THE USGG2003 README FILE ------------------------ Version: January 16, 2004 drr The USGG2003 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) XNTG.EXE (PC) or Program for extracting, translating (ascii/binary) XNTG (Sun) and yielding statistics of geoid files (source code is XNTG.FOR) The following file names are valid for binary files (if, however, you downloaded the ASCII versions of these files, the suffix will be ".asc" rather than ".bin"): s2003u01.bin USGG2003 grid #1 for CONUS (40-58N, 230-249E) s2003u02.bin USGG2003 grid #2 for CONUS (40-58N, 247-266E) s2003u03.bin USGG2003 grid #3 for CONUS (40-58N, 264-283E) s2003u04.bin USGG2003 grid #4 for CONUS (40-58N, 281-300E) s2003u05.bin USGG2003 grid #5 for CONUS (24-42N, 230-249E) s2003u06.bin USGG2003 grid #6 for CONUS (24-42N, 247-266E) s2003u07.bin USGG2003 grid #7 for CONUS (24-42N, 264-283E) s2003u08.bin USGG2003 grid #8 for CONUS (24-42N, 281-300E) (note: the USGG2003 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 USGG2003 model files, you may safely place them in the same directory as GEOID03, if you like.) To Execute (PC or Sun) Type INTG , and follow the prompts. To Terminate You can stop the program at any time using C. 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: g2003u**.bin 4,933,728 bytes ASCII Data: g2003u**.asc 12,488,896 bytes (uncompressed) FORTRAN Source Code: INTG.FOR 88,980 bytes XNTG.FOR 31,158 bytes PC executables: INTG.EXE 299,008 bytes XNTG.EXE 282,624 bytes Sun executables: INTG 466,692 bytes XNTG 426,924 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 8 for USGG2003 and 14 for GEOID99. 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 GEOID03 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, ITRF00, 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 ITRF00/GRS-80 Or WGS84 latitudes and longitudes is merely an acceptable horizontal approximation. GEOID03 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 USGG2003 Model ---------------- The USGG2003 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 USGG2003 model refers to a GRS-80 shaped ellipsoid, centered at the ITRF00 origin. It does not support direct conversion between NAD 83 GPS ellipsoidal heights and NAVD 88 orthometric heights. When comparing the USGG2003 model with GPS ellipsoidal heights in the ITRF00 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 EGM96 geopotential surface. 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 USGG2003 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. 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, USGG2003 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 USGG2003 height to exactly match the vertical datum you need. However, one can combine the precision of differential carrier phase GPS with the precision of USGG2003 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 USGG2003 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 USGG2003 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 preceding models, 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 ------------ New gravimetric and hybrid geoid models will be generated in the next year (USGG2004 and GEOID04) for all U.S. regions including the conterminous United States. These models may adopt newer global gravity field models derived from the GRACE mission as well as improved terrain and gravity field information. 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 USGG2003 and Future Research: Dr. Daniel Roman National Geodetic Survey, NOAA, N/NGS6 301-713-3202 Internet: dan.roman@noaa.gov Dr. Yan Ming Wang National Geodetic Survey, NOAA, N/NGS6 301-713-3202 Internet: yan.wang@noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/USGG2003/index.html