GEOIDEVAL(1) GeographicLib Utilities GEOIDEVAL(1)NAMEGeoidEval-- look up geoid heights
SYNOPSISGeoidEval [ -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 ]
DESCRIPTIONGeoidEval reads in positions on standard input and prints out the
corresponding heights of the geoid 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 use geoid name instead of the default "egm96-5". See "GEOIDS".
-d 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 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 when reading geographic coordinates, longitude precedes latitude
(this can be overridden by a hemisphere designator, N, S, E, W).
-z 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 on standard error before
processing the input.
--comment-delimiter
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 geoid path and name, and exit.
--help
print full documentation and exit.
--input-file
read input from the file infile instead of from standard input; a
file name of "-" stands for standard input.
--input-string
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
set the line separator character to linesep. By default this is a
semicolon.
--output-file
write output to the file outfile instead of to standard output; a
file name of "-" stands for standard output.
GEOIDSGeoidEval 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 -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 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
<http://geographiclib.sf.net/html/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,
<http://earth-info.nga.mil/GandG/wgs84/gravitymod/wgs84_180/wgs84_180.html>.
EGM96
An earth gravity model published by the NGA in 1996,
<http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/egm96.html>.
EGM2008
An earth gravity model published by the NGA in 2008,
<http://earth-info.nga.mil/GandG/wgs84/gravitymod/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 ALSOGeoConvert(1), Gravity(1), geographiclib-get-geoids(8).
An online version of this utility is availbable at
<http://geographiclib.sourceforge.net/cgi-bin/GeoidEval>.
AUTHORGeoidEval was written by Charles Karney.
HISTORYGeoidEval was added to GeographicLib, <http://geographiclib.sf.net>, in
2009-09.
GeographicLib 1.45 2015-09-30 GEOIDEVAL(1)
