GeoidEval -- look up geoid heights
[ -n name
] [ -d dir
] [ -l
] [ -a
| -c south west north east
] [ -z zone
] [ --msltohae
] [ -v
] [ --comment-delimiter
] [ --version
| --input-string instring
] [ --output-file outfile
reads in positions on standard input and prints out the
corresponding heights of the geoid above the WGS84 ellipsoid on standard
Positions are given as latitude and longitude, UTM/UPS, or MGRS, in any of the
formats accepted by GeoConvert
(1). (MGRS coordinates signify the
of the corresponding MGRS square.) If the -z
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
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,
, 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.
- -n name
- use geoid name instead of the default
"egm96-5". See "GEOIDS".
- -d dir
- read geoid data from dir instead of the default. See
- use bilinear interpolation instead of cubic. See
- cache the entire data set in memory. See
- -c south west north
- 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
- 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.
- 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.
- 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
- print information about the geoid 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.
- print usage, the default geoid path and name, and
- 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-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
- --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
computes geoid heights by interpolating on the data in a
regularly spaced table (see "INTERPOLATION"). The following geoid
tables 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 is used. This may changed by setting
the environment variable "GEOGRAPHICLIB_GEOID_NAME" or with the
option. The errors listed here are estimates of the quantization and
interpolation errors in the reported heights compared to the specified geoid.
The geoid 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
Instructions for downloading and installing geoid data are available at
: all the geoids above apply to the WGS84 ellipsoid (a
6378137 m, f
= 1/298.257223563) only.
Cubic interpolation is used to compute the geoid height unless -l
specified in which case bilinear interpolation is used. The cubic
interpolation is based on a least-squares fit of a cubic polynomial to a
. 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.
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
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
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.
- 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.
- 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.
- 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.
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
terminate; following lines will be converted.
The geoid is usually approximated by an "earth gravity model". The
models published by the NGA are:
- An earth gravity model published by the NGA in 1984,
- An earth gravity model published by the NGA in 1996,
- An earth gravity model published by the NGA in 2008,
- World Geodetic System 1984,
- Height above the WGS84 ellipsoid.
- Mean sea level, used as a convenient short hand for the
geoid. (However, typically, the geoid differs by a few meters from mean
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
An online version of this utility is availbable at
was written by Charles Karney.
was added to GeographicLib,
<https://geographiclib.sourceforge.io>, in 2009-09.