Provided by: geographiclib-tools_2.3-1build1_amd64 
NAME
GeoidEval -- look up geoid heights
SYNOPSIS
GeoidEval [ -n name ] [ -d dir ] [ -l ] [ -a | -c south west north east ] [ -w ] [ -z zone ] [ --msltohae
] [ --haetomsl ] [ -v ] [ --comment-delimiter commentdelim ] [ --version | -h | --help ] [ --input-file
infile | --input-string instring ] [ --line-separator linesep ] [ --output-file outfile ]
DESCRIPTION
GeoidEval reads in positions on standard input and prints out the corresponding heights of the geoid
model above the WGS84 ellipsoid on standard output.
Positions are given as latitude and longitude, UTM/UPS, or MGRS, in any of the formats accepted by
GeoConvert(1). (MGRS coordinates signify the center of the corresponding MGRS square.) If the -z option
is specified then the specified zone is prepended to each line of input (which must be in UTM/UPS
coordinates). This allows a file with UTM eastings and northings in a single zone to be used as standard
input.
More accurate results for the geoid height are provided by Gravity(1). This utility can also compute the
direction of gravity accurately.
The height of the geoid above the ellipsoid, N, is sometimes called the geoid undulation. It can be used
to convert a height above the ellipsoid, h, to the corresponding height above the geoid (the orthometric
height, roughly the height above mean sea level), H, using the relations
h = N + H, H = -N + h.
OPTIONS
-n name
use name for the geoid model instead of the default "egm96-5". See "GEOIDS".
-d dir
read geoid data from dir instead of the default. See "GEOIDS".
-l use bilinear interpolation instead of cubic. See "INTERPOLATION".
-a cache the entire data set in memory. See "CACHE".
-c south west north east
cache the data bounded by south west north east in memory. The first two arguments specify the SW
corner of the cache and the last two arguments specify the NE corner. The -w flag specifies that
longitude precedes latitude for these corners, provided that it appears before -c. See "CACHE".
-w toggle the longitude first flag (it starts off); if the flag is on, then when reading geographic
coordinates, longitude precedes latitude (this can be overridden by a hemisphere designator, N, S, E,
W).
-z zone
prefix each line of input by zone, e.g., "38n". This should be used when the input consists of
UTM/UPS eastings and northings.
--msltohae
standard input should include a final token on each line which is treated as a height (in meters)
above the geoid and the output echoes the input line with the height converted to height above
ellipsoid (HAE). If -z zone is specified then the third token is treated as the height; this makes
it possible to convert LIDAR data where each line consists of: easting northing height intensity.
--haetomsl
this is similar to --msltohae except that the height token is treated as a height (in meters) above
the ellipsoid and the output echoes the input line with the height converted to height above the
geoid (MSL).
-v print information about the geoid 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).
--version
print version and exit.
-h print usage, the default path and name for geoid models, and exit.
--help
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
begins.
--line-separator linesep
set the line separator character to linesep. By default this is a semicolon.
--output-file outfile
write output to the file outfile instead of to standard output; a file name of "-" stands for
standard output.
GEOIDS
GeoidEval computes geoid heights by interpolating on the data in a regularly spaced table (see
"INTERPOLATION"). The following geoid grids are available (however, some may not be installed):
bilinear error cubic error
name geoid grid max rms max rms
egm84-30 EGM84 30' 1.546 m 70 mm 0.274 m 14 mm
egm84-15 EGM84 15' 0.413 m 18 mm 0.021 m 1.2 mm
egm96-15 EGM96 15' 1.152 m 40 mm 0.169 m 7.0 mm
egm96-5 EGM96 5' 0.140 m 4.6 mm .0032 m 0.7 mm
egm2008-5 EGM2008 5' 0.478 m 12 mm 0.294 m 4.5 mm
egm2008-2_5 EGM2008 2.5' 0.135 m 3.2 mm 0.031 m 0.8 mm
egm2008-1 EGM2008 1' 0.025 m 0.8 mm .0022 m 0.7 mm
By default, the "egm96-5" geoid model is used. This may changed by setting the environment variable
"GEOGRAPHICLIB_GEOID_NAME" or with the -n option. The errors listed here are estimates of the
quantization and interpolation errors in the reported heights compared to the specified geoid.
