README file 9-oct-96 dgm/das updated 7-may-97 das/dgm -- Documentation updated for the new version (3.1) of GEOID.EXE -- Program GEOID.EXE was updated to version 3.1 to support Blue Book codes for Mexico and Caribbean geoid models. If you are working in the conterminous United States, Alaska, Hawaii, Puerto Rico or the Virgin Islands, you do not need to change from version 3.0 to 3.1 of GEOID.EXE. The GEOID96 GEOID MODELS ------------------------ You have received these models on CD-ROM, or downloaded them from the National Geodetic Survey (NGS) web site, the NGS FTP site, the NGS bulletin board system, or have received the models on individual floppy disks. Among the files you have received are: GEOID.EXE the geoid interpolation program (GEOID.FOR is source code) (version 3.1) DOSXMSF.EXE 32-bit DOS extender (needed for GEOID.EXE) AREA.PAR text file of the filenames of geoid height grids GEO96NE.GEO the GEOID96 grid for the Northeastern U.S. 36-50N, 89- 66W GEO96NC.GEO the GEOID96 grid for the Northcentral U.S. 36-50N, 107- 84W GEO96NW.GEO the GEOID96 grid for the Northwestern U.S. 36-50N, 125-102W GEO96SE.GEO the GEOID96 grid for the Southeastern U.S. 24-38N, 89- 66W GEO96SC.GEO the GEOID96 grid for the Southcentral U.S. 24-38N, 107- 84W GEO96SW.GEO the GEOID96 grid for the Southwestern U.S. 24-38N, 125-102W GEO96AN.GEO the GEOID96 grid for North Alaska 60-72N, 179-128W GEO96AS.GEO the GEOID96 grid for South Alaska 51-63N, 179-128W GEO96HW.GEO the GEOID96 grid for the Principal Hawaiian Islands GEO96PR.GEO the GEOID96 grid for Puerto Rico - Virgin Islands GEOGRD.EXE utility program for sub-area extraction and format conversion (GEOGRD.FOR is the source code) To Install (after uncompressing the files) 1) Make a subdirectory on your hard disk (example: mkdir c:\geoid96). 2) Copy the various geoid files into that subdirectory. copy *.* c:\geoid96 /v (for example) 3) Repeat step 2) as required for your other sets of geoid files. (If you have also received G96SSS model files, do not place them in the same subdirectory as your GEOID96 files.) (If you have installed from floppies, the files are put in the designated subdirectory.) 4) Check your AUTOEXEC.BAT and CONFIG.SYS files to insure compliance with the following notes: Note 1: DOSXMSF.EXE must either be present in the same directory as GEOID.EXE, or, it must be in a directory in your DOS PATH environment variable. (such as: c:\dos, for example) DOSXMSF.EXE may be freely reproduced and distributed, without royalty. Note 2: You must have a statement FILES=25 (or a number greater than 25) in your CONFIG.SYS file. To Execute Type GEOID , and follow the prompts. To Terminate You can stop the program at any time by the Control C key combination. BUT, PLEASE DON'T START YET. PLEASE KEEP READING THIS DOCUMENT. How Program GEOID Works ----------------------- The various geoid height grids are stored in the ".GEO" files. Program GEOID will assume that the files in your local directory with a .GEO extension are geoid height files. You can operate with as few as one .GEO file, or as many as 15. When the program interpolates a given point, it checks an internal list of .GEO boundaries, and uses the earliest list entry whose boundaries contain that point. The order in which the .GEO file names appear on the opening screen indicates the order in which the .GEO files are searched. When running program GEOID.EXE, the latitude and longitude of each point must be input. The GEOID96 models are heights above the NAD 83 ellipsoid, while all other recent geoid models (G96SSS, MEXICO97, and CARIB97) are heights above a GRS-80 ellipsoid, centered at the ITRF94(1996.0) origin. However, latitudes and longitudes in the ITRF94/GRS-80 system are very close to those of the NAD 83 system (with only 1-2 meters of horizontal shift.) So either type of latitude and longitude (NAD 83 or ITRF94) may be input, without affecting the interpolated geoid value. This does *not* imply that the geoid heights are heights above a different ellipsoid. Using NAD 83 latitudes and longitudes interchangably with ITRF94/GRS-80 latitudes and longitudes is merely an acceptable horizontal approximation. Each geoid model will always contain geoid heights above the specific ellipsoid, stated above. Do *NOT* use NAD 27 latitudes and longitudes. The horizontal shifts between NAD 83 and NAD 27 can exceed 100 meters, causing a noticable difference in the interpolated geoid value. To convert from NAD 27 to NAD 83 latitudes and longitudes you may use programs NADCON or CORPSCON, available from NGS. The AREA.PAR File ----------------- AREA.PAR is a plain, ASCII text file. It specifies the order in which .GEO files are to be used. If you have a favorite .GEO file, put it at the top of the AREA.PAR list. There is no problem in having overlapping .GEO files, nor is there any problem in having nested .GEO files. The AREA.PAR file specifies which geoid files are available and their priority of use. PLEASE NOTE: The AREA.PAR file we distribute contains the names of all the GEOID96 grid files. You may not have received them all; you may not want them all. This is not a problem. If a .GEO file name is in the AREA.PAR file, but not in the local directory, then a warning message is issued, and program GEOID proceeds with