Distance is the part of navigation people get wrong most often and worry about least. Direction errors are usually obvious within a few hundred metres; a time estimate that is thirty per cent optimistic is invisible until the light goes.
This page is mostly about being honest with yourself, which turns out to be the difficult part.
The base rule
Naismith's rule, which is more than a century old and still the working standard:
Count ascent from the contours. Add up the contour lines you cross going uphill along the route, multiply by the interval, and you have your climb. On a route that goes up and down repeatedly, count every climb, not just the difference between the start and the summit.
Descent is not really free. Gentle descent is quicker than flat; steep descent is slower and much harder on knees, and on rough or wet ground it is often slower than the ascent was.
This is a fit individual, on good ground, with a light pack, in good conditions. Almost nothing about a real day matches that.
Adjusting it honestly
These multiply. A mixed group, on rough ground, in the dark, is not slightly slower: it can easily be less than half the base speed, and the arithmetic surprises people every time.
Then add a margin on top. A third is not pessimism, it is normal. Stops to eat, to look at the map, to adjust boots, to deal with a blister and to make a decision all take real time that no rule accounts for.
A group moves at the pace of its slowest member, and that pace declines through the day. The estimate that matters is the one for the last hour, not the first.
Measuring distance on the map
- Follow the route, not a straight line. A winding path can be half as long again as the direct distance.
- Use the edge of a compass, a piece of string, or the marks on a map case and lay them along the route in short straight segments.
- The grid squares are the fastest method. On British 1:25,000 and 1:50,000 mapping the grid lines are one kilometre apart, so counting squares along a route gives you the distance immediately, and a diagonal across a square is about 1.4 kilometres.
- Add for ground you cannot see. A path that appears straight on the map may weave considerably around obstacles.
Judging distance by eye
Useful for checking, and less reliable than people believe.
Distance is routinely underestimated across water, snow, and uniform ground, and when looking uphill at a large object. It is overestimated when looking downhill, into a low sun, and across broken ground.
Rough visibility markers for an adult figure:
- At 100 metres you can see the eyes and the mouth.
- At 200 metres the face is a blur but clothing colours are clear.
- At 400 metres the body outline is clear and the head is a dot.
- At 800 metres you can tell a person from a post by the movement.
Sound gives you distance too. Light travels effectively instantly and sound covers roughly a kilometre in three seconds, so counting between a flash and a bang gives a usable distance to a thunderstorm.
Check your judgement against the map, deliberately, when you can. Most people find they are consistently wrong in one direction, which is a correction you can then apply.
How much daylight is left
This is rough and it is genuinely useful, particularly because it works without knowing the date or the sunset time.
It also underestimates twilight, which at British latitudes is not brief. Around midsummer there is usable light long after the sun has gone; in December it goes quickly. Know roughly what the sunset time is for the season rather than relying on the sun's position alone.
Cloud, valleys and hills all steal light early. A deep valley or a north-facing slope goes dark noticeably before the sky does.
The turnaround time
The single most useful discipline on this page.
Before you set out, work out the time at which you must turn back in order to reach safety in daylight, and commit to it out loud.
Then honour it regardless of how close you feel to the objective. The pull to carry on when the destination is nearly in sight is exactly what produces benightings, and it is strongest at the point where the decision matters most.
Build the turnaround from the return leg, not the outward one. The way back is usually slower, because everybody is more tired.
Pace on the ground
Knowing distance covered, rather than distance planned, is what keeps an estimate honest.
- Pace counting for short legs. Measure your own count over 100 metres and write it on your compass. See taking a bearing.
- Timing legs for longer distances. Note the time at every identifiable feature.
- Tick off features as you pass them and compare the real time against your estimate. If you are already 20% behind at the first checkpoint, you will be 20% behind at the end, and that is the moment to change the plan rather than at dusk.
- Recalculate at every rest stop, using the pace you have actually achieved rather than the one you assumed.
The estimate that matters is the one made from real data an hour in. It is nearly always slower than the one made at the kitchen table, and it is the one to plan the rest of the day around.