Universal Transverse Mercator system

Divide the world into sixty vertical strips, each spanning six degrees of longitude. Apply a custom Transverse Mercator projection to each strip and use false eastings and northings to make all projected coordinates positive. That's the Universal Transverse Mercator coordinate system—a comprehensive system for identifying locations and making measurements over most of the earth's surface.

The strips, called zones, are numbered one through sixty, starting from the 180th meridian and moving west to east. (Zone 1 covers 180º to 174ºW, Zone 2 covers 174ºW to 168ºW and so on.) Each zone is further subdivided to indicate whether it is north or south of the equator.

The world divided into UTM zones.

The zones of the northern hemisphere as seen from the north pole.

The zones of the southern hemisphere as seen from the south pole.

In the graphics above, the blue circles show the areas not covered by the UTM system. The zones do not extend beyond 84°N or 80°S. (These areas are covered by the Universal Polar Stereographic coordinate system, which complements the UTM system.)

About the projection
Each zone uses a custom Transverse Mercator projection with its own central meridian. (The central meridian for Zone 1 is 177ºW, the central meridian for Zone 2 is 171ºW, and so on.) Ordinarily, in a Transverse Mercator projection, the central meridian is a line of true scale. In the UTM system, however, the projection is secant, so two lines of true scale are created—one on each side of the central meridian. As you learned in the previous module, this is a distortion-balancing technique that compresses scale between the secant lines. The central meridian in each UTM zone has a scale factor of 0.9996, which means that measurements along it fall short of true scale by 4 units in 10,000 (or 1 unit in 2,500). This is the maximum scale error anywhere within the zone and is the accuracy standard that the system was designed to meet.

The central meridian and lines of true scale in a UTM zone. Scale is less than 1.0 between the red lines and greater than 1.0 outside them. Maximum scale distortion within the zone is 1/25th of one percent.

About the map units
Any point within a zone hemisphere can be identified by an x,y coordinate pair that tells you how many meters east and north the point lies from the coordinate system origin. For example, Bekins Hall (a dormitory at the University of Redlands in California) has the following UTM coordinates:

UTM Zone 11N, 484,617 meters east, 3,769,192 meters north

Where is the coordinate system origin? The natural origin of a UTM zone is the intersection of its central meridian with the equator, but this origin would assign negative coordinates to some locations. To avoid this, false easting and northing values are applied.

	False easting	False northing
North zones	500,000 m	none
South zones	500,000 m	10,000,000 m

More about UTM false easting and false northing

False easting is needed to avoid negative x-coordinates west of each zone's central meridian. False northing is needed to avoid negative y-coordinates for all locations in southern hemisphere zones. (The value of ten million meters guarantees that every point above the south pole has a positive y-value.) Northern hemisphere zones do not use false northing because their y-coordinates are naturally positive.

Data that crosses zones
UTM is a precise point referencing system that allows accurate spatial analysis within a zone. Data that crosses zones, however, is subject to rapidly increasing distortion. Suppose you are working with data that spans zones 11N, 12N, and 13N. Since you have to project all the data using the parameters of a single zone—say, 12N—data in the other zones will be distorted well beyond the 1 in 2,500 accuracy standard. As far as possible, you should use UTM for data that lies within a single zone.

Universal Polar Stereographic system

The UTM system leaves round holes at the top and bottom of the world. These are filled in by the two zones of the Universal Polar Stereographic (UPS) coordinate system. UPS North covers the area from 84ºN to the north pole; UPS South covers the area from 80ºS to the south pole.

UPS North covers the area inside the white circle.

UPS South covers the area inside the circle marked 80S.

The UPS system is based the Stereographic projection customized for each pole. As with UTM, a secant form of the projection is used. For both the north and south zones, the scale factor at the pole is 0.994, and a parallel of true scale is created at 81º06'52.3''. The system achieves the same accuracy (scale error of one unit in 2,500) as the UTM system.

False easting and false northing are applied to both zones.

	False easting	False northing
UPS North	2,000,000 m	2,000,000 m
UPS South	2,000,000 m	2,000,000 m