THE G99BM README FILE --------------------- Version: October 30, 2000 drr/das The G99BM GEOID MODELS ------------------------ You have received these models on CD-ROM, or downloaded them from the National Geodetic Survey (NGS) web site or the NGS FTP site. Files you may have received include: INTG.EXE (PC) or The geoid interpolation program (source code is INTG (Sun) INTG.FOR (PC) or INTG.F (Sun)) XNTG.EXE (PC) or Program for extracting, translating (ascii/binary) XNTG (Sun) and yielding statistics of geoid files (source code is XNTG.FOR (PC) or INTG.F (Sun)) DOSXMSF.EXE (PC) 32-bit DOS extender (needed for running INTG.EXE on a 386 PC) The following file names are valid for PC or Sun (if, however, you downloaded the ASCII versions of these files, the suffix will be ".asc" rather than ".bin"): g99bmu01.bin G99BM grid #1 for CONUS (40-58N, 230-249E) g99bmu02.bin G99BM grid #2 for CONUS (40-58N, 247-266E) g99bmu03.bin G99BM grid #3 for CONUS (40-58N, 264-283E) g99bmu04.bin G99BM grid #4 for CONUS (40-58N, 281-300E) g99bmu05.bin G99BM grid #5 for CONUS (24-42N, 230-249E) g99bmu06.bin G99BM grid #6 for CONUS (24-42N, 247-266E) g99bmu07.bin G99BM grid #7 for CONUS (24-42N, 264-283E) g99bmu08.bin G99BM grid #8 for CONUS (24-42N, 281-300E) To Install: 1) Make a subdirectory on your hard disk. 2) Copy the various geoid files into that subdirectory. You need not put the geoid files in the same directory as the programs. (If you have also received G99SSS and GEOID99 model files, you may safely place them in the same directory as G99BM, if you like.) 3) If you are using a PC, check your AUTOEXEC.BAT and CONFIG.SYS files to insure compliance with the following notes: Note 1: If you are running a 16-bit PC (such as a 386), then DOSXMSF.EXE must either be present in the same directory as INTG.EXE, or, it must be in a directory in your DOS PATH environment variable (such as: c:\dos). 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 (PC or Sun) Type INTG , 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. Check The Byte Counts of all Downloaded Files --------------------------------------------- Before beginning, it will be useful to ensure that all files you have received are the correct size. (Download problems are often manifested by incorrect byte counts in the files). Check with the list below to make sure your files match these numbers exactly. PC or Sun Data: g99bmu**.bin 4,933,728 bytes ASCII Data: g99bmu**.asc 12,488,896 bytes (uncompressed) PC executables: INTG.EXE 169,472 bytes XNTG.EXE 165,888 bytes DOSXMSF.EXE 393,942 bytes Sun executables: INTG 122,388 bytes XNTG 67,840 bytes How Program INTG Works ----------------------- The various geoid height grids are stored in the ".bin" files. Program INTG will prompt you for the name of the directory where you have chosen to store the .bin files, as well as prompting you for which geoid model you wish to use. You can operate with as few as one .bin file, or as many as 14. When the program interpolates a given point, it checks an internal list of .bin boundaries, and uses the earliest list entry whose boundaries contain that point (EXCEPTION: In the northwest USA, there is a small overlap between the CONUS g1999u01.bin file and the Alaska g1999a04.bin file In this case, if the CONUS file is available, it will take precedence over the Alaska file). The order in which the .bin file names appear on the opening screen indicates the order in which the .bin files are searched. When running program INTG.EXE (PC) or INTG (Sun), the latitude and longitude of each point must be input. The G99BM models are heights, latitudes and longitudes are in the ITRF96 (1997.0)/GRS-80 reference system. These are very close to the WGS84(G873) system and within 1-2 meters horizontally of the NAD 83 system. So any of these types of latitude and longitude (NAD 83, ITRF96, WGS84) may be input, without affecting the interpolated geoid value. This does *not* imply that the geoid heights are heights above a different ellipsoid. Using ITRF96/GRS-80 latitudes and longitudes interchangeably with NAD 83 or WGS84 latitudes and longitudes is merely an acceptable horizontal approximation. G99BM geoid heights, will always be above the ITRF96/GRS-80 ellipsoid. Do *NOT* use NAD 27 latitudes and longitudes. The horizontal shifts between NAD 83 and NAD 27 can exceed 100 meters, causing a noticeable 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. 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 that you wish to use an input file. Data Output ----------- Results may be collected into an output file. There is no default output file name. The format of the output file is linked to the format of the input file to maintain consistency. If, however, you input your data by keyboard, and ask for an output file, the format of that output file will be in the format known as "Free Format, Type 1". The G99BM Model ----------------- This one-arcminute model is based on the G99SSS but also reflects the information contained in the GPSBM data, much as the GEOID99 model does. However, this model references the ITFR96(1997.0) coordinate frame rather than NAD83, hence it can be readily compared to other data sets & models that also reference the ITRF96 (1997.0) coordinate frame. As with GEOID99, caution should be used whenever a comparison is made with data outside of the conterminous United States (CONUS). Since the extents of the GPSBM data set only reach the CONUS boundary, regions outside of CONUS are extrapolated and are increasingly invalid further outside of this boundary. In the conterminous United States, geoid heights range from a low of -52.2 meters in the Atlantic (magenta) to a high of 3.0 meters (red) in the Quebec, Canada region with an average value of -29.9 meters, an RMS difference of 31.6 meters, and a standard deviation of 10.3 meters. GPS on Bench Mark Coverage ----------------------------------------- As of the date of computation of GEOID99, all 48 of the Conterminous United States had a complete High Accuracy Reference Network (HARN). Additionally, a few states had already had re-observations of their HARNs completed. 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 and must be converted into the ITRF96 (1997.0). Also, note that it may be necessary to convert from ITRF96 (1997.0) used here to more recent versions as desired. In addition, G99BM 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 G99BM height to exactly match the vertical datum you need. The results will be close when converting ITRF96 (1997.0) 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 GEOID99 height differences, to approach that of leveling. Include at least one existing bench mark in your GPS survey (preferably many bench marks). The difference between the published elevation(s) and the height obtained from differencing your adopted GPS ellipsoidal height and the G99BM 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 bench marks, then you might detect a trend in the corrections up to a one part-per-million level. This may be error in the G99BM 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 models based on the NAD 83 datum: GEOID99, GEOID96, GEOID93 and GEOID90. The XNTG Utility Program ------------------------ The XNTG program can perform various functions, none of which are required to use the INTG program. The functions of XNTG are the extraction of sub-grids from the provided geoid grids, the translation between ASCII and binary grids, and the reporting of basic statistics for geoid grids. Future Plans ------------ Due to the quickly expanding availability of GPS data on leveled bench marks in the NGS database, there is some incentive to providing annual geoid models which reflect the latest GPS/BM data. In addition, from the gravimetric geoid side, there is always ongoing research to improve the theory of geoid determination. This research moves at a slower pace than GPS/BM data acuqisition, so while annual *hybrid* geoid models (like GEOID99) may be provided, there may be bi- or tri-annual *gravimetric* geoid models (like G99SSS). 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 GEOID99 and Future Research: Dr. Dru A. Smith National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: dru@ngs.noaa.gov Dr. Daniel R. Roman National Geodetic Survey, NOAA, N/NGS5 301-713-3202 Internet: droman@ngs.noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/GEOID99/geoid99.html