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Articles > Map & Compass

Using Map & Compass

I'm including this brief overview for people who are unfamiliar with map reading and the use of a compass. Many a hiker has found himself in the woods with a compass or map, or both, and when unsure of their location, been struck by the realization that even though they have the appropriate navigational tools, they do not have the knowledge to use them. This overview should be considered just that, an overview. For a more complete explanation of using a map and compass in the wilderness, I suggest reading June Fleming's excellent book, Staying Found.

Topographic Maps

There are several kinds of maps available on the marketplace; too many to list here. The type of map I'm going to discuss here is the topographic map. A topographic map uses contour lines to portray the shape and elevation of the land. These lines are the twisted, curved brown lines that connect points of equal elevation on the map. The contour interval is the set distance between contour lines on a given map, measures in feet or meters. This interval can be found near the map's scale, which is usually located in the map legend. These contour lines make it possible for a topographic map to render the tree-dimensional ups and downs of the terrain on the two-dimensional surface of the map.

Topographic maps show both natural and man-made (cultural) features. They use symbols, lines and colors to portray map features. Topo maps show areas shaded in various colors to represent land cover. Areas on a topo map that are shaded green represent vegetation, usually wooded cover (trees) or brush. Areas of blue and blue lines indicate bodies of water. Areas that are white are usually areas with little or no vegetation, such as desert or rocky alpine areas. Wilderness, national park, and national forest boundaries consist of black, dashed, dotted lines tinted with green, brown or gray shades. Consult the map legend for specific tints. Features that are denoted by lines are: topographic contours, shown in brown, streams, rivers and lakes in blue, and roads, trails, and boundary lines, usually in black. Some lines are solid, some are dashed, and the width and darkness of lines often vary in order to distinguish features from one another.

The Compass

It is important to know how to use a compass if you plan on venturing into the backcountry. First, a quick explanation on what a compass is. A map compass has a magnetized needle in a liquid-filled vial that can be rotated relative to it's clear base.

Around this vial is an azimuth ring that indicates 0-360 degrees. Declination (explained below) can be accounted for by offsetting the orienting arrow the appropriate number of degrees east or west. Map compasses can be used for navigating precisely with or without a map.

The magnetic needle on a compass points to magnetic north. The dial, orienting arrow, and sighting line are used to help you find East, West, South, and the points in between.

Declination

Unfortunately, true north and magnetic north are not located at the same place. A compass needle points to magnetic north; the number of degrees East or West that magnetic north and true north are separated by is known as declination. Many compasses have built-in declination adjustments; others do not. For accurate compass readings, it is necessary to account for declination. The isogonic chart at right roughly shows declination for the United States.

If, for example, you are in the White Mountains of New Hampshire, you will have a west declination of about 16 degees, meaning that magnetic north is 16 degrees west of true north. By aligning your magnetic needle 16 degrees west of true north (360 degrees minus 16 degrees, or 344 degrees), the azimuth ring will then be correctly oriented for true north. For compasses that do not have built-in declination adjustments, it is common practice to place a small piece of tape at the declination point of the azimuth ring. In other words, if you are in an area with a westerly declination, align the magnetic end of your compass needle to 360 minus the declination. If, however, you are in an area with an easterly declination, align the magnetic end of your compass needle to the number (degree) on your dial that is the declination.

Taking a Bearing

A bearing is a degree reading taken from your position to another object. If, for example, a mountain peak was directly east of your position, the bearing of the mountain would be 90 degrees. If the peak was directly south, its bearing would be 180 degrees, and if the peak was directly west, 270 degrees. To take a bearing, hold the compass level in front of you. With the sighting line on the base of the compass pointing straight at the object you are taking a bearing on, turn the dial until the magnetic needle is lined up with your declination tape (or the orienting arrow, if your compass has a built-in adjustment for declination). The degree reading indicated at the sighting line is the bearing to your object.

Using Map & Compass for Navigation

I'd like to explain how to pinpoint your location while you are backcountry. You can locate yourself by the intersection of lines. If you are already on a marked trail, or along a stream, or on a ridgeline, then you already have one line, and need only one more to determine your location on the map. If you aren't on a known line, then you'll need two lines to determine your location. To get the line(s) you need, take bearings on identifiable landmarks. When you plot these bearings on your map, their point of intersection indicates your location.

First, use your compass to orient your map and place the map on a flat surface (the ground) and anchor the corners with rocks. Then, take a bearing on a landmark some distance away that is also identified on the map. Next, set the compass on the map with a front corner of the baseplate aligned with the landmark. Now rotate the compass, keeping the pivot point on the landmark, until the magnetic needle is again aligned with your declination tape. Draw a line along the edge of the compass base, from the pivot point back. Repeat this procedure with another landmark; the intersection of the lines on the map indicate your location.


Copyright © 1996, David Lister


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Document last modified on Friday, 19-Jan-2007 07:17:39 MST.