THE GEOID12 README FILE ----------------------- Original: July 16, 2012 (drr) The GEOID12 MODEL ----------------- You have downloaded these files from the National Geodetic Survey (NGS) web site. Files you may have received include: INTG.EXE (PC) or The geoid interpolation program INTG (Solaris) (source code is available) XNTG.EXE (PC) or Program for extracting, translating (ASCII/binary) XNTG (Solaris) and yielding statistics of geoid files (source code is available) Ensure that you have downloaded these programs from this page as these are the most recent versions that will definitely read the GEOID12 and USGG2012 models. Previous versions of INTG and XNTG may not. The following file names are valid for binary files. However, if you downloaded the ASCII versions of these files, the suffix will be ".asc" rather than ".bin"): Grid Name Model Region Covered (latitude & both East/West longitude) g2012u00.bin GEOID12 entire grid for CONUS (24-58N, 230-300E/130-60W) g2012u01.bin GEOID12 grid #1 for CONUS (40-58N, 230-249E/130-111W) g2012u02.bin GEOID12 grid #2 for CONUS (40-58N, 247-266E/113-94W) g2012u03.bin GEOID12 grid #3 for CONUS (40-58N, 264-283E/96-77W) g2012u04.bin GEOID12 grid #4 for CONUS (40-58N, 281-300E/79-60W) g2012u05.bin GEOID12 grid #5 for CONUS (24-42N, 230-249E/130-111W) g2012u06.bin GEOID12 grid #6 for CONUS (24-42N, 247-266E/113-94W) g2012u07.bin GEOID12 grid #7 for CONUS (24-42N, 264-283E/96-77W) g2012u08.bin GEOID12 grid #8 for CONUS (24-42N, 281-300E/79-60W) g2012a00.bin GEOID12 entire grid for Alaska (60-72N, 172-204E/188-126W) g2012a01.bin GEOID12 grid #1 for Alaska (60-72N, 172-204E/188-156W) g2012a02.bin GEOID12 grid #2 for Alaska (60-72N, 202-234E/158-126W) g2012a03.bin GEOID12 grid #3 for Alaska (49-61N, 172-204E/188-156W) g2012a04.bin GEOID12 grid #4 for Alaska (49-61N, 202-234E/158-126W) g2012h00.bin GEOID12 entire grid for Hawaii (18-24N, 199-206E/161-154W) g2012g00.bin GEOID12 entire grid for Guam/CNMI (11-18N, 143-146E/217-214W) g2012p00.bin GEOID12 entire grid for Puerto Rico and U.S. Virgin Islands (13-21N, 291-296E/69-64W) g2012s00.bin GEOID12 entire grid for American Samoa (17-11S, 186-192E/174-168W) 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 INTG and XNTG programs though you will need to then specify the path. If you have also received USGG2012 or any other model files, you may safely place them in the same directory as GEOID12, if you like. To Execute (PC or Sun) Type INTG, and follow the prompts in a Command Prompt shell in Windows or a Unix terminal to ensure that it doesn't close immediately after completion to permit examination of the output statistics. To Terminate You can stop the program at any time using C. File Structure --------------- The files (ASCII and binary) all follow the same structure of a one line header followed by the data in row-major format. The one line header contains 4 double (real*8) words followed by three long (int*4) words. These parameters define the geographic extent of the area: SLAT: Southernmost North latitude in whole degrees. Use a minus sign (-) to indicate South latitudes. WLON: Westernmost East longitude in whole degrees. DLAT: Distance interval in latitude in whole degrees (point spacing in E-W direction) DLON: Distance interval in longitude in whole degrees (point spacing in N-S direction) NLAT: Number of rows (starts with SLAT and moves northward DLAT to next row) NLON: Number of columns (starts with WLON and moves eastward DLON to next column) IKIND: Always equal to one (indicates data are real*4 and endian condition) After this one line header, the data follow. The first row represents the southernmost row of data with the first data point being in the SW corner. The row is NLON values wide spaced at DLAT intervals, and then increments to the next row which is DLAT to the north. This continues until the last row where the last value represents the northeast corner. The easternmost longitude is = WLON + (NLON - 1) * DLON, while the northernmost latitude is = SLAT + (NLAT - 1) * DLAT. 