1GRAVITY(1) GeographicLib Utilities GRAVITY(1)
2
6 Gravity -- compute the earth's gravity field
7
9 Gravity [ -n name ] [ -d dir ] [ -N Nmax ] [ -M Mmax ] [ -G | -D | -A |
10 -H ] [ -c lat h ] [ -w ] [ -p prec ] [ -v ] [ --comment-delimiter
11 commentdelim ] [ --version | -h | --help ] [ --input-file infile |
12 --input-string instring ] [ --line-separator linesep ] [ --output-file
13 outfile ]
14
16 Gravity reads in positions on standard input and prints out the
17 gravitational field on standard output.
18 The input line is of the form lat lon h. lat and lon are the latitude
20 and longitude expressed as decimal degrees or degrees, minutes, and
21 seconds; for details on the allowed formats for latitude and longitude,
22 see the "GEOGRAPHIC COORDINATES" section of GeoConvert(1). h is the
23 height above the ellipsoid in meters; this quantity is optional and
24 defaults to 0. Alternatively, the gravity field can be computed at
25 various points on a circle of latitude (constant lat and h) via the -c
26 option; in this case only the longitude should be given on the input
27 lines. The quantities printed out are governed by the -G (default),
28 -D, -A, or -H options.
29 All the supported gravity models, except for grs80, use WGS84 as the
31 reference ellipsoid a = 6378137 m, f = 1/298.257223563, omega =
32 7292115e-11 rad/s, and GM = 3986004.418e8 m^3/s^2.
33
35 -n name
36 use gravity field model name instead of the default "egm96". See
37 "MODELS".
38 -d dir
40 read gravity models from dir instead of the default. See "MODELS".
41 -N Nmax
43 limit the degree of the model to Nmax.
44 -M Mmax
46 limit the order of the model to Mmax.
47 -G compute the acceleration due to gravity (including the centrifugal
49 acceleration due the the earth's rotation) g. The output consists
50 of gx gy gz (all in m/s^2), where the x, y, and z components are in
51 easterly, northerly, and up directions, respectively. Usually gz
52 is negative.
53 -D compute the gravity disturbance delta = g - gamma, where gamma is
55 the "normal" gravity due to the reference ellipsoid . The output
56 consists of deltax deltay deltaz (all in mGal, 1 mGal = 10^-5
57 m/s^2), where the x, y, and z components are in easterly,
58 northerly, and up directions, respectively. Note that deltax = gx,
59 because gammax = 0.
60 -A computes the gravitational anomaly. The output consists of 3 items
62 Dg01 xi eta, where Dg01 is in mGal (1 mGal = 10^-5 m/s^2) and xi
63 and eta are in arcseconds. The gravitational anomaly compares the
64 gravitational field g at P with the normal gravity gamma at Q where
65 the P is vertically above Q and the gravitational potential at P
66 equals the normal potential at Q. Dg01 gives the difference in the
67 magnitudes of these two vectors and xi and eta give the difference
68 in their directions (as northerly and easterly components). The
69 calculation uses a spherical approximation to match the results of
70 the NGA's synthesis programs.
71 -H compute the height of the geoid above the reference ellipsoid (in
73 meters). In this case, h should be zero. The results accurately
74 match the results of the NGA's synthesis programs. GeoidEval(1)
75 can compute geoid heights much more quickly by interpolating on a
76 grid of precomputed results; however the results from GeoidEval(1)
77 are only accurate to a few millimeters.
78 -c lat h
80 evaluate the field on a circle of latitude given by lat and h
81 instead of reading these quantities from the input lines. In this
82 case, Gravity can calculate the field considerably more quickly.
83 If geoid heights are being computed (the -H option), then h must be
84 zero.
85 -w toggle the longitude first flag (it starts off); if the flag is on,
87 then on input and output, longitude precedes latitude (except that,
88 on input, this can be overridden by a hemisphere designator, N, S,
89 E, W).
90 -p prec
92 set the output precision to prec. By default prec is 5 for
93 acceleration due to gravity, 3 for the gravity disturbance and
94 anomaly, and 4 for the geoid height.
