is anticipated through geoid undulation infonnation derived from GPS and leveling data.
Profiles and networks of such data have provided, in the past, quality measures of a
potential coefficient model to represent geoid undulations. The results of these tests will be
made available to the joint project team who will have the responsibility of selecting the
fmal potential coefficient model of the project
GEOID UNDULATION DETERMINATION
As noted in the first sections of this paper, the underlying goal of this project is to
estimate the best set of geoid undulations based on a degree 360 potential coefficient model,
consistent with the data available. These undulations can be considered to define a refmed
WGS-84 geoid. As such, it is anticipated that the geoid undulations will be computed in
the WGS-84 reference frame (Malys and Slater, 1994) with respect to the WGS-84
ellipsoid. A gridded geoid undulation file, with appropriate interpolation software, will be
made available to all interested parties.
The precise manner in which the geoid undulations will be detennined has not been
defined. The classical procedure calculates the geoid undulations purely as a function of
the potential coefficients. However, geodetic boundary value concerns may suggest the
calculation of a height anomaly at the surface of the Earth and its transfonnation to a geoid
undulation using the Bouguer anomaly and the elevation of the grid point as suggested in
Rapp (1992). Preliminary evidence (Milbert, 1994, private communication) suggests the
correction tenn may play an important role in creating an accurate geoid model from a set of
high degree potential coefficients. Additional research is needed on this topic.
SUMMARY
This paper has described a joint project, now under way, between NASNGSFC and
DMA that will result in a new degree 360 potential coefficient model and enhanced WGS84 geoid undulation values. The project takes advantage of three main developments:
1. A significant amount of new and updated terrestrial gravity data that is available to the
project.
2. The release of 3~' mean anomalies derived from Geosat Geodetic Mission data taking
into account ocean dynamic topography effects.
3. The availability of new satellite tracking data, including altimeter data (Geosat, TIP, and
ERS-l), not previously used in a high degree model estimation.
The model estimation will follow procedures previously used, with new methods tested
as time and resources permits. Input from the scientific community will be sought for the
testing and evaluation of several preliminary models. This testing will take place over a 3
month time period with the project defining the final model. The adopted model will be
used to define the geoid undulations in the WGS-84 system. Gridded values will be
provided to all interested users. These undulations can be used for a variety of purposes
related to geodetic, oceanographic, mapping, and charting applications. The geoid defmed
by the adopted model will fonn the reference surface used for the definition of a world
vertical datum that would be complementary to WGS-84.
Acknowledgments
The research being carried out for this joint project is funded by the Defense Mapping
Agency and NASA Headquarters. Initial interest in carrying out this project was fostered
105
Profiles and networks of such data have provided, in the past, quality measures of a
potential coefficient model to represent geoid undulations. The results of these tests will be
made available to the joint project team who will have the responsibility of selecting the
fmal potential coefficient model of the project
GEOID UNDULATION DETERMINATION
As noted in the first sections of this paper, the underlying goal of this project is to
estimate the best set of geoid undulations based on a degree 360 potential coefficient model,
consistent with the data available. These undulations can be considered to define a refmed
WGS-84 geoid. As such, it is anticipated that the geoid undulations will be computed in
the WGS-84 reference frame (Malys and Slater, 1994) with respect to the WGS-84
ellipsoid. A gridded geoid undulation file, with appropriate interpolation software, will be
made available to all interested parties.
The precise manner in which the geoid undulations will be detennined has not been
defined. The classical procedure calculates the geoid undulations purely as a function of
the potential coefficients. However, geodetic boundary value concerns may suggest the
calculation of a height anomaly at the surface of the Earth and its transfonnation to a geoid
undulation using the Bouguer anomaly and the elevation of the grid point as suggested in
Rapp (1992). Preliminary evidence (Milbert, 1994, private communication) suggests the
correction tenn may play an important role in creating an accurate geoid model from a set of
high degree potential coefficients. Additional research is needed on this topic.
SUMMARY
This paper has described a joint project, now under way, between NASNGSFC and
DMA that will result in a new degree 360 potential coefficient model and enhanced WGS84 geoid undulation values. The project takes advantage of three main developments:
1. A significant amount of new and updated terrestrial gravity data that is available to the
project.
2. The release of 3~' mean anomalies derived from Geosat Geodetic Mission data taking
into account ocean dynamic topography effects.
3. The availability of new satellite tracking data, including altimeter data (Geosat, TIP, and
ERS-l), not previously used in a high degree model estimation.
The model estimation will follow procedures previously used, with new methods tested
as time and resources permits. Input from the scientific community will be sought for the
testing and evaluation of several preliminary models. This testing will take place over a 3
month time period with the project defining the final model. The adopted model will be
used to define the geoid undulations in the WGS-84 system. Gridded values will be
provided to all interested users. These undulations can be used for a variety of purposes
related to geodetic, oceanographic, mapping, and charting applications. The geoid defmed
by the adopted model will fonn the reference surface used for the definition of a world
vertical datum that would be complementary to WGS-84.
Acknowledgments
The research being carried out for this joint project is funded by the Defense Mapping
Agency and NASA Headquarters. Initial interest in carrying out this project was fostered
105