The primary function of this page is to link to resources of interest in geoid research.
However, we keep that interpretation rather broad. So, for example, one might find links related to
accuracy of GPS ellipsoid heights, or vertical datum issues. Sometimes, commentary is provided,
to help place the information in a context.
Gravity Recovery and Climate Experiment (GRACE)
The
Gravity Recovery and Climate Experiment (GRACE) is a satellite mission
of NASA with the Mission Objective of: "A new model of the Earth's gravity
field with unprecedented accuracy every 30 days for five years".
Challenging Mini-Satellite Payload (CHAMP)
The Challenging Mini-Satellite Payload (CHAMP) is a satellite mission of GFZ with three Mission Objectives:
Gravity Field and Steady-state Ocean Circulation Explorer (GOCE)
The
Gravity Field and Steady-state Ocean Circulation Explorere (GOCE) is a satellite mission
of ESA with the Mission Objective of: "Provide measurements with high spatial resolution
AND high accuracy AND homogeneous accuracy such that global and regional models of the (static)
Earth's gravity field and of the geoid (the equipotential surface
of the Earth's gravity field potential) can be decuced with unprecedented precision."
Research and Production Software
The
The Research Software page contains programs which scientists at NGS have
used in the research and production of geoid models. Most are written in FORTRAN77.
NGSDEM99
The
NGSDEM99 data set is a 1 x 1 arcsecond DEM created at NGS
from raw DEM data of USGS and NIMA. It covers the area 39 to 49
degrees latitude and 234 to 256 degrees longitude.
The Shuttle Radar Topography Mission
The
Shuttle Radar Topography Mission
is a joint project between The National
Imagery and Mapping Agency (NIMA)
and the National Aeronautics and Space Administration
(NASA). It is an 11 day space shuttle mission
to map the Earth's surface between +60 and -60 degrees latitude.
The GPS Benchmark Data Set of 23-July-98
This
GPS benchmark data set
for the United States has been developed
as a product to support research on geoid and global geopotential models.
The data set consists of 5379 points occupied by GPS receivers on leveled
benchmarks. 211 points have been flagged as outliers after the data
cleansing procedures. Be sure to read the extensive
documentation
on these data.
The 2nd Joint Meeting of the Gravity and Geoid Commissions (IGC/IGeC)
The International Gravity Commission (IGC) and the International Geoid Commission (IGeC) are organizing a joint meeting in Trieste, Italy from Sept. 7 to Sept. 12, 1998. The meeting will have sessions in the global gravity field, the geoid, altimetry, and several other topics.
Detailed information is available at http://www-dinma.univ.trieste.it/igc. Information is also available from IGC@theoffice.it
Local Organizing Committee -------------------------- Prof. I. Marson DINMA Via Valerio 10 34127 Trieste MARSON@UNIV.TRIESTE.IT
Marc Veronneau's Canadian Geoid Home Page
I can't say enough nice things about Marc's page . It has been updated with results on his experiments with the primary and secondary indirect terrain effects, ellipsoidal corrections, global high-resolution geoid models, and more. It also retains the material on the GSD95 geoid model, GPS on benchmarks, deflections of the vertical, satellite altimetry, miscellaneous reports and figures, and Marc's analysis of the beta GSFC/DMA geopotential models.
Highly recommended!
CARIB97 and DCAR97 -- New Geoid Height and Deflection Models
CARIB97 and DCAR97 have been released. The data are available on WWW, anonymous FTP (ftp.ngs.noaa.gov), our BBS (301-713-4181/4182), and floppy disk. Be sure to browse the gallery of images, since the gravity and geology of the Caribbean Sea region is very complex and interesting.
The CARIB97 model is a high resolution gravimetric geoid model covering
the region 9-28N, 86-58W. It has been prepared in partnership with the
National Imagery and Mapping Agency
(NIMA).
As with
G96SSS,
CARIB97 does not
incorporate GPS heights on leveled benchmarks. When using CARIB97 in
comparisons, be sure that your GPS ellipsoid heights are in a geocentric (ITRF94)
reference frame. Please be aware, that the vertical datums on the various
islands are based on tidal benchmarks, and that these vertical relationships
can vary from island to island.
