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, 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.
- -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).
AUTHOR¶
Gravity was written by Charles Karney.
HISTORY¶
Gravity was added to GeographicLib, <
http://geographiclib.sf.net>,
in version 1.16.