# 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.