NAME

       Gravity -- compute the earth's gravity field

SYNOPSIS

       Gravity [ -n name ] [ -d dir ] [ -G | -D | -A | -H ] [ -c lat h ] [ -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; see GeoConvert(1) for
       details.  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 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  use gravity field model name instead of the default "egm96".  See MODELS.

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

       -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  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.

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

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

       --output-file
           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
             http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html
           egm96, earth gravity model 1996.  See
             http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html
           egm2008, earth gravity model 2008.  See
             http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008
           wgs84, world geodetic system 1984.  This returns the normal
             gravity for the WGS84 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 "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 "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
       <http://geographiclib.sf.net/html/gravity.html#gravityinst>.

ENVIRONMENT

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

       GRAVITY_PATH
           Override the compile-time default gravity path.  This is typically
           "/usr/local/share/GeographicLib/gravity" on Unix-like systems and "C:/Documents and
           Settings/All Users/Application Data/GeographicLib/gravity" on Windows systems.  The -h
           option reports the value of 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
           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).

AUTHOR

       Gravity was written by Charles Karney.

HISTORY

       Gravity was added to GeographicLib, <http://geographiclib.sf.net>, in version 1.16.