Estimating distances by eye

The brain judges distance from many clues at once: how big does an object whose real size I know appear? How much detail can be seen? How much haze lies in front of it? What lies in between? In everyday life this works well over short distances. Over longer distances in open terrain, though, we often lack practice, and the clues can mislead us.

The reference-distance method turns an unknown distance into a calculation. A football pitch is roughly a hundred metres long – a length many people know from experience. If you have internalised it, you lay it out several times in your mind across the terrain. For distant targets you first estimate up to a point halfway and then double; shorter distances are easier to estimate than long ones.

The amount of visible detail is a second, independent clue. Whether you can still make out windows on a house or single trunks at a forest edge tells you something about the distance. Which details you can see at what distance, however, depends heavily on light, weather and your own eyes. Fixed tables for this are therefore not very reliable – your own experience is better.

Targets often seem closer than they are: in very clear air (for example in the mountains or in dry regions), over snow, water or a uniform surface, when the sun is behind you and the target is brightly lit, and when you look uphill. Over a surface without intermediate points the eye lacks reference points and underestimates the distance.

Targets often seem further away: in haze, fog or against the light, at dusk, when the target is small or hardly stands out in colour from its background, and when you look across a hollow or valley that hides part of the route. These hidden valleys and dips in particular often make routes in the terrain longer than you think.

An estimate by eye is useful for roughly judging how long it will take you to reach a visible target, whether you will make it before dark, or whether a measurement on the map is plausible. It also helps in an emergency call: 'I can see the hut about a kilometre away below me' is a valuable description.

It is not suitable for decisions that depend on exact distances. And remember: distance alone says little about time. A target that looks close may lie beyond a deep valley, and every metre of height gain costs extra time and energy. Be generous when planning time, especially in clear mountain air or across snowfields.

Go to a sports field and memorise the length of the pitch from different angles – from the corner, from the middle, standing and sitting. After that, estimate distances in everyday life: to the next traffic light, to the church, to the forest edge. Check your estimate with the map, a map app or by counting your paces.

On hikes, before a visible target, estimate how far it is and how long it will take you, then note the actual time. In a group, each person estimates on their own without hearing the others; then you compare. The average of several estimates is often closer to the truth than a single one. With every trip your eye gets a little better.

Estimating distance by eye is taught, for example, in military navigation training, with reference distances and recognising detail as tools. For hikers, the exact figure matters less than plausibility: does what I see match what the map says? If, according to the map, the hut should be three kilometres away but it looks within reach, it is time to look more closely.

In an emergency, an estimated distance can help rescuers too – together with a direction and a landmark: 'About half a kilometre east of the summit cross, on the path down to the valley.'