READ_MEX.TXT file 7-may-1997 das/dgm The MEXICO97 GEOID MODELS ------------------------- The MEXICO97 model is a high resolution geoid height model covering the region 14-33N, 119-86W. It has been prepared in cooperation with the Instituto Nacional de Estadistica, Geografia e Informatica (INEGI). The MEXICO97 geoid model has been designed specifically for Mexico. Due to data coverage and computational issues, one will find offsets between MEXICO97 and either GEOID96 or G96SSS in regions of overlap. 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 -- Written specifically for distribution with MEXICO97, but equally compatable with previous geoid models) DOSXMSF.EXE 32-bit DOS extender (needed for GEOID.EXE) AREA.PAR text file of the filenames of geoid height grids MEX97W.GEO the MEXICO97 grid for Western Mexico 14-33N, 119-106W MEX97C.GEO the MEXICO97 grid for Central Mexico 14-33N, 109- 96W MEX97E.GEO the MEXICO97 grid for Eastern Mexico 14-33N, 99- 86W 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:\mexico97). 2) Copy the various geoid files into that subdirectory. copy *.* c:\mexico97 /v (for example) 3) Repeat step 2) as required for your other sets of geoid 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. Note 3: Do NOT put MEXICO97 files in the same directory as other geoid models (such as G96SSS or GEOID96). This will cause an error in program GEOID.EXE. 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 15. When the program interpolates a given point, it checks the AREA.PAR file for .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 their priority of use. PLEASE NOTE: The AREA.PAR file we distribute with the MEXICO97 model contains the names of all the MEXICO97 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 MEXICO97 - West 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 MEXICO97 Model ----------------- The MEXICO97 model was computed on March 28, 1997 using over one 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, referred to the Geodetic Reference System 1980 (GRS 80) normal ellipsoid in an International Terrestrial Reference System 1994 (ITRF94(1996.0)) frame. The EGM96 model was evaluated to conform to a "tide-free" system, insofar as the Earth's permanent tide effect is concerned. When comparing the MEXICO97 model with GPS ellipsoidal heights in the ITRF94 reference frame and leveling in the NAVD 88 datum, one may discern a systematic offset at a -25 cm level. This offset is a combination of an offset inherent in the definition of the NAVD88 datum, and an offset associated with the treatment of offshore gravity data in the MEXICO97 model. In addition to the -25 cm offset, long-wavelength systematic errors are evident in the comparisons. These errors are a composite of error in the NAVD 88 elevations, error in the GPS ellipsoidal heights, and error in the MEXICO97 model itself. Since the errors are long-wavelength, they can be modeled locally as a plane; perhaps at a 5 to 6 part-per-million level. One may expect to see larger tilts in the MEXICO97 model south of 20 degrees North latitude, due to a lack of adequate gravity coverage. To further exemplify this point, an ASCII map of the data coverage in MEXICO97 is included below. Data gaps show up as a blank (a 30' cell with less than 13 gravity measurement points). This is an ASCII map of file: mex97.ns8.released.b It is bounded in latitude by: 14.0 to 33.0 N(degrees) and in longitude by: 241.0 to 274.0 E(degrees) Cell sizes mapped are 30.0 minutes in lat and 30.0 minutes in lon For a cell to have an X, instead of a space, it must contain a minimum of 13.0 points and a maximum of 999999.0 points. There are a total of 38 cells in latitude and 66 cells in longitude. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXX XXXXXXX XX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXX XXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXX XX XXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXX X XXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX X XXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XX XX X XXXXXX XXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XX XXXXXXXXXXX XXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXX XXXXX XX XXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXX XXXX X XXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX X XXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XX XXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX (As an example on reading the above map, the cell which is bounded on the north by 17.0 degrees, on the south by 16.5 degrees, on the west by 269.5 and on the east by 270.0 degrees has an "X", but all 8 surrounding cells do not. This cell is in central Guatamala.) The Instituto Nacional de Estadistica, Geografia e Informatica (INEGI) ---------------------------------------------------------------------- Mexico's Instituto Nacional de Estadistica, Geografia e Informatica (INEGI) was very helpful in providing the observed Mexican gravity data used in the development of this geoid. Most of this information is also contained in the global gravity data base managed by NIMA (see later). INEGI also assisted in various aspects related to quality control when observed gravity values had anomalous behavior relative to surrounding points. It is expected that INEGI's participation in these areas will continue if future revisions of MEXICO97 and DMEX97 models are prepared. 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. In addition, MEXICO97 contains local and regional vertical datum offsets, itself. This leads to a warning: Do not expect the difference of a GPS ellipsoidal height at a point and the associated MEXICO97 height to exactly match the vertical datum you need. However, one can combine the precision of differential carrier phase GPS with the relative precision of MEXICO97 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 MEXICO97 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 perhaps a six part-per-million level. This may be error in the MEXICO97 model. The United States does not currently consider geoid-corrected GPS orthometric heights as a substitute for geodetic leveling in meeting our 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, GEOID96, 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 Quintana Roo by using MEX97E.GEO, "converting" from binary (.GEO) into binary (.GEO) and specifying an area that surrounds Quintana Roo. Remember two things: 1) If you do create new .GEO grids, you must update your AREA.PAR file. And, 2) a total of only 15 geoid grid files can be open at once with the PC version of GEOID. If you construct many different grid pairs, consider creating different sub- directories with different combinations of grid pairs in them. 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'. This is accomodated in program GEOID 3.00 (and later versions, including the 3.10 version supplied with MEXICO97) 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. Acknowledgements ---------------- We would like to thank all of the agencies that have contributed to the NGS databases. We thank the United Nations Environment Programme (UNEP) for their GTOPO30 global 30 arcsecond digital terrain data, which was used south of latitude 24N. We also thank the National Geophysical Data Center (NGDC) for their TOPO30 digital terrain data (TOPO30) which was used north of latitude 24N. 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 MEXICO97 and Future Research: Dr. Dru A. Smith National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: dru@ngs.noaa.gov Dr. Dennis G. Milbert National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: dennis@ngs.noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/geoid.html Best Wishes! README file 8-apr-1997 das/dgm