Can't be far to the top now." How often have you thought that as you trudge on up seemingly endless grassy slopes that disappear into the mist ahead? And when descending, how do you know exactly where to turn off the ridge, or at what point there should be a path when you can only see a short distance?
Knowing the height you're at can be a great advantage - and a comfort! Coming down a misty featureless hillside not so long ago, I knew from the map that there was a path lower down and at what height it started - my altimeter told me when I'd reached it. Even when visibility is good knowing the altitude can be useful, as you can check your progress and work out how long it will take to reach a summit.
Electronic altimeters do more than announce the height. They can also give you ascent/descent rates, the total ascent or descent over a given period, barometric pressure and, usually, barometric trend - useful for weather forecasting. One model even measures windspeed. Other features can include thermometer, compass, pedometer and more. Oh, and they tell the time!
How useful these features are depends on how much data you like to collect, and what sort of trips you do. On walks of more than a day I find the barometric trend information valuable as I can make decisions as to the next dayâs route based on what I think the weather might do. On occasion I've set off on a high level walk in a storm because the pressure had risen rapidly overnight and then had the expected good weather later in the day. Seeing a rapid rise in the pressure after days of wet weather is always encouraging. Similarly a fast plunge in pressure has seen me choose a sheltered route despite the clear skies early on, a decision usually justified when the weather deteriorates later.
Air pressure is what altimeters measure and translate into height above sea level. So when the air pressure changes, an altimeter will show a change in height despite not having been moved. A rise in pressure is recorded as a fall in altitude and vice versa. So if your camp appears to be higher in the morning than it was the night before, the pressure has dropped. To compensate, an altimeter should be reset every time you know the exact height you're at; otherwise it can become very inaccurate. As an example, I set an altimeter to the height of my house then recorded the apparent changes in altitude over the next 24 hours. This showed that the house apparently rose by 140m, then fell 45m, to finish 95m above its actual height. Over a three-day period I established a 280m difference between the highest and lowest heights recorded, without the altimeter being moved a centimetre. Clearly, if an altimeter isn't reset regularly it can't be relied on.
Different instruments display the pressure in millibars (mb) or hecto-Pascal (hPa). As 1mb equals 1hPa this can be ignored. Some will also give barometric readings in inHg (inches of mercury) if you prefer. All the altimeters are metric but some can be changed to imperial units. Pressure can be given as sea level pressure, used in weather forecasts, or the pressure at your current altitude, known as the absolute pressure.
All the altimeters come with detailed instruction manuals that need studying if all the functions are to be used fully.