clutch skill Take Waypoints From a Map
After summiting Canada’s remote Twins Tower, gear trouble forced alpinist Steve House and his partner to tackle an unplanned route to the highway: 17 miles across the Columbia Icefield—in a whiteout. House pulled new coordinates from his map, entered them into his GPS, and safely navigated the nine-hour journey to the road. Although it’s easier and more accurate to plot waypoints at home using software, you may need to do it in the field if your course changes due to weather, an accident, or altered objectives. Here’s how:
1. Some maps have a UTM overlay grid, usually with each square representing one-square kilometer. Find the grid lines just to the west and south of each waypoint you plan to mark. If there’s no grid, find the blue tick marks next to the UTM easting and northing coordinates along the map’s edges, and use them to draw grid lines. Note the lines’ coordinates (e.g., 0363000E and 4029000N (A)).
2. Measure the distance east and north of these lines to the waypoint. Use a UTM grid reader or compass baseplate ruler appropriate for your map scale. Or, trace your map’s one-kilometer bar scale on scrap paper, and divide it into 10 increments of 100 meters each (B). Replace the final three numbers in the coordinates with the distances east and north to the waypoint in meters (e.g. 0363274E and 4029262N (C)).
3. Find the map’s zone number (usually in the lower-left corner), and add it to the easting (e.g. 12S 0363274E).
4. Enter the coordinates for each waypoint into your GPS. (House plotted eight.) Be sure to match the GPS datum with the map datum. On the GPS, select the first waypoint, press GoTo, and follow the displayed direction. To save power, note the bearing and navigate by compass, checking the GPS periodically. No GPS? Double the number of waypoints (since following a compass raises the likelihood for error), take the bearing for each leg from the map, and follow the compass bearings in turn.