/usr/bin/GeoidEval

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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 model 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 name for the geoid model 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 model 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 path and name for geoid models, 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
       grids 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 model 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 model 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/C++/doc/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,
           <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84>.

       EGM96
           An earth gravity model published by the NGA in 1996,
           <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96>.

       EGM2008
           An earth gravity model published by the NGA in 2008,
           <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_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 available at
       <https://geographiclib.sourceforge.io/cgi-bin/GeoidEval>.

AUTHOR
       GeoidEval was written by Charles Karney.

HISTORY