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PROGRAM:

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

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