95 -v print information about the gravity model on standard error before
97 processing the input.
98 --comment-delimiter commentdelim
100 set the comment delimiter to commentdelim (e.g., "#" or "//"). If
101 set, the input lines will be scanned for this delimiter and, if
102 found, the delimiter and the rest of the line will be removed prior
103 to processing and subsequently appended to the output line
104 (separated by a space).
105 --version
107 print version and exit.
108 -h print usage, the default gravity path and name, and exit.
110 --help
112 print full documentation and exit.
113 --input-file infile
115 read input from the file infile instead of from standard input; a
116 file name of "-" stands for standard input.
117 --input-string instring
119 read input from the string instring instead of from standard input.
120 All occurrences of the line separator character (default is a
121 semicolon) in instring are converted to newlines before the reading
122 begins.
123 --line-separator linesep
125 set the line separator character to linesep. By default this is a
126 semicolon.
127 --output-file outfile
129 write output to the file outfile instead of to standard output; a
130 file name of "-" stands for standard output.
131
133 Gravity computes the gravity field using one of the following models
134 egm84, earth gravity model 1984. See
136 https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84
137 egm96, earth gravity model 1996. See
138 https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96
139 egm2008, earth gravity model 2008. See
140 https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008
141 wgs84, world geodetic system 1984. This returns the normal
142 gravity for the WGS84 ellipsoid.
143 grs80, geodetic reference system 1980. This returns the normal
144 gravity for the GRS80 ellipsoid.
145 These models approximate the gravitation field above the surface of the
147 earth. By default, the "egm96" gravity model is used. This may
148 changed by setting the environment variable
149 "GEOGRAPHICLIB_GRAVITY_NAME" or with the -n option.
150 The gravity models will be loaded from a directory specified at compile
152 time. This may changed by setting the environment variables
153 "GEOGRAPHICLIB_GRAVITY_PATH" or "GEOGRAPHICLIB_DATA", or with the -d
154 option. The -h option prints the default gravity path and name. Use
155 the -v option to ascertain the full path name of the data file.
156 Instructions for downloading and installing gravity models are
158 available at
159 <https://geographiclib.sourceforge.io/html/gravity.html#gravityinst>.
160
162 GEOGRAPHICLIB_GRAVITY_NAME
163 Override the compile-time default gravity name of "egm96". The -h
164 option reports the value of GEOGRAPHICLIB_GRAVITY_NAME, if defined,
165 otherwise it reports the compile-time value. If the -n name option
166 is used, then name takes precedence.
167 GEOGRAPHICLIB_GRAVITY_PATH
169 Override the compile-time default gravity path. This is typically
170 "/usr/local/share/GeographicLib/gravity" on Unix-like systems and
171 "C:/ProgramData/GeographicLib/gravity" on Windows systems. The -h
172 option reports the value of GEOGRAPHICLIB_GRAVITY_PATH, if defined,
173 otherwise it reports the compile-time value. If the -d dir option
174 is used, then dir takes precedence.
175 GEOGRAPHICLIB_DATA
177 Another way of overriding the compile-time default gravity path.
178 If it is set (and if GEOGRAPHICLIB_GRAVITY_PATH is not set), then
179 $GEOGRAPHICLIB_DATA/gravity is used.
180
182 An illegal line of input will print an error message to standard output
183 beginning with "ERROR:" and causes Gravity to return an exit code of 1.
184 However, an error does not cause Gravity to terminate; following lines
185 will be converted.
186
188 The gravity field from EGM2008 at the top of Mount Everest
189 echo 27:59:17N 86:55:32E 8820 | Gravity -n egm2008
191 => -0.00001 0.00103 -9.76782
192
194 GeoConvert(1), GeoidEval(1), geographiclib-get-gravity(8).
195
197 Gravity was written by Charles Karney.
198
200 Gravity was added to GeographicLib,
201 <https://geographiclib.sourceforge.io>, in version 1.16.
202GeographicLib 1.52 2021-06-21 GRAVITY(1)