Essentials of Geography

Dig Deeper into Locational Systems: The Universal Transverse Mercator (UTM) Grid

Though a spherical coordinate system like latitude and longitude is convenient for large areas, it is cumbersome for small areas. The need for accurate measurements within small areas was realized with the increasing range of artillery during the First and Second World Wars. This spurred the development of rectangular coordinate grid systems as plane rectangular geometry formulas were simpler than spherical geometry formulas upon which latitude and longitude is derived from. The Universal Transverse Mercator (UTM) Grid is one such system.

The Universal Transverse Mercator Grid divides the world into 60 north-south zones, each covering a strip 6° wide in longitude between 84N and 80S. Poleward the Universal Stereographic grid (UPS) is used. These longitude zones are numbered (called the "zone number") from Zone 1, between 180° and 174° west longitude, progressing eastward to Zone 60, between 174° and 180° east longitude. Each longitude zone is subdivided into a latitude zone is 8 degrees high north and south of the equator, and lettered starting from "C" at 80° S, increasing up to "X" (called the "zone designator"). The letters "I" and "O" are omitted because of their similarity to the digits one (1) and zero(0). X spans 12° of latitude.

Figure DD 1-I. The UTM Zones (Source Wikimedia Commons) The grid reference is read "right, up", and written with the longitude zone first. The example identified by the arrows is "17T".

Each longitude zone is subdivided into an eastern and western half by drawing a line down the middle called the "central meridian". A point can be described by its distance east of the origin, called its "easting" value. The central meridian is assigned a false easting of 500,000 meters to insure positive coordinates. Any easting value greater than 500,000 meters indicates a point east of the central meridian. Any easting value less than 500,000 meters indicates a point west of the central meridian. A point's north-south origin depends on the hemisphere it is in. In the northern hemisphere, the origin is the equator and all distances north (or ‘northings’) are measured from the equator. In the southern hemisphere the origin is the south pole and all northings are measured from there. When writing UTM coordinates, the easting is always first and the northing is after it.

Figure DD 1-J. UTM coordinates on topographic map.

The UTM grid is printed on all U.S. Geological Survey (USGS) maps as blue tick marks in the margins, or as full grid lines (Figure DD1-H). The spacing of the tick marks or grid lines depends on the scale of the map. Point A on the topopgraphic map is located at 599000 E., 4887000 N.

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