A Basic Land Compass Course

Land Navigation and Compass Skills

Frank R. Leslie, 4/4/2001

Maps are laid out with left-to-right lines of latitude in degrees. From top to bottom are lines of longitude, also in degrees. (Some maps use minutes of angle, where there are 60 minutes between degrees.) Only near the equator do the lines form a square. Since the longitude lines come together (converge) at the poles, the lines form tall rectangles at other latitudes. Latitude varies from 90 degrees north to 90 degrees south, while longitude varies from 180 degrees west to 180 degrees east. The longitude origin of 0 degrees is essentially at the Greenwich Observatory in England. In Lander WY, for example, the latitude is approximately 42.8 degrees north, and longitude is approximately 108.75 degrees west.

On a topographic map, local features such as rivers, mountains, and vegetation are shown on contour lines of constant elevation above mean sea level. When you know you are on a certain trail at a stream crossing and you can recognize a known mountain peak, you can easily find and verify your location on the map.

Deep in the woods, you cannot see distant landmarks. You must then rely upon where you last knew where you were, what direction you where going, and how fast. This extrapolation or "deduced reckoning" provides an approximate location until you arrive at a location where you can determine or "fix" your position.

In an open field or on a peak, there are many landmarks to choose from. In fact, there may be a confusing number of landmarks, and care in choosing and identifying them is required.

A magnetic compass is used to determine the direction of magnetic north, shown on maps by a half-headed arrow. True north, which is used for map azimuths and locations, differs from magnetic north by the variation (nautical name) or declination (land name). In Lander WY, the declination is approximately 16 degrees east of true north. The compass needle thus points 16 degrees east of true north, and true (geographic) north is at 360 degrees - 16° = 344 degrees magnetic azimuth on your compass. (The declination changes with time, and is printed on the topo map margin as of a certain date.)

To make sightings or map reading easy, rotate the compass base until the north end of the needle points at 16 degrees. The compass "rose" (scale of 360 degrees) will then be oriented north. Place the compass over your map location. The horizontal map can now be rotated under the compass so that 0 degrees true aligns with the longitude lines and the directions to landmarks are certain. In practice, this is done quickly and approximately.

Once you know where you are and where you want to go, you can use the compass as a protractor to measure the desired azimuth angle (clockwise from north). Orienting the compass to point at the declination line, you can sight over the desired azimuth angle mark to select a distant object to serve as a landmark. You then can hike to the landmark without a lot of reference to the compass. When you get to that landmark, use the compass again, repeat the process, and choose a new landmark at the correct azimuth.

Distances are harder to estimate on a winding trail. Four miles per hour walking on a sidewalk turns into two miles an hour with a daypack on a trail, only one mile per hour with a full pack, and perhaps one-half mile per hour in dense undergrowth "bushwhacking" across country. From the estimated speed times the elapsed time, you can estimate the distance traveled and the direction from the last known location.

Occasionally, you may want to find your position more accurately. If you recognize a mountain peak at 200 degrees true azimuth, then you are on the reciprocal azimuth line of 200 - 180 degrees = 20 degrees. If you draw a line on the map at an azimuth of 20 degrees from the peak towards and well past your position, you are nearly on the line. Suppose that you drew a line from another peak in the same manner. These two lines will cross, giving you an initial position estimate. Repeating the process will show a line crossing the previous lines, forming a "small" triangle. The more accurate the measuring process, the smaller the error triangle. Your best estimated position is somewhere near the middle of the triangle. There may still be significant error remaining, thus a fourth sighting will reduce the confusion and error further.

This process of comparison of your map with the real world should be practiced especially when you know where you are. These skills will be invaluable when the time comes that you don't!

From www.oocities.org/Yosemite/1270/general/compass.html by Frank R. Leslie , copyright 1996, 2001