NAME

SYNOPSIS

DESCRIPTION

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

OPTIONS

-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 "GEOIDS".

-l

use bilinear interpolation instead of cubic. See "INTERPOLATION".

-a

cache the entire data set in memory. See "CACHE".

-c south west north east

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

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.

--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 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).

--version

print version and exit.

-h

print usage, the default geoid path and name, and exit.

--help

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.

--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 reading begins.

--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 output.

GEOIDS

                                  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 <https://geographiclib.sourceforge.io/html/geoid.html#geoidinst>.

NOTE: all the geoids above apply to the WGS84 ellipsoid (a = 6378137 m, f = 1/298.257223563) only.

INTERPOLATION

   . 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

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

ABBREVIATIONS

EGM84

An earth gravity model published by the NGA in 1984, <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm84>.

EGM96

An earth gravity model published by the NGA in 1996, <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_egm96>.

EGM2008

An earth gravity model published by the NGA in 2008, <https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84#tab_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

    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

An online version of this utility is availbable at <https://geographiclib.sourceforge.io/cgi-bin/GeoidEval>.

AUTHOR

HISTORY