Adventure racers love it. The military teaches it. Rescue groups swear by it, and professional orienteers don't use anything else. We're talking about UTM, a method of plotting coordinates that's fast replacing lat/long as the system of choice for off-trail travel.
UTM stands for Universal Transverse Mercator. It's a reference grid overlaid on topo maps to divide terrain into standardized chunks. Each line of the grid represents an east/west or north/south position, like lat/long. But there's a difference: Instead of degrees, hours, and minutes, UTM uses a decimal-based metric system to number coordinates. A UTM grid divides your map into 1 km squares. Positions within each square are expressed in meters. To plot coordinates, you'll need a UTM-gridded map and a UTM tool-a clear plastic card with a tiny grid printed on it. Download a free tool (and find a more detailed tutorial) at www.maptools.com.