THE G99SSS README FILE ---------------------- Version: January 12, 2000 das/drr The G99SSS GEOID MODEL ---------------------- 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"): s1999u01.bin G99SSS grid #1 for CONUS (40-58N, 230-249E) s1999u02.bin G99SSS grid #2 for CONUS (40-58N, 247-266E) s1999u03.bin G99SSS grid #3 for CONUS (40-58N, 264-283E) s1999u04.bin G99SSS grid #4 for CONUS (40-58N, 281-300E) s1999u05.bin G99SSS grid #5 for CONUS (24-42N, 230-249E) s1999u06.bin G99SSS grid #6 for CONUS (24-42N, 247-266E) s1999u07.bin G99SSS grid #7 for CONUS (24-42N, 264-283E) s1999u08.bin G99SSS grid #8 for CONUS (24-42N, 281-300E) (note: the G99SSS model only exists for the conterminous United States) 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 GEOID99 model files, you may safely place them in the same directory as G99SSS, 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. These values are good for the PC and Sun versions of the data. PC or Sun Data: s1999u**.bin 4,933,728 bytes ASCII Data: s1999u**.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. 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 GEOID99 models are heights above the NAD 83 ellipsoid. However, latitudes and longitudes in the ITRF97/GRS-80 and WGS84(G873) systems are very close to those of the NAD 83 system (with only 1-2 meters of horizontal shift.) So any of these types of latitude and longitude (NAD 83, ITRF97, 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 NAD 83 latitudes and longitudes interchangeably with ITRF97/GRS-80 or WGS84 latitudes and longitudes is merely an acceptable horizontal approximation. GEOID99 geoid heights, will always be above the NAD83 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 G99SSS Model ---------------- The G99SSS model is known as a gravimetric geoid model, as it makes use purely of gravimtric information, and does not rely on GPS ellipsoid heights on leveled bench marks. The G99SSS model refers to a GRS-80 shaped ellipsoid, centered at the ITRF97 origin. It does not support direct conversion between NAD 83 GPS ellipsoidal heights and NAVD 88 orthometric heights. When comparing the G99SSS model with GPS ellipsoidal heights in the ITRF97 reference frame and leveling in the NAVD 88 datum, one can discern a systematic offset at a 50 cm level. It is likely that this offset is inherent in the definition of the NAVD 88 datum; where the NAVD 88 zero reference is below the current estimate of global mean sea level. In addition, 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 G99SSS model itself. Since the errors are long-wavelength, they can be modeled locally as a plane; usually at a 1 to 2 part-per-million level. Alaska, Hawaii, Puerto Rico and the Virgin Islands -------------------------------------------------- The difference between G99SSS and GEOID99 is primarily a difference of reference frames. G99SSS is a gravimetric geoid relative to a geocentric GRS80 ellipsoid. GEOID99, in the conterminous United States, is computed through a conversion using knowledge of reference frame differences and GPS ellipsoidal heights on leveled bench marks. In Alaska, Hawaii, Puerto Rico and the Virgin Islands, only the gravimetric, geocentric, geoid models are available. These models are designated as GEOID99 to facilitate ordering by our users, though they have no GPS/Bench Mark data in their production. 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, G99SSS 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 G99SSS height to exactly match the vertical datum you need. However, one can combine the precision of differential carrier phase GPS with the precision of G99SSS 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 G99SSS 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 G99SSS 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, 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 (hybrid) 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 G99SSS 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/G99SSS/g99sss.html