Dead reckoning (DR) is as old as navigation itself. Highly logical and ridiculously simple, the idea behind it is this: "If I go in this direction for this distance, I must be right here." For example "If I go north from my starting point for two kilometers, I must be two kilometers north of my starting point."
Aaaaargh! Where Might We Be?
DR almost certainly started with mariners, and has been used to cross entire oceans. Thanks to DR, Christopher Columbus made it to what would later become known as the New World, and back.
Before the invention of a reliable sea-going chronomoter, one's longitude was seemingly next to impossible to determine. But it was--and still is--as easy as pie with a good sextant to find your latitude. Many ships have navigated the high seas, and some probably still do, knowing only their latitude and their best guess at longitude determined by DR.
There is, as you might imagine, an element of navigation error that inevitably creeps into DR. In fact, the US Coast Guard says the average DR position error is around 15 percent. Even with that amount of error, a good navigator can still have a fairly accurate, if not exact, idea of position. And a fairly accurate idea is usually good enough.
Dead Reckoning for Land Navigation
GPS and other fancy instruments aside, there are only two ways to navigate:
Normally you'll use terrain association, making good use of baselines, checkpoints, and other elements of pathfinding to find your way. Whenever you can see what's around you and can relate it back to your map, you can know exactly where you are.
Sometimes, however, you can't see more than a few feet in any direction. This might be during a whiteout when fog or blowing snow reduces visibility to only a few feet. Or it could be during a "greenout" when you're surrounded by dense forest. At these times, terrain association doesn't work because you can't associate your position with the terrain.
So, what's a poor land navigator to do? As long as you at least know starting point, you can dead reckon from there.
As the explanations below will tell you, you'll keep up with where you are by knowing not only the direction in which you're traveling, but also the distance you've traveled in that direction.
Because you're an imperfect two-legged, land-bound creature making your way as best you can, you'll hardly ever travel in the exact direction you think your traveling in, and you'll almost certainly always be off somewhat in your distance calculation.
In spite of these errors, if you're careful, you can still have a pretty good idea of where you are. And a pretty good idea is a whole lot better than no idea. That's the idea behind DR--not necessarily to know exactly where you are, but to maintain a pretty good idea. The longer you must navigate solely by DR, the greater your navigation error is likely to be.
But once you again find some recognizable terrain features around you and can relate those features to your map, you can then pinpoint your exact position. Any accrued navigation error from your dead reckoning instantly evaporates because you now know exactly where you are.
In this manner you'll make your way across the terrain going from an exact terrain-associated position and when necessary to an inexact DR position, then back to an exact terrain-associated position. So, you don't dead reckon all the time, only when it makes sense to do so.
If you carefully keep up with your direction and distance, you can have faith in your DR position. If mariners can cross entire oceans using DR, you can certainly use it to find your way overland a few kilometers.
So, let's get into the nitty gritty of how dead reckoning works, and see how it's done...