xenial (1) Gravity.1.gz

Provided by: geographiclib-tools_1.45-2_amd64 bug

NAME

       Gravity -- compute the earth's gravity field

SYNOPSIS

       Gravity [ -n name ] [ -d dir ] [ -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  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.

       -w  on input and output, longitude precedes latitude (except that on input this can be overridden by a
           hemisphere designator, N, S, E, W).

       -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.
           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
       <http://geographiclib.sf.net/html/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

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