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GRAVITY(1)		    GeographicLib Utilities		    GRAVITY(1)

       Gravity -- compute the earth's gravity field

       Gravity [ -n name ] [ -d	dir ] [	-N Nmax	] [ -M Mmax ] [	-G | -D	| -A |
       -H ] [ -c lat h ] [ -w ]	[ -p prec ] [ -v ] [ --comment-delimiter
       commentdelim ] [	--version | -h | --help	] [ --input-file infile	|
       --input-string instring ] [ --line-separator linesep ] [	--output-file
       outfile ]

       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;	for details on the allowed formats for latitude	and longitude,
       see the "GEOGRAPHIC COORDINATES"	section	of GeoConvert(1).  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.

       -n name
	   use gravity field model name	instead	of the default "egm96".	 See

       -d dir
	   read	gravity	models from dir	instead	of the default.	 See "MODELS".

       -N Nmax
	   limit the degree of the model to Nmax.

       -M Mmax
	   limit the order of the model	to Mmax.

       -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 lat h
	   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

       -w  toggle the longitude	first flag (it starts off); if the flag	is on,
	   then	on input and output, longitude precedes	latitude (except that,
	   on input, this can be overridden by a hemisphere designator,	N, S,
	   E, W).

       -p prec
	   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 commentdelim
	   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).

	   print version and exit.

       -h  print usage,	the default gravity path and name, and exit.

	   print full documentation and	exit.

       --input-file infile
	   read	input from the file infile instead of from standard input; a
	   file	name of	"-" stands for standard	input.

       --input-string instring
	   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

       --line-separator	linesep
	   set the line	separator character to linesep.	 By default this is a

       --output-file outfile
	   write output	to the file outfile instead of to standard output; a
	   file	name of	"-" stands for standard	output.

       Gravity computes	the gravity field using	one of the following models

	   egm84, earth	gravity	model 1984.  See
	   egm96, earth	gravity	model 1996.  See
	   egm2008, earth gravity model	2008.  See
	   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
       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

	   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.

	   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.

	   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.

       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.

       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

       GeoConvert(1), GeoidEval(1), geographiclib-get-gravity(8).

       Gravity was written by Charles Karney.

       Gravity was added to GeographicLib,
       <>, in version 1.16.

GeographicLib 1.51		  2020-11-22			    GRAVITY(1)


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