README file 12-jul-94 dgm/dgs The GEOID93 and ALASKA94 GEOID MODELS You have probably received one to five floppies, depending upon your request. Among the files on the floppies are GEOID.EXE the geoid interpolation program (GEOID.FOR is source code) AREA.PAR text file of the filenames of geoid height grids ALA94.GEO the ALASKA-94 grid for Alaska 51-72N, 179-128W GEOID93E.GEO the GEOID93 grid for the Eastern U.S. 24-50N, 89- 66W GEOID93C.GEO the GEOID93 grid for the Central U.S. 24-50N, 107- 84W GEOID93W.GEO the GEOID93 grid for the Western U.S. 24-50N, 125-102W HAW93.GEO the GEOID93 grid for the Principal Hawaiian Islands PRVI93.GEO the GEOID93 grid for Puerto Rico - Virgin Islands To Install 1) Make sure your original floppies are write protected! 2) Make a subdirectory on your hard disk (GEOID will also run on floppies) 3) Go into the subdirectory . cd geoid93 (for example) 4) Copy the first floppy disk into your subdirectory. copy A:*.* *.* /v 5) Repeat step 4) as required for your other floppies. 6) Put the original floppies in a safe place! 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 any file in your local directory with a .GEO extension is a geoid height file. You can operate with as few as one .GEO file, or as many as eight. 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 indicate the order in which the .GEO files are searched. 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 their priority of use. PLEASE NOTE: The AREA.PAR on your floppies contains the names of the six GEOID93 grid files. You may not have gotten all six; you may not want all six. 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. And, if a .GEO file is in your local directory, but is not listed in your AREA.PAR file, then the .GEO file will still be used. As mentioned above, all .GEO files in your local directory will be used. The AREA.PAR file allows you to specify the priority of use. An Example: You just wish to work with the GEOID93 - West file. So, load AREA.PAR into your favorite line editor, and delete the lines referring to GEOID93C, GEOID93E, HAW93, ALA94, and PRVI93. 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. 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 GEOID93 Model The GEOID93 model was computed on January 26, 1993 using over 1.8 million terrestrial and ship gravity values. The method of computation uses a Fast Fourier Transform (FFT) technique to compute the detailed geoid structure, which is combined with an underlying OSU91A geopotential model. The result is a geoid height grid with a 3' X 3' spacing in latitude and longitude, referred to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid. By comparing the GEOID93 model with combined GPS and leveling, the GEOID93 has roughly a 10-cm accuracy (one sigma) over length scales of 100 km. Better accuracy is seen over shorter lengths. At transcontinental spacings the accuracy of GEOID93 will be governed by the accuracy of the underlying global geopotential model, OSU91A. In some locations of the country, long-wavelength errors in GEOID93 up to a 1 to 2 part-per-million level may occur. Because of better data quality and coverage, and better computational procedures, GEOID93 possesses better accuracy in mountains when compared to GEOID90. The ALASKA94 Model The ALASKA94 model was computed using the same basic methods as GEOID93. Because of meridian convergence the geoid height grid has a 3' by 6' spacing interval in latitude and longitude respectively. The same general trends in accuracy issues are found. Because of poorer overall data coverage compared to GEOID93 for the continental US, error estimates for Alaska are larger. Long-wavelength errors may be as large as 4-5 parts-per-million in some areas. In addition, over short distances (100km), the Alaskan geoid model approximately has a 20 cm. accuracy. Particular care must be used in establishing GPS-derived orthometric heights using ALASKA94 in the tectonically active areas in southern Alaska. Crustal motion may exceed 1 meter even after coseismic events such as the 1964 Prince William Sound Earthquake are modeled and removed. Improvement in modeling the causative effects of crustal motion will change this situation in the next couple of years. The Defense Mapping Agency The Defense Mapping Agency (DMA) has been of immense help in this endeavor. DMA has provided a major portion of the NGS land gravity data set. DMA has also been instrumental in the creation of the various 30" elevation grids in existence. Although the work of the DMA generally precludes public recognition, their cooperation in this work is gratefully acknowledged. The OSU91A Model The OSU91A model was computed by Prof. Richard H. Rapp, Yan Ming Wang, and Nikolaos K. Pavlis, at the Department of Geodetic Science and Surveying, The Ohio State University, Columbus, Ohio. This is a global geopotential model using spherical harmonics 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. Its accuracy is estimated at around 60-cm (one sigma). A Useful 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 GEOID93W.GEO, "converting" from binary, .GEO into binary, .GEO, and specifying the Colorado state boundaries. Deriving Orthometric Heights From GPS One key problem is deciding which orthometric height datum to use. NGVD29 is not a sea-level datum, and the heights are not true orthometric heights. The datum of NGVD88 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, an ellipsoidal height at a single point does not display the same precision. In addition, GEOID93 rests upon an underlying OSU91A global geopotential model, and OSU91A 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 GEOID93 height to exactly match the vertical datum you need. But, one can exploit the precision of differential carrier phase GPS with the precision of GEOID93 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 GEOID93 model, could be considered a "local orthometric height datum correction". If your are surveying an extensive area (100+ km), and you occupy a lot of benchmarks, then you might detect a trend in the corrections at a one part-per-million level. This may be error in the GEOID93 model. We do not currently consider geoid-corrected GPS heights as a substitute for geodetic leveling in meeting the FGCS standards for vertical control networks. Work is underway, and many less stringent requirements can be satisfied by geoid modeling. Widespread success has been achieved with the preceeding model, GEOID90. 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 measurments. It is likely that this research, in conjunction with greater availability of high precision GPS surveys, will yield a significant upgrade to our geoid model in 1996. For More Information For Products Available From the National Geodetic Survey: National Geodetic Information Center 301-713-3242 For Information on GEOID93 and Future Research: Dr. Dennis G. Milbert 301-713-3202 Internet: dennis@hecate.ngs.noaa.gov Donald G. Schultz 301-713-3202 Internet: donald@charybdis.ngs.noaa.gov Best Wishes! README file 12-jul-94 dgm/dgs