UTM is an acronym for Universal Transverse Mercator; a system of 120+ coordinate systems devised by the U.S. military shortly after the end of World War II. Presumably, Universal comes from the fact that the UTM system was designed to map the entire world. The ‘Transverse Mercator’ part comes from the fact that the Transverse Mercator, more specifically the Gauss Kruger variation thereof, is the predominant projection used in this system.

Choosing the Transverse Mercator projection was quite easy since it is the most widely used map projection in the world. It is especially well adapted for mapping regions whose extent is predominantly north/south. In order to keep the distortion introduced by the projection below a desired level, the inventors of UTM chose to define 60 separate zones around the earth. Quite naturally, each zone is, then, six degrees wide (6 × 60 = 360).

Zone 1 is defined as that region between 180° west longitude and 174° west longitude. Zone 2 covers 174° through 168° west longitude; and so on. Given a longitude, the formula which calculates the UTM zone number is:

where l is the longitude in degrees and must be negative for west longitude. Ignore the fractional part of the result to obtain the zone number. Calculating the longitude of the central meridian of the UTM zone, given the zone number, is also quite simple:

where, again, l is longitude of the zone central meridian in degrees and negative values indicate west longitude.

While the zones defined above apply to both the northern and southern hemispheres, separate coordinate systems were defined for the two hemispheres for each of the 60 zones. These definitions differ only in the false northing, enabling Y/Northing coordinates in the southern hemisphere to be positive. Thus, there are 120 coordinate system definitions in the UTM system which are actually based on the Transverse Mercator projection. Additional coordinate systems based on the polar aspect of the Stereographic projection are also defined for regions covering the north and south poles.

The following reference has a map of the UTM zones for the conterminous 48 states.

http://www.mentorsoftwareinc.com/resource/imagedir.htm

We still have a hundred or so postcards with this map printed on it. Should you desire copy, please send your request and snail mail address to: