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/1.51/Gravity.1.html\n";
  else
    ( retval ? std::cerr : std::cout ) << "Man page:\n"
"NAME\n"
"       Gravity -- compute the earth's gravity field\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"
"             http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html\n"
"           egm96, earth gravity model 1996.  See\n"
"             http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html\n"
"           egm2008, earth gravity model 2008.  See\n"
"             http://earth-info.nga.mil/GandG/wgs84/gravitymod/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/html/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"
"       Gravity was added to GeographicLib,\n"
"       <https://geographiclib.sourceforge.io>, in version 1.16.\n"
;
  return retval;
}
