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int usage(int retval, bool brief) {
if (brief)
( retval ? std::cerr : std::cout ) << "Usage:\n"
" Gravity [ -n name ] [ -d dir ] [ -N Nmax ] [ -M Mmax ] [ -G | -D | -A |\n"
" -H ] [ -c lat h ] [ -w ] [ -p prec ] [ -v ] [ --comment-delimiter\n"
" commentdelim ] [ --version | -h | --help ] [ --input-file infile |\n"
" --input-string instring ] [ --line-separator linesep ] [ --output-file\n"
" outfile ]\n"
"\n"
"For full documentation type:\n"
" Gravity --help\n"
"or visit:\n"
" https://geographiclib.sourceforge.io/C++/2.6/Gravity.1.html\n";
else
( retval ? std::cerr : std::cout ) << "Man page:\n"
"\n"
"SYNOPSIS\n"
" Gravity [ -n name ] [ -d dir ] [ -N Nmax ] [ -M Mmax ] [ -G | -D | -A |\n"
" -H ] [ -c lat h ] [ -w ] [ -p prec ] [ -v ] [ --comment-delimiter\n"
" commentdelim ] [ --version | -h | --help ] [ --input-file infile |\n"
" --input-string instring ] [ --line-separator linesep ] [ --output-file\n"
" outfile ]\n"
"\n"
"DESCRIPTION\n"
" Gravity reads in positions on standard input and prints out the\n"
" gravitational field on standard output.\n"
"\n"
" The input line is of the form lat lon h. lat and lon are the latitude\n"
" and longitude expressed as decimal degrees or degrees, minutes, and\n"
" seconds; for details on the allowed formats for latitude and longitude,\n"
" see the \"GEOGRAPHIC COORDINATES\" section of GeoConvert(1). h is the\n"
" height above the ellipsoid in meters; this quantity is optional and\n"
" defaults to 0. Alternatively, the gravity field can be computed at\n"
" various points on a circle of latitude (constant lat and h) via the -c\n"
" option; in this case only the longitude should be given on the input\n"
" lines. The quantities printed out are governed by the -G (default),\n"
" -D, -A, or -H options.\n"
"\n"
" All the supported gravity models, except for grs80, use WGS84 as the\n"
" reference ellipsoid a = 6378137 m, f = 1/298.257223563, omega =\n"
" 7292115e-11 rad/s, and GM = 3986004.418e8 m^3/s^2.\n"
"\n"
"OPTIONS\n"
" -n name\n"
" use gravity field model name instead of the default \"egm96\". See\n"
" \"MODELS\".\n"
"\n"
" -d dir\n"
" read gravity models from dir instead of the default. See \"MODELS\".\n"
"\n"
" -N Nmax\n"
" limit the degree of the model to Nmax.\n"
"\n"
" -M Mmax\n"
" limit the order of the model to Mmax.\n"
"\n"
" -G compute the acceleration due to gravity (including the centrifugal\n"
" acceleration due the the earth's rotation) g. The output consists\n"
" of gx gy gz (all in m/s^2), where the x, y, and z components are in\n"
" easterly, northerly, and up directions, respectively. Usually gz\n"
" is negative.\n"
"\n"
" -D compute the gravity disturbance delta = g - gamma, where gamma is\n"
" the \"normal\" gravity due to the reference ellipsoid . The output\n"
" consists of deltax deltay deltaz (all in mGal, 1 mGal = 10^-5\n"
" m/s^2), where the x, y, and z components are in easterly,\n"
" northerly, and up directions, respectively. Note that deltax = gx,\n"
" because gammax = 0.\n"
"\n"
" -A computes the gravitational anomaly. The output consists of 3 items\n"
" Dg01 xi eta, where Dg01 is in mGal (1 mGal = 10^-5 m/s^2) and xi\n"
" and eta are in arcseconds. The gravitational anomaly compares the\n"
" gravitational field g at P with the normal gravity gamma at Q where\n"
" the P is vertically above Q and the gravitational potential at P\n"
" equals the normal potential at Q. Dg01 gives the difference in the\n"
" magnitudes of these two vectors and xi and eta give the difference\n"
" in their directions (as northerly and easterly components). The\n"
" calculation uses a spherical approximation to match the results of\n"
" the NGA's synthesis programs.\n"
"\n"
" -H compute the height of the geoid above the reference ellipsoid (in\n"
" meters). In this case, h should be zero. The results accurately\n"
" match the results of the NGA's synthesis programs. GeoidEval(1)\n"
" can compute geoid heights much more quickly by interpolating on a\n"
" grid of precomputed results; however the results from GeoidEval(1)\n"
" are only accurate to a few millimeters.\n"
"\n"
" -c lat h\n"
" evaluate the field on a circle of latitude given by lat and h\n"
" instead of reading these quantities from the input lines. In this\n"
" case, Gravity can calculate the field considerably more quickly.\n"
" If geoid heights are being computed (the -H option), then h must be\n"
" zero.\n"
"\n"
" -w toggle the longitude first flag (it starts off); if the flag is on,\n"
" then on input and output, longitude precedes latitude (except that,\n"