New Global Marine Gravity, 2 arc-minute density
This June, Per Knudsen and Ole Andersen at the Geodetic Division of Kort og Matrikelstyrelsen (KMS) (the National Survey and Cadastre of Denmark) have produced a new global marine gravity field by processing of Geosat and ERS-1 satellite altimetry data.
These data are the gravity data set we used to supplement the ship gravity in the CARIB97 geoid height model. We have found this altimetry-derived, marine gravity anomaly data set to have very nice properties in the Gulf of Mexico and the Caribbean Sea. In addition, we understand Prof. Richard Rapp has also done studies in the Gulf of Califorina with the KMS data. One nice feature of the KMS anomaly set is that a model of permanent dynamic topography (sea surface topography) was removed from the satellite altimeter measurments prior to the conversion into gravity anomalies.
Gravity files for FTP download.
For more information, you can e-mail:
Per Knudsen (pk@kms.min.dk), and
Ole Baltazar Andersen (oa@kms.min.dk).
GTOPO30 -- Global Digital Terrain, 30 arc-second density
The GTOPO30 data set appears to be more extensive than the current Level 0 DTED(tm)
data set by NIMA. So, you have an interest in a area that was not in the NIMA set,
do investigate this link.
MEXICO97 and DMEX97 -- Geoid Height and Deflection Models for Mexico
MEXICO97 and DMEX97 have been released. The data are available on WWW, anonymous FTP (ftp.ngs.noaa.gov), our BBS (301-713-4181/4182), and floppy disk. Be sure to browse the gallery of images, since the gravity and geology in and around Mexico is very complex and interesting.
The MEXICO97 model is a high resolution gravimetric geoid model covering
the region 14-33N, 119-86W. It has been prepared in cooperation with the
Instituto Nacional de Estadistica, Geografia e Informatica
(INEGI).
As with
G96SSS,
MEXICO97 does not
incorporate GPS heights on leveled benchmarks. When using MEXICO97 in
comparisons, be sure that your GPS ellipsoid heights are in a geocentric (ITRF94)
reference frame. Please be aware, if you use NAVD 88 orthometric heights,
the NAVD 88 datum is offset from global mean sea level by about 31 cm in the
United States.
Geoid, Gravity, Deflection, and Data Coverage Grids
We've been working with the Solid Earth Geophysics Division of the National Geophysical Data Center (NGDC), NOAA, to provide data for an update to their Gravity CD-ROM. Some of the datasets we've submitted to NGDC are ASCII grids of geoid, gravity, deflection, and gravity data coverage. The geoid height and deflection of the vertical models are from GEOID96, DEFLEC96, MEXICO97, DMEX97, and G96SSS. However, each model is organized as a single, big, ASCII file; rather than as the overlapping binary files used in our standard distributions. In addition, we provide grids of terrain-corrected free air gravity anomalies, and gravity data coverage grids.
We encourage everyone to consider obtaining the new Gravity CD-ROM from NGDC
when it is available. But, this will give you a chance to get an early
look at a few of the datasets.
GEOID96 -- The Latest U.S. Geoid Model from NGS
GEOID96 has been released. The release encompasses three models,
GEOID96,
DEFLEC96, and
G96SSS.
The data are available on WWW, anonymous FTP (ftp.ngs.noaa.gov), our BBS (301-713-4181/4182),
floppy disk, and CD-ROM. Dru Smith and I have built a
gallery of images
with short captions about the models and data sets. We will be adding to
this page over time.
The refinements over the old GEOID93 model are:
The G96SSS geoid is a geocentric, gravimetric model. As such, it does not
incorporate GPS heights on leveled benchmarks. When using this model in
comparisons, be sure that your GPS ellipsoid heights are in a geocentric (ITRF94)
reference frame. Please be aware, if you use NAVD 88 orthometric heights,
the NAVD 88 datum is offset from global mean sea level by about 31 cm.
In addition, the G96SSS geoid heights need to be reduced by a constant 12.3
cm to correspond to the best estimate of the global equipotential surface.
GSFC/DMA Geopotential Model, EGM96
The EGM96 geoid grid has been released (has been for a while now, actually).
