The UTM (Universal Transverse Mercator) coordinate system divides the world into sixty north-south zones, each 6 degrees of longitude wide. UTM zones are numbered consecutively beginning with Zone 1, which includes the westernmost point of Alaska, and progress eastward to Zone 19, which includes Maine. If UTM ticks are shown on a USGS topographic map, the zone is indicated in the credit legend in the lower left corner of the map collar.
Within each zone, coordinates are measured as northings and eastings in meters. The northing values are measured from zero at the equator in a northerly direction. Each zone has a central meridian that is assigned an easting value of 500,000 meters. In Zone 16, for example, the central meridian is at 87 degrees longitude west. One meter east of that central meridian is 500,001 meters easting.
Almost all USGS topographic maps produced after 1977 show UTM tick marks on the sides of the map (or a full-line grid) every 1,000 meters. Some maps, including all those produced after 2009 (US Topo maps) include full UTM grid lines. To make UTM measurements, subdivide the 1,000-meter grid squares into tenths or hundredths. This narrows down the coordinate to a 100 meter or 10 meter square. Measurements can be made using a gridded mylar overlay, a paper scale, or a coordinate reader.
Note that the large numbers adjacent to the tick marks around the perimeter of the map represent tens of thousands and thousands of meters. The millions and hundreds of thousands of meters are shown with small numbers and are sometimes dropped when giving UTM coordinate positions. The military implementation of UTM (Military Grid Reference System or MGRS) drops the small digits and indicates the 100,000 meter square by a two letter identifier. Most UTM users and GPS (Global Positioning System) units use the full value of the UTM coordinates.