README file 21-dec-90 dgm 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 GEOID90E.GEO the GEOID90 grid for the Eastern U.S. 24-50N, 90- 66W GEOID90C.GEO the GEOID90 grid for the Central U.S. 24-50N, 107- 83W GEOID90W.GEO the GEOID90 grid for the Western U.S. 24-50N, 125-101W CONUS.GEO the OSU89B grid for the conterminous U.S. ALASKA.GEO the OSU89B grid for Alaska HAWAII.GEO the OSU89B grid for Hawaii PRVI.GEO the OSU89B 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 geoid90 (for example) 4) Copy the first floppy disk into your subdirectory. copy A:*.* *.* /v 5) Repeate 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 vanilla, 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 three GEOID90 grid files. You may not have gotten all three; you may not want all three. 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 GEOID90 - West file. So, load AREA.PAR into your favorite line editor, and delete the lines referring to GEOID90C and GEOID90E. You may now delete those .GEO files without receiving the warning messages on the opening screen of program GEOID. 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 GEOID90 Model The GEOID90 model was computed on December 19, 1990 using nearly 1.5 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 OSU89B 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 GEOID90 model with combined GPS and leveling, the GEOID90 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 GEOID90 will be governed by the accuracy of the underlying global geopotential model, OSU89B. In some locations of the country, long-wavelength errors in GEOID90 up to a one part-per-million level may occur. The OSU89B Model The OSU89B model was computed by Prof. Richard H. Rapp 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 60-cm (one sigma). Other Programs Depending on which disks you requested, you may have received some other programs which are nice to have. 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 GEOID90W.GEO, "converting" from binary, .GEO into binary, .GEO, and specifying the Colorado state boundaries. REDO -- Converts from binary, .GEO format into an ASCII format suitable for a commercial contouring program. GRDCON -- Converts from an alternate ASCII format back to .GEO format. Works in conjunction with REDO. 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. Add to this the fact that GEOID90 rests upon an underlying OSU89B global geopotential model, and that OSU89B 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 GEOID90 height to match the vertical datum you require. But, one can exploit the precision of differential GPS. 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 GEOID90 model, could be considered a "local orthometric height datum correction". If your are surveying an extensive area (100+ km), then you might detect a trend in the corrections at a one part-per-million level. This may be due to error in the GEOID90 model. We do not currently consider geoid-corrected GPS heights as a substitute for geodetic leveling in meeting the FGCC standards for Vertical Control Networks. Work is underway, and many less stringent requirements can be satisfied by geoid modeling. Future Plans A research effort is underway to improve geoid height estimates in the future, perhaps at the 4-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 the next five years. For More Information For Products Available From the National Geodetic Survey: National Geodetic Information Center N/CG174, Rockwall Bldg, Room 26 National Geodetic Survey, NOAA Rockville, MD 20852 301-443-8631 For Information on GEOID90 and Future Research: Dr. Dennis G. Milbert N/CG113, Rockwall Bldg, Room 513 National Geodetic Survey, NOAA Rockville, MD 20852 301-443-8528 Internet: dennis@hecate.ngs.noaa.gov Best Wishes! README file 21-dec-90 dgm