1GRAVITY(1) GeographicLib Utilities GRAVITY(1)
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6 Gravity -- compute the earth's gravity field
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9 Gravity [ -n name ] [ -d dir ] [ -G | -D | -A | -H ] [ -c lat h ] [ -w
10 ] [ -p prec ] [ -v ] [ --comment-delimiter commentdelim ] [ --version |
11 -h | --help ] [ --input-file infile | --input-string instring ] [
12 --line-separator linesep ] [ --output-file outfile ]
13
15 Gravity reads in positions on standard input and prints out the
16 gravitational field on standard output.
17 The input line is of the form lat lon h. lat and lon are the latitude
19 and longitude expressed as decimal degrees or degrees, minutes, and
20 seconds; for details on the allowed formats for latitude and longitude,
21 see the "GEOGRAPHIC COORDINATES" section of GeoConvert(1). h is the
22 height above the ellipsoid in meters; this quantity is optional and
23 defaults to 0. Alternatively, the gravity field can be computed at
24 various points on a circle of latitude (constant lat and h) via the -c
25 option; in this case only the longitude should be given on the input
26 lines. The quantities printed out are governed by the -G (default),
27 -D, -A, or -H options.
28 All the supported gravity models, except for grs80, use WGS84 as the
30 reference ellipsoid a = 6378137 m, f = 1/298.257223563, omega =
31 7292115e-11 rad/s, and GM = 3986004.418e8 m^3/s^2.
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34 -n name
35 use gravity field model name instead of the default "egm96". See
36 "MODELS".
37 -d dir
39 read gravity models from dir instead of the default. See "MODELS".
40 -G compute the acceleration due to gravity (including the centrifugal
42 acceleration due the the earth's rotation) g. The output consists
43 of gx gy gz (all in m/s^2), where the x, y, and z components are in
44 easterly, northerly, and up directions, respectively. Usually gz
45 is negative.
46 -D compute the gravity disturbance delta = g - gamma, where gamma is
48 the "normal" gravity due to the reference ellipsoid . The output
49 consists of deltax deltay deltaz (all in mGal, 1 mGal = 10^-5
50 m/s^2), where the x, y, and z components are in easterly,
51 northerly, and up directions, respectively. Note that deltax = gx,
52 because gammax = 0.
53 -A computes the gravitational anomaly. The output consists of 3 items
55 Dg01 xi eta, where Dg01 is in mGal (1 mGal = 10^-5 m/s^2) and xi
56 and eta are in arcseconds. The gravitational anomaly compares the
57 gravitational field g at P with the normal gravity gamma at Q where
58 the P is vertically above Q and the gravitational potential at P
59 equals the normal potential at Q. Dg01 gives the difference in the
60 magnitudes of these two vectors and xi and eta give the difference
61 in their directions (as northerly and easterly components). The
62 calculation uses a spherical approximation to match the results of
63 the NGA's synthesis programs.
64 -H compute the height of the geoid above the reference ellipsoid (in
66 meters). In this case, h should be zero. The results accurately
67 match the results of the NGA's synthesis programs. GeoidEval(1)
68 can compute geoid heights much more quickly by interpolating on a
69 grid of precomputed results; however the results from GeoidEval(1)
70 are only accurate to a few millimeters.
71 -c lat h
73 evaluate the field on a circle of latitude given by lat and h
74 instead of reading these quantities from the input lines. In this
75 case, Gravity can calculate the field considerably more quickly.
76 If geoid heights are being computed (the -H option), then h must be
77 zero.
78 -w toggle the longitude first flag (it starts off); if the flag is on,
80 then on input and output, longitude precedes latitude (except that,
81 on input, this can be overridden by a hemisphere designator, N, S,
82 E, W).
83 -p prec
85 set the output precision to prec. By default prec is 5 for
86 acceleration due to gravity, 3 for the gravity disturbance and
87 anomaly, and 4 for the geoid height.
88 -v print information about the gravity model on standard error before
90 processing the input.
