/usr/bin/Gravity

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SYNOPSIS
       Gravity [ -n name ] [ -d dir ] [ -N Nmax ] [ -M Mmax ] [ -G | -D | -A |
       -H ] [ -c lat h ] [ -w ] [ -p prec ] [ -v ] [ --comment-delimiter
       commentdelim ] [ --version | -h | --help ] [ --input-file infile |
       --input-string instring ] [ --line-separator linesep ] [ --output-file
       outfile ]

DESCRIPTION
       Gravity reads in positions on standard input and prints out the
       gravitational field on standard output.

       The input line is of the form lat lon h.  lat and lon are the latitude
       and longitude expressed as decimal degrees or degrees, minutes, and
       seconds; for details on the allowed formats for latitude and longitude,
       see the "GEOGRAPHIC COORDINATES" section of GeoConvert(1).  h is the
       height above the ellipsoid in meters; this quantity is optional and
       defaults to 0.  Alternatively, the gravity field can be computed at
       various points on a circle of latitude (constant lat and h) via the -c
       option; in this case only the longitude should be given on the input
       lines.  The quantities printed out are governed by the -G (default),
       -D, -A, or -H options.

       All the supported gravity models, except for grs80, use WGS84 as the
       reference ellipsoid a = 6378137 m, f = 1/298.257223563, omega =
       7292115e-11 rad/s, and GM = 3986004.418e8 m^3/s^2.

OPTIONS
       -n name
           use gravity field model name instead of the default "egm96".  See
           "MODELS".

       -d dir
           read gravity models from dir instead of the default.  See "MODELS".

       -N Nmax
           limit the degree of the model to Nmax.

       -M Mmax
           limit the order of the model to Mmax.

       -G  compute the acceleration due to gravity (including the centrifugal
           acceleration due the the earth's rotation) g.  The output consists
           of gx gy gz (all in m/s^2), where the x, y, and z components are in
           easterly, northerly, and up directions, respectively.  Usually gz
           is negative.

       -D  compute the gravity disturbance delta = g - gamma, where gamma is
           the "normal" gravity due to the reference ellipsoid .  The output
           consists of deltax deltay deltaz (all in mGal, 1 mGal = 10^-5
           m/s^2), where the x, y, and z components are in easterly,
           northerly, and up directions, respectively.  Note that deltax = gx,
           because gammax = 0.

       -A  computes the gravitational anomaly.  The output consists of 3 items
           Dg01 xi eta, where Dg01 is in mGal (1 mGal = 10^-5 m/s^2) and xi
           and eta are in arcseconds.  The gravitational anomaly compares the
           gravitational field g at P with the normal gravity gamma at Q where
           the P is vertically above Q and the gravitational potential at P
           equals the normal potential at Q.  Dg01 gives the difference in the
           magnitudes of these two vectors and xi and eta give the difference
           in their directions (as northerly and easterly components).  The
           calculation uses a spherical approximation to match the results of
           the NGA's synthesis programs.

       -H  compute the height of the geoid above the reference ellipsoid (in
           meters).  In this case, h should be zero.  The results accurately
           match the results of the NGA's synthesis programs.  GeoidEval(1)
           can compute geoid heights much more quickly by interpolating on a
           grid of precomputed results; however the results from GeoidEval(1)
           are only accurate to a few millimeters.

       -c lat h
           evaluate the field on a circle of latitude given by lat and h
           instead of reading these quantities from the input lines.  In this
           case, Gravity can calculate the field considerably more quickly.
           If geoid heights are being computed (the -H option), then h must be
           zero.

       -w  toggle the longitude first flag (it starts off); if the flag is on,
           then on input and output, longitude precedes latitude (except that,
           on input, this can be overridden by a hemisphere designator, N, S,
           E, W).

       -p prec
           set the output precision to prec.  By default prec is 5 for
           acceleration due to gravity, 3 for the gravity disturbance and
           anomaly, and 4 for the geoid height.

       -v  print information about the gravity 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 gravity 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.

MODELS
       Gravity computes the gravity field using one of the following models

           egm84, earth gravity model 1984.  See
             https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84
           egm96, earth gravity model 1996.  See
             https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96
           egm2008, earth gravity model 2008.  See
             https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008
           wgs84, world geodetic system 1984.  This returns the normal
             gravity for the WGS84 ellipsoid.
           grs80, geodetic reference system 1980.  This returns the normal
             gravity for the GRS80 ellipsoid.

       These models approximate the gravitation field above the surface of the
       earth.  By default, the "egm96" gravity model is used.  This may
       changed by setting the environment variable
       "GEOGRAPHICLIB_GRAVITY_NAME" or with the -n option.

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

       Instructions for downloading and installing gravity models are
       available at
       <https://geographiclib.sourceforge.io/C++/doc/gravity.html#gravityinst>.

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

       GEOGRAPHICLIB_GRAVITY_PATH
           Override the compile-time default gravity path.  This is typically
           "/usr/local/share/GeographicLib/gravity" on Unix-like systems and
           "C:/ProgramData/GeographicLib/gravity" on Windows systems.  The -h
           option reports the value of GEOGRAPHICLIB_GRAVITY_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 gravity path.
           If it is set (and if GEOGRAPHICLIB_GRAVITY_PATH is not set), then
           $GEOGRAPHICLIB_DATA/gravity is used.

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

EXAMPLES
       The gravity field from EGM2008 at the top of Mount Everest

           echo 27:59:17N 86:55:32E 8820 | Gravity -n egm2008
           => -0.00001 0.00103 -9.76782

SEE ALSO
       GeoConvert(1), GeoidEval(1), geographiclib-get-gravity(8).

AUTHOR
       Gravity was written by Charles Karney.

HISTORY