NAME¶
GeoidEval -- look up geoid heights
SYNOPSIS¶
GeoidEval [
-n name ] [
-d dir ] [
-l
] [
-a |
-c south west north east ]
[
-g ] [
-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 geoid heights on standard output. Optionally, it also prints the
northerly and easterly gradients of the geoid height.
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.
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. See "CACHE".
- -g
- print the northerly and easterly gradients after the geoid height (i.e.,
the rate at which the geoid height changes per unit distance along the
WGS84 ellipsoid in the specified directions). As a result of the way that
the geoid data is stored, the calculation of gradients can result in large
quantization errors. This is particularly acute at high latitudes and for
the easterly gradient.
- -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.
GEOIDS¶
GeoidEval 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. The gradients are computed by differentiating the
interpolated results.
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,
<http://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).
AUTHOR¶
GeoidEval was written by Charles Karney.
HISTORY¶
GeoidEval was added to GeographicLib,
<
http://geographiclib.sf.net>, in 2009-09.