2. NEXT LEVEL Make Your GPS Talk to Your Topo
Use satellite data to find your location on a map.
>> Match map data and your GPS display
Inconsistencies between navigation tools can put you miles off target. Before translating GPS data onto paper, identify your map’s data sets, then program your
receiver’s settings to correspond: Datum Map datum describes the survey grid used to create the map. Look for the citation in the margin, then update your GPS unit’s datum on the
main settings menu. The most common North American datum are WGS 84 and NAD 27. Coordinate grid Most wilderness navigators use the UTM (Universal Transverse Mercator) reference grid, which is also BACKPACKER’s standard.
Another common choice is the latitude/longitude grid. Maps usually have references to both. North preference Switch your receiver to true north mode if you’re working with a map only. If you’re using a compass, too, consider toggling
between true and magnetic readings to smooth declination calculations.
>> Mark a waypoint and note your coordinates
Your GPS unit’s waypoint screen, which displays after you create a new point, shows a precise location as a coordinate set. To translate a pair of UTM coordinates
from your GPS to your map, note the last seven digits of both the easting (listed first) and northing (listed second).
>> Find your position
Identify your map’s UTM grid, drawn in blue (older maps may have only tick marks on the margin). Match the first four digits of the easting and northing with the
grid. The corresponding 1-by-1-km square marks your general location. Use the last three digits of each coordinate, which represent meters, to pinpoint your position
within the 1,000-meter square. Measure east (easting) and north (northing) from the square’s bottom left corner; your position is where the measurements intersect.
Take Bearings and Triangulate
Use a map, compass, and nearby landmarks to orient yourself.
1. Orient the map. Align your map’s north-orienting arrow with the compass baseplate’s direction-of-travel pointer. Place the plate’s edge parallel to the magnetic
north line (in the margin). Rotate map and compass together until the needle is ‘boxed’ by the baseplate.
2. Take a bearing. Hold the compass at waist height and point the direction-of-travel arrow at a known land feature. Rotate the bezel until the north-seeking
arrow is boxed. Your bearing is the number on the bezel that lines up with your direction-of-travel arrow.
3. Transfer bearing to map. Keeping the north-seeking arrow boxed, place your compass on the map with the baseplate’s edge
running through the center of the landmark; mark that line on your map in pencl.
4. Triangulate a position. Take at least three bearings using distinct landmarks. The point where they intersect (it may be a small triangle) is