Provided by: geographiclib-tools_1.49-4_amd64 bug

NAME

       GeoidEval -- look up geoid heights

SYNOPSIS

       GeoidEval [ -n name ] [ -d dir ] [ -l ] [ -a | -c south west north east ] [ -w ] [ -z zone
       ] [ --msltohae ] [ --haetomsl ] [ -v ] [ --comment-delimiter commentdelim ] [ --version |
       -h | --help ] [ --input-file infile | --input-string instring ] [ --line-separator linesep
       ] [ --output-file outfile ]

DESCRIPTION

       GeoidEval reads in positions on standard input and prints out the corresponding heights of
       the geoid above the WGS84 ellipsoid on standard output.

       Positions are given as latitude and longitude, UTM/UPS, or MGRS, in any of the formats
       accepted by GeoConvert(1).  (MGRS coordinates signify the center of the corresponding MGRS
       square.)  If the -z option is specified then the specified zone is prepended to each line
       of input (which must be in UTM/UPS coordinates).  This allows a file with UTM eastings and
       northings in a single zone to be used as standard input.

       More accurate results for the geoid height are provided by Gravity(1).  This utility can
       also compute the direction of gravity accurately.

       The height of the geoid above the ellipsoid, N, is sometimes called the geoid undulation.
       It can be used to convert a height above the ellipsoid, h, to the corresponding height
       above the geoid (the orthometric height, roughly the height above mean sea level), H,
       using the relations

           h = N + H,   H = -N + h.

OPTIONS

       -n name
           use geoid name instead of the default "egm96-5".  See "GEOIDS".

       -d dir
           read geoid data from dir instead of the default.  See "GEOIDS".

       -l  use bilinear interpolation instead of cubic.  See "INTERPOLATION".

       -a  cache the entire data set in memory.  See "CACHE".

       -c south west north east
           cache the data bounded by south west north east in memory.  The first two arguments
           specify the SW corner of the cache and the last two arguments specify the NE corner.
           The -w flag specifies that longitude precedes latitude for these corners, provided
           that it appears before -c.  See "CACHE".

       -w  toggle the longitude first flag (it starts off); if the flag is on, then when reading
           geographic coordinates, longitude precedes latitude (this can be overridden by a
           hemisphere designator, N, S, E, W).

       -z zone
           prefix each line of input by zone, e.g., "38n".  This should be used when the input
           consists of UTM/UPS eastings and northings.

       --msltohae
           standard input should include a final token on each line which is treated as a height
           (in meters) above the geoid and the output echoes the input line with the height
           converted to height above ellipsoid (HAE).  If -z zone is specified then the third
           token is treated as the height; this makes it possible to convert LIDAR data where
           each line consists of: easting northing height intensity.

       --haetomsl
           this is similar to --msltohae except that the height token is treated as a height (in
           meters) above the ellipsoid and the output echoes the input line with the height
           converted to height above the geoid (MSL).

       -v  print information about the geoid on standard error before processing the input.

       --comment-delimiter commentdelim
           set the comment delimiter to commentdelim (e.g., "#" or "//").  If set, the input
           lines will be scanned for this delimiter and, if found, the delimiter and the rest of
           the line will be removed prior to processing and subsequently appended to the output
           line (separated by a space).

       --version
           print version and exit.

       -h  print usage, the default geoid path and name, and exit.

       --help
           print full documentation and exit.

       --input-file infile
           read input from the file infile instead of from standard input; a file name of "-"
           stands for standard input.

       --input-string instring
           read input from the string instring instead of from standard input.  All occurrences
           of the line separator character (default is a semicolon) in instring are converted to
           newlines before the reading begins.

       --line-separator linesep
           set the line separator character to linesep.  By default this is a semicolon.

       --output-file outfile
           write output to the file outfile instead of to standard output; a file name of "-"
           stands for standard output.

GEOIDS

       GeoidEval computes geoid heights by interpolating on the data in a regularly spaced table
       (see "INTERPOLATION").  The following geoid tables are available (however, some may not be
       installed):

                                         bilinear error    cubic error
          name         geoid    grid     max      rms      max      rms
          egm84-30     EGM84    30'      1.546 m  70 mm    0.274 m  14 mm
          egm84-15     EGM84    15'      0.413 m  18 mm    0.021 m  1.2 mm
          egm96-15     EGM96    15'      1.152 m  40 mm    0.169 m  7.0 mm
          egm96-5      EGM96     5'      0.140 m  4.6 mm   .0032 m  0.7 mm
          egm2008-5    EGM2008   5'      0.478 m  12 mm    0.294 m  4.5 mm
          egm2008-2_5  EGM2008   2.5'    0.135 m  3.2 mm   0.031 m  0.8 mm
          egm2008-1    EGM2008   1'      0.025 m  0.8 mm   .0022 m  0.7 mm

       By default, the "egm96-5" geoid is used.  This may changed by setting the environment
       variable "GEOGRAPHICLIB_GEOID_NAME" or with the -n option.  The errors listed here are
       estimates of the quantization and interpolation errors in the reported heights compared to
       the specified geoid.