the files that are available. You must have an entry in AREA.PAR for each .GEO file to be searched. An Example: You just wish to work with the GEOID96 - Northwest file. So, load AREA.PAR into your favorite line editor, and delete the lines referring to the other geoid regions. You may now delete those .GEO files without receiving the warning messages on the opening screen of program GEOID. Save the updated AREA.PAR as plain ASCII text. Data Input ---------- You can key data by hand, point by point, or you can create an input file using a text editor. Several file formats are provided, including the NGS "Blue Book" format. These formats are detailed in a "Help" menu option which appears if you specify an input file name. That file doesn't need to exist if you are only going to look at the supported formats in the "Help". Data Output ----------- Results are collected into an output file. The default name of these files is GEOID.OUT, but you can use any legal file name you choose. (A word of advice: Don't use misleading extensions such as .EXE, .GEO, .BAT, etc.) The format of the output file is linked to the format of the input file to maintain consistency. The GEOID96 Model ----------------- The GEOID96 model was computed on October 1, 1996 using over 1.8 million terrestrial and marine gravity values. The method of computation uses a Fast Fourier Transform (FFT) technique to compute the detailed geoid structure, which is then combined with an underlying EGM96 geopotential model. The result is a gravimetric geoid height grid with a 2' X 2' spacing in latitude and longitude (2' x 4' in Alaska), referred to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid in an International Terrestrial Reference System 1994 (ITRF94) frame. Then, by means of NAD 83 GPS ellipsoidal heights on NAVD 88 benchmark data, plus known relationships between NAD 83 and the ITRF94 reference frames, a conversion is applied to generate the final GEOID96 geoid model. This conversion causes the GEOID96 model to be biased relative to a geocentric ellipsoid; but, this bias is deliberate. The GEOID96 model was developed to support direct conversion between NAD 83 GPS ellipsoidal heights a nd NAVD 88 orthometric heights. When comparing the GEOID96 model with GPS ellipsoidal heights in the NAD 83 reference frame and leveling in the NAVD 88 datum, it is seen that GEOID96 has roughly a 3-cm accuracy (one sigma) in the regions of GPS benchmark coverage. In those states with sparse (150km+) GPS benchmark coverage, less point accuracy may be evident; but relative accuracy at about a 1 to 2 part-per-million level, or better, should still be obtained. For users with less stringent accuracy requirements, simple height conversions with GEOID96 in the conterminous United States can be sufficient. For users with more stringent accuracy requiements, please see the section entitled "Deriving Orthometric Heights From GPS", later in this document. Users should be aware that GPS ellipsoid height error, by itself, can be significantly greater than error in geoid height differences. States with Sparse GPS Benchmark Coverage ----------------------------------------- As of the date of computation of GEOID96, the states with sparse GPS benchmark coverage are: Arkansas, Illinois, Indiana, Iowa, Kansas, Minnesota, Missouri, North Dakota, South Dakota, and West Virginia. This does not mean that the GEOID96 model can not be used in these states. It does mean that users may not see the same absolute accuracy when compared to other parts of the United States with denser GPS benchmark coverage. As stated above, relative accuracy may reach 1-2 PPM. Even so, the major components of the datum relationships between NAD 83 and NAVD 88 in all of the lower 48 states have been incorporated into the GEOID96 geoid model. As a rule, one can expect better results with GEOID96, relative to GEOID93, in any part of the United States. Alaska, Hawaii, Puerto Rico and the Virgin Islands -------------------------------------------------- It must be emphasized that the GEOID96 models in Alaska, Hawaii, Puerto Rico, and the Virgin Islands were NOT, repeat, NOT computed by incorporating a conversion surface based on GPS benchmarks. This was due to a shortage of reliable NAD 83 GPS ellipsoidal heights on NAVD 88 benchmarks in these regions. The GEOID96 geoid models provided in these areas are relative to a geocentric, GRS80 ellipsoid; as were earlier GEOID93 and GEOID90 models. For this reason, users should refer to the section entitled "Deriving Orthometric Heights From GPS", later in this document. Due to poorer data coverage, error estimates for GEOID96 in these regions are larger. Long-wavelength errors may be as large as 4-5 parts-per-million in some areas. Particular care must be used in computing heights in the tectonically active areas in southern Alaska. Crustal motion may exceed 1 meter even after accounting for the shift of the 1964 Prince William Sound Earthquake. The National Imagery and Mapping Agency --------------------------------------- The National Imagery and Mapping Agency (NIMA), which incorporates the former Defense Mapping Agency (DMA), has been of immense help in this endeavor. NIMA has provided a major portion of the NGS land gravity data set. NIMA has also