The Goddard Space Flight Center (GSFC) and the former Defense Mapping Agency (now in the National Imagery and Mapping Agency -- NIMA) have been engaged in a joint project to compute an improved global spherical harmonic model of the Earth's geopotential. This model incorporates the latest satellite tracking data, as well as altimeter data from TOPEX/Poseidon, ERS-1, and the Geosat Geodetic Mission. EGM96 also incorporates new surface and ship gravity data covering the globe, including the former Soviet Union.
The geopotential coefficients, a correction coefficient file, and a
15' x 15' geoid grid can be found at the
WGS 84 EGM Public Home Page
web site. GSFC has a very extensive (and informative) page on the joint model,
in addition to solution products and links to other data files, at the page
EGM96: The NASA GSFC and NIMA Joint Geopotential Model.
Both sites are highly recommended!
Evaluation of final EGM96 model
The Special Working Group of the International Geoid Service (IGeS) has extended their evaluation effort to include the final EGM96 model, supplementing their evaluation of beta test versions. Both the beta and the EGM96 evaluations will be published in a forthcoming IGeS bulletin.
Read the
NGS evaluation results, here on the Web.
Evaluation of beta test versions
The GSFC/DMA group has asked the International Geoid Service, headed by Prof. Fernando Sanso, to organize an effort to evaluate preliminary, "beta" models, in support of the final model computation. The chair of the Special Working Group , Prof. Mike Sideris, has organized an international team to evaluate these models.
Read the NGS evaluation results for the beta models, here on the Web.
Since HTML has problems with Greek characters, you can also download the
NGS evaluation results for the beta models,
in binary format, Word Perfect for Windows 6.1 (96K).
Marine Gravity, 2 arc-minute density
Sandwell and Smith have computed a global marine gravity field by processing of satellite altimetry data. In addition, they have predicted topography (bathymetry) of the seafloor from the gravity data.
You can read about the dataset in:
Marine Gravity Anomaly from Geosat and ERS-1 Satellite Altimetry, David T. Sandwell and Walter H. F. Smith, Journal of Geophysical Research, 102(B5), 10039-10054, 1997.
Resolution of the Scripps/NOAA Marine Gravity Field from Satellite Altimetry, Karen M. Marks, Geophysical Research Letters, 23(16), 2069-2072, August 1, 1996.
Gravity files for FTP download.
GLOBE -- Global Digital Terrain, 30 arc-second density
The data are stored on two CD-ROM's, and so are too large to download.
The GLOBE 0.1 announcement page.
The NGDC Solid Earth Geophysics site.
The development of the data set has been performed in close cooperation
with the National Imagery and Mapping Agency (NIMA). NIMA has announced
the planned release of a more extenisve 30 arc-second digital terrain data set, the
Level 0 DTED(tm),
on CD-ROM in early 1997. My NIMA source indicates it is likely that this
product will be disseminated in cooperation with NGDC.
International Geoid Commission Report, 1991-1995
Yuki Kuroishi's Japanese Geoid Report
It should be no surprise that Japan has done work with high-resolution
geoid models. What might be a surprise is that some of that work
as performed by Dr. Yuki Kuroishi during a year-long stay at the
National Geodetic Survey. Yuki has published a
report
on the JGEOID93 model in the Bulletin of the
Geographical Survey Institute
(the sponsoring agency for his stay). The gravity field
is particularly interesting in this area, given the presence of
the Japan and Izu-Bonin trenches and the mountainous terrain of Japan itself.
GEOID96 Paper for GIS/LIS'96 Conference
Converting GPS Height into NAVD88 Elevation with the GEOID96 Geoid Height Model
This is the first paper which discusses the GEOID96 model. Due to deadline requirements, it was written before GEOID96 was completed. While the numbers are preliminary, they still give the "flavor" of the model. This paper is written to be accessible to the widest possible audience, and scientific papers on GEOID96 will be written in the near future.
This paper will appear in the GIS/LIS'96 conference proceedings.
Read the paper, here on the Web (about 100K of html and gif's).
You can also download the paper, in binary format, Word Perfect for Windows 6.1 (Warning! -- 1,270Kb).