naomi

North American Overlay Mapping

Layer Notes - UTM Zones

The Universal Transverse Mercator (UTM) projection and grid system was adopted by the U.S. Army in 1947 for designating rectangular coordinates on large scale military maps. With the advent of inexpensive GPS receivers, many map users besides the military are adopting the UTM grid system for coordinates that some find easier to use than latitude and longitude.

The UTM system divides the earth into 60 zones, each 6 degrees of longitude wide. These zones define the centre reference point for UTM grid coordinates within the zone. UTM coverage only extends from a latitude of 80S to 84N; in polar regions, the Universal Polar Stereographic (UPS) grid system is used instead.

UTM zones are numbered vertically 1-60, starting at longitude 180°W, and proceeding east in 6° bands to 180°E. Each of these zones are then divided horizontally into bands that span 8 degrees of latitude. These bands begin at 80°S with the letter C, and continue north to letter X at 84°N. The letters I and O are omitted, to avoid any confusion with the numbers one and zero. Band 'X' is larger than the others, and spans 12° of latitude.

UTM coordinates are of two types: those refering to a distance in meters to the east are referred to as the "Easting", and those refering to a distance in meters to the north are referred to as the "Northing". The center of each zone is called the Central Meridian.

UTM easting coordinates are referenced to the Central Meridian, which is assigned an easting value of 500,000 meters East of its actual position. Since this 500,000 meter value is arbitrarily assigned, eastings are sometimes referred to as "False Eastings". An easting of zero will never occur, since a 6° wide zone is never more than 674,000 meters wide. The range of minimum and maximum easting values are 160,000m - 834,000m at the equator; and 465,000m - 515,000m at 84°N.

UTM northing coordinates are measured relative to the equator. For locations north of the equator the equator is assigned the northing value of 0 meters North. To avoid negative numbers, locations south of the equator are made with the equator assigned a value of 10,000,000 meters North. Some UTM northing values are valid both north and south of the equator. In order to avoid confusion the full coordinate needs to specify if the location is north or south of the equator, usually done by including the letter for the latitude band C-X.

Frequently, in land navigation, the zone information and the digits representing 1,000,000m, and 100,000m are dropped. The 1m, 10m and 100m digits are used only to the extent of accuracy desired. Note that it's the smaller digits that are dropped in the notation used by the USGS on the edges of their maps. For example 4282000 mN. becomes 82.

To change the Status Bar position reading so that it shows positions in UTM, see the Options page. This position-format will also be used in many of the Map Tools.