Frequently Asked Questions About The Geoid

Dennis G. Milbert, Ph.D. and Dru A. Smith, Ph.D.

"A Work In Progress ... " -- 7-october-98

Do you have "README" files for your geoid and deflection models? Any on-line papers where I can get more information?
Yes, indeed! We have "README" files for GEOID96, DEFLEC96, MEXICO97, DMEX97, and CARIB97, DCAR97, and G96SSS. We also have a collection of on-line papers that is expanding.

I downloaded a "GEO" file, but neither GEOID.EXE nor GEOGRD.EXE can open the file.
This is a symptom that the "GEO" file (a binary file in PC format) has been corrupted somehow. Check the file size of the "GEO" files. The file sizes have to be: 
          US (lower 48)      1168096 bytes
          Alaska             1110616 bytes
          Caribbean          1034176 bytes
          Mexico              896896 bytes
          Hawaii              154336 bytes
          Puerto Rico/V.I.     88816 bytes
If your file sizes don't match exactly, please try another download. Oh, by the way, just to be complete: 
          GEOID.EXE           143872 bytes
          DOSXMSF.EXE         393942 bytes
          DEFLEC.EXE          145408 bytes
          GEOGRD.EXE           68776 bytes
          DEFGRD.EXE           73240 bytes
Your PC can lock up if these sizes don't match.

I was downloading a "GEO" file from your web page with [name of web browser deleted] and I got a screen of garbled text instead of a window prompting for where to put the file. What gives?
Some versions of web browsers identify "GEO" files as ASCII text and try to display it directly. To circumvent this, hold down the SHIFT key when you click on a link with your mouse button. This forces a download. This behavior of web browsers is probably related to some cases of corrupted downloads of the GEO files from the web pages. We find the [SHIFT][CLICK] technique gives clean downloads.

Will a "GEO" file work in a UNIX system, or is it for PC's only?
The "GEO" files are binary files for PC's. If you need to work on a different kind of system, be sure to get program GEOGRD.EXE. Bring the "GEO" file to a PC system. Run GEOGRD, and convert the input binary file into an output ASCII file. Transfer the ASCII form of the geoid model to your target system. Recompile GEOID and GEOGRD on the new system. Then run GEOGRD, and convert the ASCII back into a binary "GEO" file. By the way, don't forget to transfer the AREA.PAR file, too.

Is the format of the new "GEO" files the same as under GEOID93?
Yes! We maintained the same format. The grid spacing has changed, as well as the number of rows and columns, but this is encoded in the header part of the GEO files. As long as an application reads and processes the header information, it can access the remaining data.

Does this mean I could use the new "GEO" files with an old version of program GEOID.EXE (versions 1.0 or 2.0)?
Yes, it is that compatible. (We don't recommend this due to enhancements regarding number of files supported, updates to the Blue-Book records, etc. ...)

I heard a rumor that there is a new version of program GEOID.EXE. Do I need to get it?
Probably not. Version 3.1 of program GEOID.EXE was created May 7, 1997 (with a minor update to the interactive prompts on July 17) to support new Blue Book format codes for the geoid models of Mexico ( MEXICO97 ) and the Caribbean ( CARIB97 ). If you do not use those models, or if you do not require that Blue Book *86 records be created by GEOID.EXE, then you can use version 3.0. We have placed the latest version (3.1) of GEOID.EXE on our Web and FTP sites.

I see you have two different models for the U.S., GEOID96 and G96SSS. Which should I use?
In general, most users should work with GEOID96. It is constructed to relate GPS ellipsoid heights in NAD 83 and orthometric heights in the NAVD 88 datum. These are the datums used in many maps and charts, and it is likely that your application requires that consistency.

When would anybody want to use G96SSS?
G96SSS is appropriate for specialized studies that require a geocentric (ITRF94) reference system. One example would be to relate satellite altimetry data or GPS receivers mounted on buoys to ocean circulation models. If you use G96SSS with GPS heights in an ITRF94 reference frame, you will detect an offset of 32 to 45 cm in the NAVD 88 datum.

What is the G96SSS model relationship to sea level?
It is necessary to subtract 12.0 cm from the G96SSS values to obtain the geoid undulation between the best-fit global geopotential surface and the GRS80 ellipsoid (when both are expressed in a tide-free system). 
Schematic of vertical reference surfaces (not to scale):


     -----------------------------------------  Local mean sea level
                |
                |  (variable)
                |
     -----------------------------------------  GRS80 ellipsoid (tide-free)
                |
                |  (geoid height)
                |
     -----------------------------------------  G96SSS geoid (if uncorrected)
                |
                |  (12.0 cm)
                |
     -----------------------------------------  Global geopotential
                |
                |  (31.4 cm)
                |
     -----------------------------------------  NAVD 88
The schematic above shows the G96SSS geoid below the ellipsoid. This is the case in the conterminous United States, and corresponds to the negative values for our geoid heights. Our current estimate of the NAVD 88 vertical datum offset is 31.4 cm below the current best estimate of the Earth's best-fit global geopotential. (A piece of good news: the NAVD88 offset seems to be a constant offset; the same throughout the conterminous United States.)

None of these surfaces are the same as local mean sea level (LMSL). The LMSL surface varies from place to place due to oceanographic and meteorologic effects, such as wind setup, prevailing currents, mean temperature, and salinity variations. One can think of of LMSL as an approximation to the global geopotential (or vice versa), but departures can range up to a meter or two.

So the NAVD 88 vertical datum is not at mean sea level?
No. The NAVD 88 datum was obtained by fixing a single datum point, Father Point/Rimouski, in Quebec, Canada. A number of factors went into selecting the number to be fixed. The foremost requirement was to select a value that would minimize the need to recompile topographic maps. At the time, there were not enough GPS heights on benchmarks to establish the true offset.

Even today, getting a precise number for the NAVD 88 offset is tricky. It is sensitive to data processing procedures used to obtain gravity anomalies (derived from satellite altimetry data) in the oceans. The uncertainty in the 31.4 cm value may be at the +/- 10 cm level. Getting a more exact figure depends upon the computational treatment of ocean circulation and the tides.

Is this uncertainty in the offset of NAVD 88 a problem?
For most applications, no. GEOID96 establishes the relationship between NAVD 88 and a non-geocentric GRS80 ellipsoid to about +/- 2.5 cm (one sigma). For accurate results, one should tie into one or more nearby NAVD 88 benchmarks. By using them as vertical constraints in conjunction with GEOID96 or the G96SSS models, the accuracy of the NAVD 88 heights you get from GPS will be limited primarily by how accurately you can measure the vertical with the GPS.

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