rapp (1990). New harmonic
expansions have been made using updated and improved elevation models.
The DMA Point Gravity Anomaly (PGA) file contains over 30 million values that were
available to be used for the project and is considered to be the most extensive gravity
collection in the world. The procedure to create the terrestrial anomalies consisted of the
following:
a. All gravity anomalies have been placed on WGS-84 before referencing them to ITRF91.
b. Point gravity data are adjusted to IGSN71 (International Gravity Standardization Net).
c. Station coordinates are checked for elevation and position discrepancies.
d. Point data are contoured and analyzed for validity and detailed accuracies are assigned.
e. Molodensky free-air anomalies on the Earth's surface were created.
In addition to the PGA file, a 5' worldwide mean elevation file was created mainly from
DMA's Digital Terrain Elevation Data (OTED) file with additions from map sources, the
National Geophysical Data Center (NGDC) 5' worldwide elevation file, and NASA
altimetry sources. This file was necessary for computations of:
a. Terrain corrections.
b. Spherical harmonic models of the topography.
c. Restoration of free-air from Bouguer anomaly computations.
d. g 1 analytical reduction terms.
e. Topographic/isostatic anomaly computations.
The computational steps for each 1 0 terrestrial area consisted of the following:
1. The PGA data were selected at a 2' x 2' interval for most areas of the world. If the PGA
data were sparse or insufficient, then the interval of selection was increased.
2. A 360 x 360 spherical harmonic representation of the free-air or Bouguer anomalies was
then removed from the PGA data using the JGM-2/0SU91A geopotential harmonic
coefficient models. The Bouguer anomalies required the creation of a spherical harmonic
model of the topography from the 5' mean elevation file to degree and order 360.
98