NAME

GeoidEval -- look up geoid heights


SYNOPSIS

GeoidEval [ -n name ] [ -d dir ] [ -l ] [ -a | -c south west north east ] [ -z zone ] [ --msltohae ] [ --haetomsl ] [ -v ] [ --version | -h | --help ]


DESCRIPTION

GeoidEval in positions on standard input and print out the corresponding geoid heights on standard output. In addition print the northerly and easterly gradients of the geoid height (i.e., the rate at which the geoid height changes per unit distance along the WGS84 ellipsoid in the specified directions).

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 corresponing 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.


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.

-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).

--haetomsl

standard input should include a final token on each line which is treated as a height (in meters) above the ellipsoid and the output echoes the input line with the height converted to height above geoid.

-v

print information about the geoid on standard error before processing the input.

--version

print version.

-h

print usage.

--help

print full documentation.


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'      1.546m  70mm      0.274m  14mm
   egm84-15     EGM84    15'      0.413m  18mm      0.020m   1mm
   egm96-15     EGM96    15'      1.152m  40mm      0.169m   7mm
   egm96-5      EGM96     5'      0.140m   5mm      0.003m   1mm
   egm2008-5    EGM2008   5'      0.478m  12mm      0.294m   5mm
   egm2008-2_5  EGM2008   2.5'    0.135m   3mm      0.031m   1mm
   egm2008-1    EGM2008   1'      0.025m   1mm      0.003m   1mm

By default, the egm96-5 geoid is used. This may changed by setting the environment variable 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 variable GEOID_PATH or with the -d option. 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 = 6378137m, 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

GEOID_NAME

Override the compile-time default geoid name of egm96-5. If the -n name option is used then name takes precedence.

GEOID_PATH

Override the compile-time default geoid path. This is typically /usr/local/share/GeographicLib/geoids on Unix or Linux systems and C:/cygwin/usr/local/share/GeographicLib/geoids on Windows systems. If the -d dir option is used then dir takes precedence.


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 "16d46'33N" "3d00'34W" | 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). GeoidEval is a part of GeographicLib, http://geographiclib.sf.net.


AUTHOR

GeoidEval was written by Charles Karney.