NAME

       Geo::HelmertTransform

VERSION

       1.14

SYNOPSIS

           use Geo::HelmertTransform;

           my ($lat, $lon, $h) = ...; # from OS map
           my $airy1830 = Geo::HelmertTransform::datum('Airy1830');
           my $wgs84 = Geo::HelmertTransform::datum('WGS84');

           ($lat, $lon, $h)
               = Geo::HelmertTransform::convert_datum($airy1830, $wgs84,
                                                       $lat, $lon, $h);

DESCRIPTION

       Perform transformations between geographical coordinates in different datums.

       It is usual to describe geographical points in terms of their polar coordinates (latitude,
       longitude and altitude) referenced to a "datum ellipsoid", which is used to approximate
       the Earth's geoid. The latitude, longitude and altitude of a given physical point vary
       depending on which datum ellipsoid is in use.  Unfortunately, a number of ellipsoids are
       in everyday use, and so it is often necessary to transform geographical coordinates
       between different datum ellipsoids.

       Two different datum ellipsoids may differ in the locations of their centers, or in their
       shape; and there may be an angle between their equatorial planes or the meridians relative
       to which longitude is measured. The Helmert Transform, which this module implements, is a
       linear transformation of coordinates between pairs of datum ellipsoids in the limit of
       small angles of deviation between them.

CONVENTIONS

       Latitude is expressed in degrees, positive-north; longitude in degrees, positive-east.
       Heights (ellipsoid) and cartesian coordinates are in meters.

FUNCTIONS

       rad_to_deg RADIANS
           Convert RADIANS to degrees.

       deg_to_rad DEGREES
           Convert DEGREES to radians.

       geo_to_xyz DATUM LAT LON H
           Return the Cartesian (X, Y, Z) coordinates for the geographical coordinates (LAT, LON,
           H) in the given DATUM.

       xyz_to_geo DATUM X Y Z
           Return the geographical (LAT, LON, H) coordinates for the Cartesian coordinates (X, Y,
           Z) in the given DATUM. This is an iterative procedure.

       convert_datum D1 D2 LAT LON H
           Given geographical coordinates (LAT, LON, H) in datum D1, return the corresponding
           coordinates in datum D2. This assumes that the transformations are small, and always
           converts via WGS84.

       datum NAME
           Return the datum of the given NAME. Currently implemented are:

           Airy1830
               The 1830 Airy ellipsoid to which the British Ordnance Survey's National Grid is
               referenced.

           Airy1830Modified
               The modified 1830 Airy ellipsoid to which the Irish Grid (as used by Ordnance
               Survey Ireland and Ordnance Survey Northern Ireland); also known as the Ireland
               1975 datum.

           WGS84
               The global datum used for GPS.

SEE ALSO

       A guide to coordinate systems in Great Britain, http://www.gps.gov.uk/guidecontents.asp

       Making maps compatible with GPS,
       http://www.osni.gov.uk/downloads/Making%20maps%20GPS%20compatible.pdf

AUTHOR AND COPYRIGHT

       Written by Chris Lightfoot, team@mysociety.org

       Copyright (c) UK Citizens Online Democracy.

       This module is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.