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: g2012u00.bin 34,297,008 bytes g2012u**.bin 4,933,728 bytes g2012a00.bin 20,554,848 bytes g2012a**.bin 5,540,208 bytes g2012h00.bin 607,968 bytes g2012g00.bin 304,848 bytes g2012p00.bin 434,688 bytes g2012s00.bin 521,328 bytes ASCII Data (uncompressed): g2012u00.asc 86,816,078 bytes g2012u**.asc 12,488,895 bytes g2012a00.asc 52,030,658 bytes g2012a**.asc 14,024,273 bytes g2012h00.asc 1,539,045 bytes g2012g00.asc 771,795 bytes g2012p00.asc 1,100,430 bytes g2012s00.asc 1,319,918 bytes C Program Source Code: INTG_V314.ZIP 106,269 bytes XNTG_V12.ZIP 28,914 bytes PC executables: INTG.EXE 122,331 bytes XNTG.EXE 127,414 bytes Sun executables: INTG 186,500 bytes XNTG 95,212 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 the entire geoid model data set. If a master file is given (e.g., g2012u00.bin for CONUS), then this will be preferentially used over the individual subgrids. When the program interpolates a given point, it checks an internal list of .bin boundaries, and uses the earliest list entry whose boundaries contain the best data scheme to interpolate 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. This applies especially to where the CONUS and Alaska grids overlap. When running program INTG.EXE (PC) or INTG (Sun), the latitude and longitude of each point must be input. The GEOID12 models are heights above the NAD 83 ellipsoid. While the latitudes and longitudes in the IGS08/GRS-80 and WGS84 systems are very close to those of the NAD 83 system (with only 1-2 meters of horizontal shift), NAD 83 (2011/PA11/MA11) coordinates should exclusively be used. If necessary, use HTDP (which is available in the NGS Tool Kit) to convert into the appropriate ellipsoidal datum (2011: for CONUS, AK, PR, VI; PA11 for HI and Am. Samoa; and MA2011 for Guam and CNMI). GEOID12 geoid heights will always reference NAD 83, not IGS08/GRS-80. USGG2012 geoid heights will always reference IGS08/GRS-80, not NAD 83. 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 the NGS Tool Kit. 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 GEOID12 Model ----------------- The GEOID12 model is known as a hybrid geoid model, as it is modified from a gravimetric model to fit GPS ellipsoid heights on leveled bench marks. The GEOID12 model refers to a GRS-80 shaped ellipsoid, centered at the NAD83 origin. It supports direct conversion between NAD83 GPS ellipsoidal heights and NAVD88, GUVD04, NMVD03, PRVD03, VIVD09, and ASVD03 orthometric heights. In Hawaii, note that there is no defined vertical datum. Hence, GEOID12 in Hawaii converts between NAD 83 (PA11) and the W0 surface used to define USGG2012 (62,636,856.00 m**2/s**2). When comparing the USGG2012 model with GPS ellipsoidal heights in the IGS08 reference frame and leveling in the NAVD 88 datum, one can discern a systematic offset at a 50 cm level and a national trend of about 1.2 m. It is likely that this offset is inherent in the definition of the NAVD 88, since this same trend is seen when comparing to global geoid height models. Since the errors are long-wavelength, they can be modeled locally as a plane; usually at a 1 to 2 part-per-million level (see GEOID12 Technical Details for further information on how GEOID12 was developed from the GPSBM2012 and OPUSDBBM12 Control Data). 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. This leads to a warning: Do not expect the difference of a GPS ellipsoidal height at a bench mark vertical datum height and the associated GEOID12 height to exactly match. The modleing process allowed for random errors that could approach 10 cm. However, one can combine the precision of differential carrier phase GPS with the precision of GEOID12 height differences, to approach that of l eveling. 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 GEOID12 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 GEOID12 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. 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. 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 GEOID12 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/GEOID12