91 --comment-delimiter commentdelim
93 set the comment delimiter to commentdelim (e.g., "#" or "//"). If
94 set, the input lines will be scanned for this delimiter and, if
95 found, the delimiter and the rest of the line will be removed prior
96 to processing and subsequently appended to the output line
97 (separated by a space).
98 --version
100 print version and exit.
101 -h print usage, the default gravity path and name, and exit.
103 --help
105 print full documentation and exit.
106 --input-file infile
108 read input from the file infile instead of from standard input; a
109 file name of "-" stands for standard input.
110 --input-string instring
112 read input from the string instring instead of from standard input.
113 All occurrences of the line separator character (default is a
114 semicolon) in instring are converted to newlines before the reading
115 begins.
116 --line-separator linesep
118 set the line separator character to linesep. By default this is a
119 semicolon.
120 --output-file outfile
122 write output to the file outfile instead of to standard output; a
123 file name of "-" stands for standard output.
124
126 Gravity computes the gravity field using one of the following models
127 egm84, earth gravity model 1984. See
129 http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html
130 egm96, earth gravity model 1996. See
131 http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html
132 egm2008, earth gravity model 2008. See
133 http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008
134 wgs84, world geodetic system 1984. This returns the normal
135 gravity for the WGS84 ellipsoid.
136 grs80, geodetic reference system 1980. This returns the normal
137 gravity for the GRS80 ellipsoid.
138 These models approximate the gravitation field above the surface of the
140 earth. By default, the "egm96" gravity model is used. This may
141 changed by setting the environment variable
142 "GEOGRAPHICLIB_GRAVITY_NAME" or with the -n option.
143 The gravity models will be loaded from a directory specified at compile
145 time. This may changed by setting the environment variables
146 "GEOGRAPHICLIB_GRAVITY_PATH" or "GEOGRAPHICLIB_DATA", or with the -d
147 option. The -h option prints the default gravity path and name. Use
148 the -v option to ascertain the full path name of the data file.
149 Instructions for downloading and installing gravity models are
151 available at
152 <https://geographiclib.sourceforge.io/html/gravity.html#gravityinst>.
153
155 GEOGRAPHICLIB_GRAVITY_NAME
156 Override the compile-time default gravity name of "egm96". The -h
157 option reports the value of GEOGRAPHICLIB_GRAVITY_NAME, if defined,
158 otherwise it reports the compile-time value. If the -n name option
159 is used, then name takes precedence.
160 GEOGRAPHICLIB_GRAVITY_PATH
162 Override the compile-time default gravity path. This is typically
163 "/usr/local/share/GeographicLib/gravity" on Unix-like systems and
164 "C:/ProgramData/GeographicLib/gravity" on Windows systems. The -h
165 option reports the value of GEOGRAPHICLIB_GRAVITY_PATH, if defined,
166 otherwise it reports the compile-time value. If the -d dir option
167 is used, then dir takes precedence.
168 GEOGRAPHICLIB_DATA
170 Another way of overriding the compile-time default gravity path.
171 If it is set (and if GEOGRAPHICLIB_GRAVITY_PATH is not set), then
172 $GEOGRAPHICLIB_DATA/gravity is used.
173
175 An illegal line of input will print an error message to standard output
176 beginning with "ERROR:" and causes Gravity to return an exit code of 1.
177 However, an error does not cause Gravity to terminate; following lines
178 will be converted.
179
181 The gravity field from EGM2008 at the top of Mount Everest
182 echo 27:59:17N 86:55:32E 8820 | Gravity -n egm2008
184 => -0.00001 0.00103 -9.76782
185
187 GeoConvert(1), GeoidEval(1), geographiclib-get-gravity(8).
188
190 Gravity was written by Charles Karney.
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193 Gravity was added to GeographicLib,
194 <https://geographiclib.sourceforge.io>, in version 1.16.
195GeographicLib 1.49 2017-10-05 GRAVITY(1)