GEOID18 README FILE ------------------------ Version history: Original: September 10, 2019 The GEOID18 MODEL ----------------- These files, downloaded from geodesy.noaa.gov/GEOID/GEOID18 are a complete set of programs and grids for determining hybrid geoid heights, enabling conversions between NAD83 ellipsoid heights and NAVD88, PRVD02, and VIVD09 orthometric heights. 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) Grid Name Model Region Covered (latitude & both East/West longitude) g2018u0.bin* GEOID18 all grids 1-8 for CONUS (24-58N, 230-300E=130-60W) g2018u1.bin GEOID18 grid #1 for CONUS (40-58N, 230-249E=130-111W) g2018u2.bin GEOID18 grid #2 for CONUS (40-58N, 247-266E=113-094W) g2018u3.bin GEOID18 grid #3 for CONUS (40-58N, 264-283E=096-077W) g2018u4.bin GEOID18 grid #4 for CONUS (40-58N, 281-300E=079-060W) g2018u5.bin GEOID18 grid #5 for CONUS (24-42N, 230-249E=130-111W) g2018u6.bin GEOID18 grid #6 for CONUS (24-42N, 247-266E=113-094W) g2018u7.bin GEOID18 grid #7 for CONUS (24-42N, 264-283E=096-077W) g2018u8.bin GEOID18 grid #8 for CONUS (24-42N, 281-300E=079-060W) g2018p0.bin GEOID18 grid for PR/USVI (13-21N, 291-296E=069-064W) Please be sure you have downloaded these programs from the NGS GEOID page: https://www.ngs.noaa.gov/GEOID, as these are the most recent versions to read the GEOID18 model; previous versions of INTG and XNTG created for previous geoid models may not work properly. * If you downloaded ASCII versions of these files, the suffix will be ".asc," rather than ".bin" To Install: Make a subdirectory on your computer hard drive. Copy the various geoid files into that subdirectory. You do not need to put the geoid files in the same directory as the INTG and XNTG programs, although you will need to specify the path. If you have any other geoid model files, you may safely place them in the same directory as GEOID18 should you wish to do so. To Execute: (PC or Sun) Open a command window or powershell, type INTG, and follow the prompts. This will ensure it doesn't close immediately after completion, and you will be able to examine the output statistics. To Terminate: You can stop the program at any time using C. Check The Byte Counts of all Downloaded Files --------------------------------------------- Before beginning, it will be useful to ensure 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 binary Data: g2018u0.bin 34,297,008 bytes g2018u*.bin 4,933,728 bytes g2018p0.bin 434,688 bytes ASCII Data (uncompressed): g2018u0.asc 86,816,078 bytes g2018u*.asc 12,488,895 bytes g2018p0.asc 1,100,430 bytes C Program Source Code: INTG_V317.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 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 you would like to use an input file. Data Output: ----------- Results may be collected into an output file. There is no default output file name. To maintain consistency, the format of the output file is linked to the format of the input file. 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". File Structure: --------------- The files (ASCII and binary) follow the same structure of a one-line header followed by the data in row-major format. The one-line header contains four 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 N-S direction) DLON: Distance interval in longitude in whole degrees (point spacing in W-E 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) The data follows after this one-line header. 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 DLON 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. 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 prompt you for the geoid model grids you wish to use. You can operate with as few as one .bin file, or you may use the entire geoid model data set. The all CONUS grids file (g2018u0.bin) should be used instead of the individual subgrids when possible. 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 of the .bin file names on the opening screen indicates the order in which the .bin files are searched. This particularly applies to where the CONUS grids overlap. When running program INTG.EXE (PC) or INTG (Sun), you must input the latitude and longitude of each point. The GEOID18 models are heights above the NAD83 ellipsoid. While the latitudes and longitudes in the IGS08/GRS-80 and WGS84 systems are very close to those of the NAD83 system (with only 1 to 2 meters of horizontal shift), NAD83(2011) coordinates should exclusively be used. If necessary, use NCAT (available in the NGS Tool Kit) to convert into NAD83(2011) GEOID18 geoid heights will always reference NAD83, not IGS08/GRS-80. xGEOID19 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 NGS’s NCAT or NADCON program, available from the NGS Tool Kit. For more information on the GEOID18 model, refer to the Technical Details Page (ADD LINK) 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 https://geodesy.noaa.gov/INFO/NGSinfo.shtml or https://geodesy.noaa.gov/INFO/programs-products.shtml National Geodetic Survey, NOAA, N/NGS12 301-713-3242 fax: 301-713-4172 For Information on GEOID18, source code, and Future Research: Dr. Kevin Ahlgren National Geodetic Survey, NOAA, N/NGS4 240-533-9894 Email: kevin.ahlgren@noaa.gov Visit the NGS website: https://geodesy.noaa.gov/GEOID/GEOID18/