been instrumental in the creation of the various 30" and 3" elevation grids in existence. And, NIMA was a partner in the joint project to compute the new global geopotential model, EGM96. Although the work of the NIMA generally precludes public recognition, their cooperation is gratefully acknowledged. GSFC/NIMA Geopotential Model, EGM96 ----------------------------------- The Goddard Space Flight Center (GSFC) and 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 marine gravity data covering the globe, including the former Soviet Union. EGM96 is a global geopotential model expressed as spherical harmonic coefficients complete to degree and order 360. Therefore, the shortest wavelength this model can exhibit is one degree, and its resolution is one-half degree (about 50 km). Although this model does not reproduce geoid structure at very fine resolution, it is global. We thank the many members of the project team for making this model available. Deriving Orthometric Heights From GPS ------------------------------------- One key problem is deciding which orthometric height datum to use. NGVD 29 is not a sea-level datum, and the heights are not true orthometric heights. The datum of NAVD 88 is selected to maintain reasonable conformance with existing height datums, and its Helmert heights are good approximations of true orthometric heights. And, while differential ellipsoidal heights obtained from GPS are precise, they are often expressed in the NAD 83 datum, which is not exactly geocentric. In addition, GEOID96 rests upon an underlying EGM96 global geopotential model, and EGM96 does possess some error of commission. This leads to a warning: Do not expect the difference of a GPS ellipsoidal height at a point and the associated GEOID96 height to exactly match the vertical datum you need. The results will be close when converting NAD 83 GPS ellipsoidal heights into NAVD 88 elevations; but, maybe not accurate enough for your requirement. However, one can combine the precision of differential carrier phase GPS with the precision of GEOID96 height differences, to approach that of leveling. Include at least one existing benchmark in your GPS survey (preferably many benchmarks). The difference between the published elevation(s) and the height obtained from differencing your adopted GPS ellipsoidal height and the GEOID96 model, could be considered a "local orthometric height datum correction." If you are surveying an extensive area (100+ km), and you occupy a lot of benchmarks, then you might detect a trend in the corrections up to a one part-per-million level. This may be error in the GEOID96 model. We do not currently consider geoid-corrected GPS orthometric heights as a substitute for geodetic leveling in meeting the Federal Geodetic Control Subcommittee(FGCS) standards for vertical control networks. Studies are underway, and many less stringent requirements can be satisfied by geoid modeling. Widespread success has been achieved with the preceeding models, GEOID93 and GEOID90. The GEOGRD Utility Program -------------------------- GEOGRD -- This converts to and from ".GEO" binary files and ASCII text files. It can also be used to extract subgrids in the process of conversion. For example: one can make a .GEO grid for the state of Colorado by using GEO96NW.GEO, "converting" from binary, .GEO into binary, .GEO, and specifying the Colorado state boundaries. A Technical Note on Program GEOID --------------------------------- Some users prefer to write their own interpolation software. If you do, please be aware that there is a loss of precision in the grid file headers for grid spacings of 2' (or 4'). This is accomodated in program GEOID 3.10 by internally re-computing the grid spacing in subroutine GRIDS. You might need to place similar code in your interpolation software, depending upon how it was written. ------------------ (Example Fortran 77 code) ----------------------------- *** patch for inexact headers (due to 2' spacing) idx1=idnint(DX1*3600.d0) DX(NAREA) = dble(idx1)/3600.d0 idy1=idnint(DY1*3600.d0) DY(NAREA) = dble(idy1)/3600.d0 ***** DX(NAREA) = DX1 **** old code ***** DY(NAREA) = DY1 **** old code ---------------------------------------------------------------------------- Future Plans ------------ A research effort is underway to improve geoid height estimates in the future, perhaps at the 1-cm accuracy level. One important direction is integrating gravity data with GPS and geodetic leveling measurements, and the study of error in GPS ellipsoid heights and in the NAVD88 vertical datum. It is likely that this research, in conjunction with the completion of the state upgrade GPS surveys, will yield a significant improvement to our geoid model in 1999. For More Information -------------------- For Products Available From the National Geodetic Survey: Information Services Branch National Geodetic Survey, NOAA, N/NGS12 301-713-3242 fax: 301-713-4172 For Information on GEOID96 and Future Research: Dr. Dennis G. Milbert National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: dennis@ngs.noaa.gov Dr. Dru A. Smith National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: dru@ngs.noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/geoid.html Best Wishes! README file 9-oct-96 dgm/das