Navigating with an altimeter

A barometric altimeter contains a small pressure sensor. Because air pressure decreases with height, the device can calculate a height from the measured pressure. It assumes a 'standard' atmosphere. If the actual weather differs – for example with a high or a low – the reading shifts. Many outdoor watches have such an altimeter built in.

For navigation, height is a third piece of information alongside direction and distance. On its own it does not give a position, because countless points lie at the same height – a whole contour line. It becomes powerful in combination: if you know you are on a particular path, ridge or stream, the matching contour usually crosses that line at only one place. That is where you are.

Because weather changes shift air pressure, an altimeter gradually becomes less accurate if it is not reset. That is why you calibrate it at points of known height: summit, saddle, hut, or a junction with a height given on the map or on a sign. After that it is right again – until the weather changes further.

Set the altimeter before you start at a point of known height, and in your trip plan write down the heights of key points: junctions, saddle, hut, turnaround point. On the way, compare at every confidently identified point with a spot height and reset. If the difference is large although you have only been walking a short time, that suggests a change in the weather.

Read the map's contours carefully. Every map has a fixed height step between two lines (the contour interval), for example 10 or 20 metres; it is given in the legend. If you overlook it, you can easily be out by many metres. Some devices are also sensitive to temperature changes, for example when a watch is worn on a warm wrist or outside on a cold jacket – check the instructions for your model.

A particularly useful trick in fog: if you want to traverse a slope at the same height to reach a hut or a saddle, the altimeter helps you hold that height. If you unintentionally descend or climb, the reading shows it even when you can see nothing.

An altimeter measures air pressure, not height. When a low moves in, pressure falls and the device shows a greater height than you really are at – and vice versa. So never descend into steep terrain by altimeter alone, for example to get 'level with the turn-off'. Always check with map, terrain and compass too.

A known height also says nothing about whether the way there is safe. Go around drops, gullies, water, avalanche or rockfall terrain, whatever the reading says. Glacier and avalanche terrain need their own training; this entry does not replace it.

The altimeter can also be a weather hint. If you stay in one place, for example overnight in a hut, and the displayed height rises, the air pressure has fallen – this can point to worsening weather. It is only a hint, not a forecast; it does not replace a real weather forecast.

GPS devices and phones also show an altitude. It does not depend on the weather but can vary with reception, especially in narrow valleys or in forest. The two methods complement each other well: GPS helps with setting, while the barometric altimeter gives steady readings even without satellite reception.

Take the altimeter on a familiar hike and set it at the start. At each junction with a spot height, read how far off the display is and note it. After a few trips you will know how quickly your device drifts in different weather.

Then practise combining: find a path on the map that climbs a slope, and try to fix your position using only height and path before you reach the signpost. This teaches you to read contours and terrain together.