The geoid model data will be loaded from a directory specified at compile time. This may changed by
setting the environment variables "GEOGRAPHICLIB_GEOID_PATH" or "GEOGRAPHICLIB_DATA", or with the -d
option. The -h option prints the default geoid path and name. Use the -v option to ascertain the full
path name of the data file.
Instructions for downloading and installing geoid data are available at
<https://geographiclib.sourceforge.io/C++/doc/geoid.html#geoidinst>.
NOTE: all the geoids above apply to the WGS84 ellipsoid (a = 6378137 m, f = 1/298.257223563) only.
INTERPOLATION
Cubic interpolation is used to compute the geoid height unless -l is specified in which case bilinear
interpolation is used. The cubic interpolation is based on a least-squares fit of a cubic polynomial to
a 12-point stencil
. 1 1 .
1 2 2 1
1 2 2 1
. 1 1 .
The cubic is constrained to be independent of longitude when evaluating the height at one of the poles.
Cubic interpolation is considerably more accurate than bilinear; however it results in small
discontinuities in the returned height on cell boundaries.
CACHE
By default, the data file is randomly read to compute the geoid heights at the input positions. Usually
this is sufficient for interactive use. If many heights are to be computed, use -c south west north east
to notify GeoidEval to read a rectangle of data into memory; heights within the this rectangle can then
be computed without any disk access. If -a is specified all the geoid data is read; in the case of
"egm2008-1", this requires about 0.5 GB of RAM. The evaluation of heights outside the cached area causes
the necessary data to be read from disk. Use the -v option to verify the size of the cache.
Regardless of whether any cache is requested (with the -a or -c options), the data for the last grid cell
in cached. This allows the geoid height along a continuous path to be returned with little disk
overhead.
ENVIRONMENT
GEOGRAPHICLIB_GEOID_NAME
Override the compile-time default geoid name of "egm96-5". The -h option reports the value of
GEOGRAPHICLIB_GEOID_NAME, if defined, otherwise it reports the compile-time value. If the -n name
option is used, then name takes precedence.
GEOGRAPHICLIB_GEOID_PATH
Override the compile-time default geoid path. This is typically
"/usr/local/share/GeographicLib/geoids" on Unix-like systems and
"C:/ProgramData/GeographicLib/geoids" on Windows systems. The -h option reports the value of
GEOGRAPHICLIB_GEOID_PATH, if defined, otherwise it reports the compile-time value. If the -d dir
option is used, then dir takes precedence.
GEOGRAPHICLIB_DATA
Another way of overriding the compile-time default geoid path. If it is set (and if
GEOGRAPHICLIB_GEOID_PATH is not set), then $GEOGRAPHICLIB_DATA/geoids is used.
ERRORS
An illegal line of input will print an error message to standard output beginning with "ERROR:" and
causes GeoidEval to return an exit code of 1. However, an error does not cause GeoidEval to terminate;
following lines will be converted.
ABBREVIATIONS
The geoid is usually approximated by an "earth gravity model". The models published by the NGA are:
EGM84
An earth gravity model published by the NGA in 1984,
<https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84>.
EGM96
An earth gravity model published by the NGA in 1996,
<https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96>.
EGM2008
An earth gravity model published by the NGA in 2008,
<https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm2008>.
WGS84
World Geodetic System 1984, <https://en.wikipedia.org/wiki/WGS84>.
HAE Height above the WGS84 ellipsoid.
MSL Mean sea level, used as a convenient short hand for the geoid. (However, typically, the geoid
differs by a few meters from mean sea level.)
EXAMPLES
The height of the EGM96 geoid at Timbuktu
echo 16:46:33N 3:00:34W | GeoidEval
=> 28.7068 -0.02e-6 -1.73e-6
The first number returned is the height of the geoid and the 2nd and 3rd are its slopes in the northerly
and easterly directions.
Convert a point in UTM zone 18n from MSL to HAE
echo 531595 4468135 23 | GeoidEval --msltohae -z 18n
=> 531595 4468135 -10.842
SEE ALSO
GeoConvert(1), Gravity(1), geographiclib-get-geoids(8).
An online version of this utility is availbable at
<https://geographiclib.sourceforge.io/cgi-bin/GeoidEval>.
AUTHOR
GeoidEval was written by Charles Karney.
HISTORY
GeoidEval was added to GeographicLib, <https://geographiclib.sourceforge.io>, in 2009-09.
GeographicLib 2.3 2023-07-25 GEOIDEVAL(1)