NAME
Geo::Coordinates::UTM - Perl extension for Latitiude Lon
gitude conversions.
SYNOPSIS
use Geo::Coordinates::UTM;
($zone,$easting,$northing)=latlon_to_utm($ellipsoid,$lati
tude,$longitude);
($latitude,$longitude)=utm_to_latlon($ellip
soid,$zone,$easting,$northing);
DESCRIPTION
This module will translate latitude longitude coordinates
to Universal Transverse Mercator(UTM) coordinates and vice
versa.
Mercator Projection
The Mercator projection was first invented to help
mariners. They needed to be able to take a course and know
the distance traveled, and draw a line on the map which
showed the day's journey. In order to do this, Mercator
invented a projection which preserved length, by project
ing the earth's surface onto a cylinder, sharing the same
axis as the earth itself. This caused all Latitude and
Longitude lines to intersect at a 90 degree angle, thereby
negating the problem that longitude lines get closer
together at the poles.
Transverse Mercator Projection
A Transverse Mercator projection takes the cylinder and
turns it on its side. Now the cylinder's axis passes
through the equator, and it can be rotated to line up with
the area of interest. Many countries use Transverse Merca
tor for their grid systems.
Universal Transverse Mercator
The Universal Transverse Mercator(UTM) system sets up a
universal world wide system for mapping. The Transverse
Mercator projection is used, with the cylinder in 60 posi
tions. This creates 60 zones around the world. Positions
are measured using Eastings and Northings, measured in
meters, instead of Latitude and Longitude. Eastings start
at 500,000 on the centre line of each zone. In the North
ern Hemisphere, Northings are zero at the equator and
increase northward. In the Southern Hemisphere, Northings
start at 10 million at the equator, and decrease south
ward. You must know which hemisphere and zone you are in
to interpret your location globally. Distortion of scale,
distance, direction and area increase away from the cen
tral meridian.
UTM projection is used to define horizontal positions
world-wide by dividing the surface of the Earth into 6
degree zones, each mapped by the Transverse Mercator pro
jection with a central meridian in the center of the zone.
UTM zone numbers designate 6 degree longitudinal strips
extending from 80 degrees South latitude to 84 degrees
North latitude. UTM zone characters designate 8 degree
zones extending north and south from the equator. Eastings
are measured from the central meridian (with a 500 km
false easting to insure positive coordinates). Northings
are measured from the equator (with a 10,000 km false nor
thing for positions south of the equator).
UTM is applied separately to the Northern and Southern
Hemisphere, thus within a single UTM zone, a single X / Y
pair of values will occur in both the Northern and South
ern Hemisphere. To eliminate this confusion, and to speed
location of points, a UTM zone is sometimes subdivided
into 20 zones of Latitude. These grids can be further sub
divided into 100,000 meter grid squares with double-letter
designations. This subdivision by Latitude and further
division into grid squares is generally referred to as the
Military Grid Reference System (MGRS). The unit of mea
surement of UTM is always meters and the zones are num
bered from 1 to 60 eastward, beginning at the 180th merid
ian. The scale distortion in a north-south direction par
allel to the central meridian (CM) is constant However,
the scale distortion increases either direction away from
the CM. To equalize the distortion of the map across the
UTM zone, a scale factor of 0.9996 is applied to all dis
tance measurements within the zone. The distortion at the
zone boundary, 3 degrees away from the CM is approximately
1%.
Datums and Ellipsoids
Unlike local surveys, which treat the Earth as a plane,
the precise determination of the latitude and longitude of
points over a broad area must take into account the actual
shape of the Earth. To achieve the precision necessary for
accurate location, the Earth cannot be assumed to be a
sphere. Rather, the Earth's shape more closely approxi
mates an ellipsoid (oblate spheroid): flattened at the
poles and bulging at the Equator. Thus the Earth's shape,
when cut through its polar axis, approximates an ellipse.
A "Datum" is a standard representation of shape and offset
for coordinates, which includes an ellipsoid and an ori
gin. You must consider the Datum when working with geospa
tial data, since data with two different Datum will not
line up. The difference can be as much as a kilometer!
EXAMPLES
A description of the available ellipsoids and sample usage
of the conversion routines follows
Ellipsoids
The Ellipsoids available are numbered as follows:
1 Airy
2 Australian National
3 Bessel 1841
4 Bessel 1841 (Nambia)
5 Clarke 1866
6 Clarke 1880
7 Everest
8 Fischer 1960 (Mercury)
9 Fischer 1968
10 GRS 1967
11 GRS 1980
12 Helmert 1906
13 Hough
14 International
15 Krassovsky
16 Modified Airy
17 Modified Everest
18 Modified Fischer 1960
19 South American 1969
20 WGS 60
21 WGS 66
22 WGS-72
23 WGS-84
latlon_to_utm
Latitude values in the southern hemisphere should be sup
plied as negative values (e.g. 30 deg South will be -30).
Similarly Longitude values West of the meridian should
also be supplied as negative values. Both latitude and
longitude should not be entered as deg,min,sec but as
their decimal equivalent, e.g. 30 deg 12 min 22.432 sec
should be entered as 30.2062311
The ellipsoid value should correspond to one of the num
bers above, e.g. to use WGS-84, the ellipsoid value should
be 23
For latitude 57deg 49min 59.000sec North
longitude 02deg 47min 20.226sec West
using Clarke 1866 (Ellipsoid 5)
($zone,$east,$north)=latlon_to_utm(5,57.803055556,-2.788951667)
returns
$zone = 30V
$east = 512533.364651484
$north = 6409932.13416127
utm_to_latlon
Reversing the above example,
($latitude,$longitude)=utm_to_latlon(5,30V,512533.364651484,6409932.13416127)
returns
$latitude = 57.8330555601433
$longitude = -2.788951666974
which equates to
latitude 57deg 49min 59.000sec North
longitude 02deg 47min 20.226sec West
AUTHOR
Graham Crookham, grahamc@cpan.org
COPYRIGHT
Copyright (c) 2000,2002 by Graham Crookham. All rights
reserved.
This package is free software; you can redistribute it
and/or modify it under the same terms as Perl itself.