" on input, this can be overridden by a hemisphere designator, N, S,\n"
" E, W).\n"
"\n"
" -p prec\n"
" set the output precision to prec. By default prec is 5 for\n"
" acceleration due to gravity, 3 for the gravity disturbance and\n"
" anomaly, and 4 for the geoid height.\n"
"\n"
" -v print information about the gravity model on standard error before\n"
" processing the input.\n"
"\n"
" --comment-delimiter commentdelim\n"
" set the comment delimiter to commentdelim (e.g., \"#\" or \"//\"). If\n"
" set, the input lines will be scanned for this delimiter and, if\n"
" found, the delimiter and the rest of the line will be removed prior\n"
" to processing and subsequently appended to the output line\n"
" (separated by a space).\n"
"\n"
" --version\n"
" print version and exit.\n"
"\n"
" -h print usage, the default gravity path and name, and exit.\n"
"\n"
" --help\n"
" print full documentation and exit.\n"
"\n"
" --input-file infile\n"
" read input from the file infile instead of from standard input; a\n"
" file name of \"-\" stands for standard input.\n"
"\n"
" --input-string instring\n"
" read input from the string instring instead of from standard input.\n"
" All occurrences of the line separator character (default is a\n"
" semicolon) in instring are converted to newlines before the reading\n"
" begins.\n"
"\n"
" --line-separator linesep\n"
" set the line separator character to linesep. By default this is a\n"
" semicolon.\n"
"\n"
" --output-file outfile\n"
" write output to the file outfile instead of to standard output; a\n"
" file name of \"-\" stands for standard output.\n"
"\n"
"MODELS\n"
" Gravity computes the gravity field using one of the following models\n"
"\n"
" egm84, earth gravity model 1984. See\n"
" https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84\n"
" egm96, earth gravity model 1996. See\n"
" https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96\n"
" egm2008, earth gravity model 2008. See\n"
" https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008\n"
" wgs84, world geodetic system 1984. This returns the normal\n"
" gravity for the WGS84 ellipsoid.\n"
" grs80, geodetic reference system 1980. This returns the normal\n"
" gravity for the GRS80 ellipsoid.\n"
"\n"
" These models approximate the gravitation field above the surface of the\n"
" earth. By default, the \"egm96\" gravity model is used. This may\n"
" changed by setting the environment variable\n"
" \"GEOGRAPHICLIB_GRAVITY_NAME\" or with the -n option.\n"
"\n"
" The gravity models will be loaded from a directory specified at compile\n"
" time. This may changed by setting the environment variables\n"
" \"GEOGRAPHICLIB_GRAVITY_PATH\" or \"GEOGRAPHICLIB_DATA\", or with the -d\n"
" option. The -h option prints the default gravity path and name. Use\n"
" the -v option to ascertain the full path name of the data file.\n"
"\n"
" Instructions for downloading and installing gravity models are\n"
" available at\n"
" <https://geographiclib.sourceforge.io/C++/doc/gravity.html#gravityinst>.\n"
"\n"
"ENVIRONMENT\n"
" GEOGRAPHICLIB_GRAVITY_NAME\n"
" Override the compile-time default gravity name of \"egm96\". The -h\n"
" option reports the value of GEOGRAPHICLIB_GRAVITY_NAME, if defined,\n"
" otherwise it reports the compile-time value. If the -n name option\n"
" is used, then name takes precedence.\n"
"\n"
" GEOGRAPHICLIB_GRAVITY_PATH\n"
" Override the compile-time default gravity path. This is typically\n"
" \"/usr/local/share/GeographicLib/gravity\" on Unix-like systems and\n"
" \"C:/ProgramData/GeographicLib/gravity\" on Windows systems. The -h\n"
" option reports the value of GEOGRAPHICLIB_GRAVITY_PATH, if defined,\n"
" otherwise it reports the compile-time value. If the -d dir option\n"
" is used, then dir takes precedence.\n"
"\n"
" GEOGRAPHICLIB_DATA\n"
" Another way of overriding the compile-time default gravity path.\n"
" If it is set (and if GEOGRAPHICLIB_GRAVITY_PATH is not set), then\n"
" $GEOGRAPHICLIB_DATA/gravity is used.\n"
"\n"
"ERRORS\n"
" An illegal line of input will print an error message to standard output\n"
" beginning with \"ERROR:\" and causes Gravity to return an exit code of 1.\n"
" However, an error does not cause Gravity to terminate; following lines\n"
" will be converted.\n"
"\n"
"EXAMPLES\n"
" The gravity field from EGM2008 at the top of Mount Everest\n"
"\n"
" echo 27:59:17N 86:55:32E 8820 | Gravity -n egm2008\n"
" => -0.00001 0.00103 -9.76782\n"
"\n"
"SEE ALSO\n"
" GeoConvert(1), GeoidEval(1), geographiclib-get-gravity(8).\n"
"\n"
"AUTHOR\n"
" Gravity was written by Charles Karney.\n"
"\n"
"HISTORY\n"
;
return retval;
}
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