THE GEOID03 README FILE ----------------------- Original: 2003 December 30 (drr) Update : g2003rme.txt - 1.6 - 2006/05/05 (ccyy/mm/dd) NOTE: The GEOID03 file number 7 has been updated. A Vertical Time Dependent Positioning (VTDP) model has been developed to account for subsidence in the southern Louisiana region. As a result, many of the orthometric heights at the bench marks have been altered. The update to the GEOID03 model reflects the VTDP modified heights. The region affected is limited and the changes to GEOID03 immediately taper off outside the southern Louisiana region. For more details, follow the links on the main NGS GEOID03 web page for discussions about VTDP. The GEOID03 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) 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 data (if, however, you downloaded the ASCII versions of these files, the suffix will be ".asc" rather than ".bin"): g2003u01.bin GEOID03 grid #1 for CONUS (40-58N, 230-249E) g2003u02.bin GEOID03 grid #2 for CONUS (40-58N, 247-266E) g2003u03.bin GEOID03 grid #3 for CONUS (40-58N, 264-283E) g2003u04.bin GEOID03 grid #4 for CONUS (40-58N, 281-300E) g2003u05.bin GEOID03 grid #5 for CONUS (24-42N, 230-249E) g2003u06.bin GEOID03 grid #6 for CONUS (24-42N, 247-266E) g2003u07.bin GEOID03 grid #7 for CONUS (24-42N, 264-283E) g2003u08.bin GEOID03 grid #8 for CONUS (24-42N, 281-300E) g2003a01.bin GEOID03 grid #1 for Alaska (60-72N, 172-204E) g2003a02.bin GEOID03 grid #2 for Alaska (60-72N, 202-234E) g2003a03.bin GEOID03 grid #3 for Alaska (49-61N, 172-204E) g2003a04.bin GEOID03 grid #4 for Alaska (49-61N, 202-234E) g2003h01.bin GEOID03 grid #1 for Hawaii (18-24N, 199-206E) g2003p01.bin GEOID03 grid #1 for PR/VI (15-21N, 291-296E) 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.) 3) If you are using a PC, check your AUTOEXEC.BAT and CONFIG.SYS files to insure compliance with the following notes: Note: 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: g2003u**.bin 4,933,728 bytes g2003a**.bin 5,540,208 bytes g2003h01.bin 607,968 bytes g2003p01.bin 434,688 bytes ASCII Data: g2003u**.asc 12,488,896 bytes (uncompressed) g2003a**.asc 14,024,273 bytes (uncompressed) g2003h01.asc 1,558,212 bytes (uncompressed) g2003p01.asc 1,114,182 bytes (uncompressed) PC executables: INTG.EXE 147,950 bytes XNTG.EXE 282,624 bytes DOSXMSF.EXE 393,942 bytes Sun executables: INTG 367,344 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 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 GEOID03 models are heights above the NAD 83 ellipsoid. However, latitudes and longitudes in the ITRF00/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 program NADCON, available from NGS, or CORPSCON, available from the Army Corp of Engineers: - either http://crunch.tec.army.mil/#products select CORPSCON, - or http://crunch.tec.army.mil/software/corpscon/corpscon.html 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 GEOID03 Model ----------------- The GEOID03 model is known as a hybrid geoid model, combining gravimetric information with GPS ellipsoid heights on leveled bench marks. The GEOID03 model was developed to support direct conversion between NAD 83 GPS ellipsoidal heights and NAVD 88 orthometric heights. When comparing the GEOID03 model with GPS ellipsoidal heights in the NAD 83 reference frame and leveling in the NAVD 88 datum, it is seen that GEOID03 has roughly a 2.4 cm absolute accuracy (one sigma) in the regions of GPS on Bench Mark coverage. In those states with sparse (150km+) GPS on Bench Mark coverage, less point accuracy may be evident; but relative accuracy at about a 1 to 2 part-per-million level, or better, should still be obtained. For users with less stringent accuracy requirements, simple height conversions with GEOID03 in the conterminous United States can be sufficient. For users with more stringent accuracy requiements, please see the section entitled "Deriving Orthometric Heights From GPS", later in this document. Users should be aware that GPS ellipsoid height error, by itself, can be significantly greater than error in geoid height differences. GPS on Bench Mark Coverage ----------------------------------------- As of the date of computation of GEOID03, all 48 of the Conterminous United States had re-observations of their HARN's with respect to CORS data. This has resulted in a significant improvement in both spatial coverage and data quality. GEOID03 in Alaska --------------------------------------------------    It must be emphasized that the GEOID03 model in Alaska was NOT computed by incorporating GPS on leveled bench marks. This was due to a shortage of reliable NAD 83 GPS ellipsoidal heights on NAVD 88 bench marks in this region. The GEOID03 geoid model provided in this area is an equilpotential surface refers to NAD 83. However, this surface is not tied to NAVD 88 or any tidal datums. For this reason, users should refer to the section entitled "Deriving Orthometric Heights From GPS", later in this document.      Due to poorer data coverage, error estimates for GEOID03 in this region is larger.  Long-wavelength errors may be as large as 4-5 parts-per-million in some areas.  Particular care must be used in computing heights in the tectonically active areas in southern Alaska.  Crustal motion may exceed 1 meter even after accounting for the shift of the 1964 Prince William Sound Earthquake. 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, GEOID03 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 GEOID03 height to exactly match the vertical datum you need. The results will be close when converting NAD 83 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 GEOID03 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 GEOID03 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 GEOID03 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, 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 (USGG2006 and GEOID06) 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 GEOID03 and Future Research: Dr. Daniel Roman National Geodetic Survey, NOAA, N/NGS6 301-713-3202 email: dan.roman@noaa.gov Dr. Yan Ming Wang National Geodetic Survey, NOAA, N/NGS6 301-713-3202 email: yan.wang@noaa.gov Visit our web site: http://www.ngs.noaa.gov/GEOID/GEOID03/index.html end g2003rme.txt