Desert Hiking - Using a Map and Compass

Desert hiking usually involves faint trails and cross-country travel. Map-reading skills are essential.

The best way to learn to read topographic maps is to buy one for an area you already know. By comparing the features of the terrain with its depiction on the map, you'll quickly master the art of map reading.

Contour lines are the essential feature of topographic maps. Each line represents a constant elevation above sea level.

Where the terrain is gentle, the lines are spread apart. Closely spaced contour lines mean that the terrain is steep. Ridges and drainages are shown by U- or V-shaped lines. Thin blue lines representing an intermittent or permanent stream usually distinguish drainages.

If contour lines are missing, there's a vertical or overhanging cliff. It's especially important to note missing contours along narrow canyon bottoms, which usually means there's one or more pour offs or dry waterfalls. You may or may not be able to find a route down or around these obstacles.

Learn the symbols used on your topographic map. Wilderness and recreation area maps usually have a legend printed on the map. The U.S. Geological Survey will provide a free legend for their topographic maps on request.

Don't rely on permanent streams shown on maps of desert country. They may not be reliable sources of water. Often maps show seasonal water that just happened to be present during surveying. The same is true of springs.

Learn the difference between true north, as shown on maps, and magnetic north, the direction your compass needle points. True north is the direction to the Earth's geographic North Pole. Compass needles align themselves with the earth's magnetic field, and generally point toward a spot in northeastern Canada about a thousand miles from the geographic pole. Declination (also called variation) is the difference between true and magnetic north. Because the magnetic field changes slowly, declination in a given location changes with time. Good maps show the declination as of publication date, and may show the rate of change. Declination can be as much as 20 degrees in the western U.S., which will cause large errors if you don't correct for it. (There's yet a third "north," grid north, which is the difference between the grid lines on the map and true north. For most fieldwork, you can ignore it. It's usually only a fraction of a degree.)

To help you relate the map to the landscape around you, orient the map and yourself toward true north. All good maps are printed with north toward the top of the sheet as a standard. Use your compass to determine north. Locate visible terrain features on the map, keeping in mind that desert objects are often much farther away than they appear. After you become familiar with the country, you'll be able to relate the map to terrain features without orienting it.

Triangulation can be used to identify unknown landmarks, such as mountain peaks, but you must be sure of your own current position. Take a compass bearing on the landmark, and then plot the bearing on your map, starting from your position. Use a protractor or the baseplate of your compass. Remember that you're working with magnetic bearings to the landmarks. The line will run through the unknown landmark. If the bearing line crosses several possible landmarks, hike a bit further and take another bearing. The two map lines will intersect at the landmark.

Resection (often incorrectly called triangulation) can be used if you are unsure of your position, but have one or more known landmarks in sight. Take a bearing on each of the landmarks. If you have a single landmark to work with, you'll at least know that your position is along the bearing line. If you also know that you're on a linear feature, such as a trail, road, ridge, or drainage, then the point where the bearing line crosses that feature on your map is your position. With two or more bearings, your position is shown by the intersection of the bearings on your map.

A compass with a sighting mirror is more accurate, and an adjustable declination setting lets you work with true bearings without having to correct for magnetic variation.

The type of compass with a clear plastic baseplate and a rotating bearing ring works well for plotting bearing lines on a map. Use the straight edge of another map to extend the compass straight edge if necessary.