A transverse Mercator projection is a map projection similar to the Mercator projection in that it is a projection of Earth on a tangent cylinder by rays radial with respect to the cylinder. However, in a transverse Mercator projection, the cylinder is tangent at some meridian rather than at the equator.

This kind of projection is useful for mapping small areas or areas with a small longitude range, e.g., Chile, since the distortion grows together with the distance from the tangent meridian.

The Universal Transverse Mercator (UTM) projection is not only used for graphical representations in maps, but also for the UTM coordinate system, a geographic coordinate system that is an alternative for using longitudes and latitudes. An advantage is that quantities are in metres, as opposed to degrees/minutes/seconds longitude which vary in length.

The Earth is divided into 60 zones limited by meridians, each spanning six degrees of longitude. Each zone is then projected using the transverse Mercator with the central meridian of the zone as the tangent meridian.

The division into zones is as follows: zone 1 lies between 180° and 174° West, with the tangent meridian at 177° West, zone 2 lies between 174° and 168° West with the tangent meridian at 171° West, and so on. Zone 30 lies between 6° West and 0°, the Greenwich meridian. Zone 31 lies between the Greenwich meridian and 6° East, etc. Finally, zone 60 lies between 174° East and 180°.

The Universal Transverse Mercator is convenient because no point is far from the center meridian of its zone, so that distortions inside zones are small. However, this is achieved at the cost of discontinuity: a zone border point is projected at two different locations, unless it lies on the equator; a line connecting two points at neighboring zones is not continuous, again, unless it crosses the zone border at the equator.

To avoid this discontinuity, sometimes a zone is extended, while its tangent meridian stays the same. This allows continuous maps mostly compatible with the standard. However, towards such a zone's borders the distortion is higher than in standard zones.

External links

• UTM conversion library (http://search.cpan.org/~grahamc/Geo-Coordinates-UTM-0.04/) written in Perl