Dead-reckoning is the simplest navigation skill. It’s much easier to learn than reading a map, or operating a compass, GPS, or altimeter watch.
Yet I find it to be one of the most useful and frequently used, especially when hiking on well-maintained trails where a consistent physical effort yields consistent results. Such trails include well-known long-distance footpaths (e.g. Appalachian, John Muir, Wonderland) and most trails in National Parks and high-use Wilderness Areas (e.g. Yellowstone, Indian Peaks).
This technique is less useful when hiking off-trail or on trails with extreme vertical change. Off-trail terrain is more variable (i.e. it’s sometimes great, sometimes awful), so results are less predictable. And in extremely mountains environments, forward progress is more a function of vertical hiking speed (e.g. 30 vertical feet per minute), and the horizontal distance over which this vertical change happens is less relevant (if not entirely irrelevant).
I use dead-reckoning to:
- Predict ETA at future landmarks, like water sources, trail junctions and campsites;
- Approximate my current location relative to the last or next landmark; and,
- Rule out “false” features like false summits and unmarked trail junctions.
In general, regular dead-reckoning helps to to “stay found” because it sets expectations about what should be happening. If reality does not match expectations (e.g. did not reach a landmark when expected, or reached a different landmark than expected), it’s a cause for a map-check.
To dead-reckon, two out of three following items are needed:
- Distance between two landmarks, derived from a topographic map, databook, or guidebook;
- Rate of travel, which can be measured (e.g. with a GPS unit) or calculated on past trips; or,
- Time, using a simple watch (although if you want to get serious about backpacking, I recommend a GPS sports watch).
Advanced tip: In mountainous environments, it is useful to know the vertical change between two landmarks, because this factor may influence your calculations. Vertical change between two points can be quickly measured on a map so long as the gradient is consistently up or down; in rolling terrain, vertical change calculations are more time-consuming.
How to dead-reckon
You might recall this formula from high school physics: Distance = Rate * Time, whereby:
- Distance = miles or kilometers
- Rate = MPH or KPH
- Time = Hours
The main formula can be reconfigured to solve for other variables:
- Rate = Distance / Time
- Time = Distance / Rate
To dead-reckon, essentially you insert two known variables to determine the third.
Tip: It’s useful to remember that 2 MPH = 30 minutes/mile; 2.5 MPH = 25 minutes/mile; and 3 MPH = 20 minutes/mile.
Examples of dead reckoning
1. I’ve been been walking non-stop for 1:20 minutes from my last known landmark, Crystal Spring. On this kind of rolling terrain, I generally walk at around 3 MPH. I’m planning to camp at Buchanan Creek, which is 5.5 miles from Crystal Spring, per my guidebook. How much further until camp?
Using the formula Distance = Rate * Time, I calculate that I’ve walked about 4 miles (3 mph for 80 minutes). That would leave 1.5 miles to camp.
2. Today is the first of a 3-day trip with two new guys I met on an ultralight backpacking forum. Normally I walk at 3 MPH, but our pace feels much slower than that — these guys have dialed kits, but it looks as if they never actually do any hiking. A 5-mile section of easy trail takes us 2.5 hours.
Using the formula Rate = Distance / Time, I calculate that we’re walking at about 2 miles per hour, or about one-third slower than my usual hiking pace.
3. My databook says that it’s 2.5 miles between Pawnee Lake and Pawnee Pass, and I know that I normally hike at 3 MPH. Using the formula, Time = Distance / Rate, I determine that it should take me 50 minutes.
However, these two landmarks are separated by 2,500 vertical feet, and yesterday on a similarly steep section of trail I climbed at about 25 vertical feet per minute. So I adjust my expectations: rather than 50 minutes, it will take about 100 minutes (2,500 vertical feet divided by 25 vertical feet per minute).