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.

The geoid height, N, can be used to convert a height above the ellipsoid, h, to the corresponding height above the geoid (roughly the height above mean sea level), H, using the relations

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, 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).

An online version of this utility is availbable at http://geographiclib.sourceforge.net/cgi-bin/GeoidEval.

AUTHOR

GeoidEval was written by Charles Karney.

HISTORY

GeoidEval was added to GeographicLib, http://geographiclib.sf.net, in 2009-09.