Navigation: Dead Reckoning

by Phil Romig Jr

 

A story in the last issue told about a trip to “Lost Lake” where we missed a trail junction and ended up camping 1,000 feet directly below the lake. We had not spent enough time studying topographic maps and satellite photos to know where to expect the junction.

 

Later that year we decided to try again, and that time we did our homework. However, the planning alone would not have been enough. The trail junction was so little used and overgrown that we would have missed it again had we not been tracking our progress on the map. Using dead reckoning (backed up by GPS), we were able to determine where the junction should be and then search the area till we picked up the trail to the lake.

 

Today, people ask: “Why not just export the route to a GPS receiver and follow it?” When I was starting to teach geophysical exploration, an experienced old-timer insisted that we use four-wheel drive only to get out of trouble. If we used it routinely and got into trouble, there would be nothing left to get us out. GPS is like four-wheel drive: GPS receivers are electronic devices, subject to failure due to bad components, dead batteries, moisture, weak signals and other problems. If you rely on them and they fail, you are up the proverbial creek without an alternative. GPS receivers make navigation easy and can be lifesavers, and the next issue will focus on the use of GPS technology (just in time for Christmas). But the primary form of navigation should be dead reckoning, and GPS should be the back-up.

 

Someone once told me that the term “Dead Reckoning” came from “If you don’t Reckon right, you will end up Dead.” Wikipedia© suggests that the term may have originated as “Ded” (for deduced) Reckoning. Regardless of the origin of the name, the technique has been used by travelers for several thousand years. It simply means that, if you know where you started, what direction you have been traveling, and how far you have gone, then you can calculate where you are now. In the early days, it may have been as basic as: “Go toward the setting sun for three days, then turn right for one day.” Today, the parameters are much more precise, and the challenge is making the measurements and calculations easy enough to do in the field and accurate enough to reach the objective.

 

Dead reckoning was first used at sea where there were no landmarks. One of the earliest navigational instruments was a ship’s “log”: a large piece of wood attached to a cord that had knots at uniform intervals was thrown overboard. As the “sea anchor” (piece of wood) drifted back and pulled out the cord, the navigator counted the knots for a fixed period of time. That was the origin of the “knot” as a measure of speed and the “nautical mile” as a measure of distance. On land, the “mille passuum” (“thousand paces” in Latin), adopted by the Roman legions for their conquest of Europe, was the precursor to our mile. The “pace” (two steps— one right and one left) still is the basis for dead reckoning on hikes or pack trips today and, for most of us, a thousand paces is about a mile.

 

While there have been a variety of techniques for measuring distance, a single invention revolutionized direction-finding. The magnetic compass made it possible to determine direction even when the stars or sun were not visible and enabled travelers to follow a course, day or night and in any weather. Everyone on a backcountry trail should have two compasses—one for regular use in dead reckoning and a smaller one in their survival kit in case the first one is lost or broken.

 

Traditionally, dead reckoning requires counting paces (or using a pedometer) and watching a compass while on the trail. Whenever there is a significant change in direction, stop and convert the number of paces to the map scale, correct the direction for magnetic declination, lay a straight-edge in the right direction, mark off the distance, draw the track on the map, and plot the current location. In principle, this works well, but in practice, it is often not done. Many people don’t have the skills or are not willing to invest the time to do it. I’m in the latter category; once on the trail, I want to spend time hiking, not calculating and drawing.

 

On recent trips, a different approach has been more effective. Instead of doing the calculations on the trail, they are done while planning the trip, and the results are printed on a map. After the route has been determined, the steps are:

  1. Put waypoints at “inflection points” (where the route makes a noticeable change in direction) in the route.
  2. Enter coordinates of the waypoints in a spreadsheet.
  3. Have the spreadsheet calculate distances (in paces) and compass courses (taking into account declination) between waypoints.
         • Remember that a “pace” is two steps—one right and one left.
         • If you don’t know your pace, 5.3 feet (1,000 paces per mile) is a good guess.
  4. Export a copy of the map to a format that will allow you to draw and print on it.
         • If all else fails, print a hard copy and write on it with an indelible pen.
         • Include the waypoint markers and, if convenient, lines between the waypoints.
  5. For each trail segment between waypoints, print the compass course and distance (in paces) on the map.
  6. Note any special features (landmarks) near each waypoint that will help you recognize it and write a description on the back of the map or separate piece of paper.

With this done in advance, dead reckoning becomes quick and easy. At each waypoint, aim the compass in the direction printed on the map for the next waypoint, and walk in that direction. When you have gone the number of paces printed on the map, use the description to verify that you are at the next waypoint. If you reach a waypoint where the landmarks don’t fit, or the trail on the ground goes north while the course is west, then you have a different problem: figuring out where you are. That will be the subject of a future article. If all else fails, you can backtrack to the previous waypoint and try again.