Dead Reckoning

Excerpts from Finding Directions without a Map or a Compass,
from the U.S. Armed Forces Survival Manual, Edited by John Boswell,
published by Rawson, Wade Publishers, Inc., New York, 1980.

Before moving from where you are in a survival situation, remember that keeping a record of your time underway is as important as maintaining a given direction. A log or detailed diary is essential not only to successful dead reckoning navigation, but to survival in general. For many centuries, mariners have used dead reckoning to navigate their ships when they are out of sight of land or during bad weather, and it is just as applicable to navigation on land.

Movement on land must be carefully planned. One's starting location and destination should be known or approximated, and -- if a map is available - carefully plotted, along with any known intermediary features along the route. These intermediate features, if clearly recongizable on the ground, serve as invaluable checkpoints. If a map is not available, the plotting is done on a blank sheet of paper. A scale is selected so that the entire route will fit on one sheet. A north direction is clearly established. The starting point and destination then are plotted in accurate relationship to each other.

If the terrain permits, the ideal course is a straight line from starting point to destination. This is seldom possible or practicable. The route of travel usually consists of several courses, with an azimuth, or angle stated in degrees, established at the starting point for the first course to be followed. Distance measurement begins with the departure, and continues through the first course until a change of direction is made. A new azimuth is established for the second course and the distance is measured until a second change of direction is made, and so on. Records of all data are kept and all positions are plotted.

For determining distance over land, a "pace" is the best unit of measure. A pace is equal to one natural step, approximately 30 inches. Usually, paces are counted in hundreds, and hundreds can be kept track of in many ways: make notes in a record book; count on your fingers; place small objects such as pebbles into an empty pocket; tie knots in a string; or use a mechanical hand counter. Distances measured this way are only approximate, but with practice can become very accurate. It is important that any person who might find himself in a survival situation predetermine the length of his average pace. This is done by measuring the length of ten average paces (in feet, inches, etc.) and dividing the length by ten. In the field, an average pace must often be adjusted because of the following conditions:

Slopes

The pace lengthens on a downgrade and shortens on an upgrade.

Winds

A headwind shortens the pace, a tailwind increases it.

Surfaces

Sand, gravel, mud, or similar surface materials tends to shorten the pace.

Elements

Snow, rain, or ice cause the pace to be shortened.

Clothing

Heavy clothing shortens the pace; the type of shoe affects traction and therefore the pace length.

Stamina

Fatigue affects the length of the pace.