       The geoid data will be loaded from a directory specified at compile time.  This may
       changed by setting the environment variables "GEOGRAPHICLIB_GEOID_PATH" or
       "GEOGRAPHICLIB_DATA", or with the -d option.  The -h option prints the default geoid path
       and name.  Use the -v option to ascertain the full path name of the data file.

       Instructions for downloading and installing geoid data are available at
       <https://geographiclib.sourceforge.io/html/geoid.html#geoidinst>.

       NOTE: all the geoids above apply to the WGS84 ellipsoid (a = 6378137 m, f =
       1/298.257223563) only.

INTERPOLATION

       Cubic interpolation is used to compute the geoid height unless -l is specified in which
       case bilinear interpolation is used.  The cubic interpolation is based on a least-squares
       fit of a cubic polynomial to a 12-point stencil

          . 1 1 .
          1 2 2 1
          1 2 2 1
          . 1 1 .

       The cubic is constrained to be independent of longitude when evaluating the height at one
       of the poles.  Cubic interpolation is considerably more accurate than bilinear; however it
       results in small discontinuities in the returned height on cell boundaries.

CACHE

       By default, the data file is randomly read to compute the geoid heights at the input
       positions.  Usually this is sufficient for interactive use.  If many heights are to be
       computed, use -c south west north east to notify GeoidEval to read a rectangle of data
       into memory; heights within the this rectangle can then be computed without any disk
       access.  If -a is specified all the geoid data is read; in the case of "egm2008-1", this
       requires about 0.5 GB of RAM.  The evaluation of heights outside the cached area causes
       the necessary data to be read from disk.  Use the -v option to verify the size of the
       cache.

       Regardless of whether any cache is requested (with the -a or -c options), the data for the
       last grid cell in cached.  This allows the geoid height along a continuous path to be
       returned with little disk overhead.

ENVIRONMENT

       GEOGRAPHICLIB_GEOID_NAME
           Override the compile-time default geoid name of "egm96-5".  The -h option reports the
           value of GEOGRAPHICLIB_GEOID_NAME, if defined, otherwise it reports the compile-time
           value.  If the -n name option is used, then name takes precedence.

       GEOGRAPHICLIB_GEOID_PATH
           Override the compile-time default geoid path.  This is typically
           "/usr/local/share/GeographicLib/geoids" on Unix-like systems and
           "C:/ProgramData/GeographicLib/geoids" on Windows systems.  The -h option reports the
           value of GEOGRAPHICLIB_GEOID_PATH, if defined, otherwise it reports the compile-time
           value.  If the -d dir option is used, then dir takes precedence.

       GEOGRAPHICLIB_DATA
           Another way of overriding the compile-time default geoid path.  If it is set (and if
           GEOGRAPHICLIB_GEOID_PATH is not set), then $GEOGRAPHICLIB_DATA/geoids is used.

ERRORS

       An illegal line of input will print an error message to standard output beginning with
       "ERROR:" and causes GeoidEval to return an exit code of 1.  However, an error does not
       cause GeoidEval to terminate; following lines will be converted.

ABBREVIATIONS

       The geoid is usually approximated by an "earth gravity model". The models published by the
       NGA are:

       EGM84
           An earth gravity model published by the NGA in 1984,
           <http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html>.

       EGM96
           An earth gravity model published by the NGA in 1996,
           <http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html>.

       EGM2008
           An earth gravity model published by the NGA in 2008,
           <http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008>.

       WGS84
           World Geodetic System 1984, <https://en.wikipedia.org/wiki/WGS84>.

       HAE Height above the WGS84 ellipsoid.

       MSL Mean sea level, used as a convenient short hand for the geoid.  (However, typically,
           the geoid differs by a few meters from mean sea level.)

EXAMPLES

       The height of the EGM96 geoid at Timbuktu

           echo 16:46:33N 3:00:34W | GeoidEval
           => 28.7068 -0.02e-6 -1.73e-6

       The first number returned is the height of the geoid and the 2nd and 3rd are its slopes in
       the northerly and easterly directions.

       Convert a point in UTM zone 18n from MSL to HAE

          echo 531595 4468135 23 | GeoidEval --msltohae -z 18n
          => 531595 4468135 -10.842

SEE ALSO

       GeoConvert(1), Gravity(1), geographiclib-get-geoids(8).

       An online version of this utility is availbable at
       <https://geographiclib.sourceforge.io/cgi-bin/GeoidEval>.

AUTHOR

       GeoidEval was written by Charles Karney.

HISTORY

       GeoidEval was added to GeographicLib, <https://geographiclib.sourceforge.io>, in 2009-09.