Your altimeter is a universally useful device to help you navigate, increase personal and group safety and provide extra interest on your journey and even helps predict the future (weather!) Regardless if it is an ultrasophisticated digitial or a classic analogue style device, whether you walk, drive, ski or trek its a great lightweight addition to your equipment that used properly more than justifies its carriage.

Unlike a GPS, an altimeter does not use any external systems for its accuracy. It works just as well in any weather, with our without access to the sky (so is also useful when caving.) Analogue altimeters need no battery so are almost impervious to cold and incredibly reliable. Digital altimeters are typically very flexible and can support a range of additional helpful features. However because they are self-contained they need a frame of reference from which to work.

RELATIVE VERSUS ABSOLUTE ALTITUDE

Absolute altitude is the height above (or below) sea level. Relative altitude is the height above (or below) your reference point. The absolute method is particularly useful when locating your position on a map with appropriate contour lines or to determine how much further to go to a given landmark. Relative altitude measurement is most useful for targetting an altitude gain to avoid climbing too high in a day or on a descent to avoid overshooting and having to climb back up to camp or to avoid missing a vital navigation point. Night and poor visibility navigation can be significantly enhanced with the extra information provided by an altimeter. All altimeters are capabable of providing either of these readings depending whether the initial reference altitude is set at the height above sea level (for Absolute measurement) or set at an initial zero altitude (for Relative altitude.)

THE IMPORTANCE OF SETTING THE REFERENCE ALTITUDE

All altimeters measure atmospheric pressure and convert this to an altitude reading. Because atmospheric pressure is constantly changing due to weather effects the reading of the altitude of any given location will vary in sympathy with the barometric pressure. So, in low pressure altitude readings will be higher than normal, whereas under high pressure consitions, the reading may be lower than expected. The good news is that barometric pressure changes relatively slowly so it is possible if one has a known altitude of a landmark or other trig point to set the reference absolute altitude. From this known altitude, any increase or decrese in altitude will be relatively accurately displayed by the altimeter. The accuracy will depend on three main factors - a) the length of time since the last setting of the reference altitude and b) the barometric and temperature drift due to changing weather and/or location and c) the altitude change since the last calibration since air pressure does not change directly in proportion to height. It is therefore important to combine the use of an altimeter with an accurate map and to reset the reference altitude regularly. Temperature also has an effect on accuracy with lower temperatures causing greater air density and so causing a slight under-reading of any altitude gain or loss. In hot weather the reverse is the case with slightly less dense than normal air leading to inflated gain/loss meaturements. Temperature compensated altimeters will lesses this effect to a negligible level though.

ALTITUDE AS A PERFORMANCE INDICATOR

Most digital altimeters include memory functions to enable the user to record a combination of time and altitude metrics that can be later analysed to compare performance or be used in conjunction with a map to work out which route was taken or the total ascent/descent - especially useful for mountain bikers and off-piste skiiers. Ascent rate is also very useful as an indication of fitness level and in Alpine and high altitude mountaineering to ensure the rate of ascent leaves time for a safe descent is sufficient to ensure the next camp can be reached before nightfall. As well as providing onformation as to whether you are travelling faster or slower than anticipated it can also be used to help pace an ascent or guage the likely time to reach the next location.

NAVIGATION WITH AN ALTIMETER

Contouring (travelling across a slope while maintaining constant altitude) is very difficult, particularly in poor visibility. The tendancy to walk slightly downhill in these situations can be removed by simply monitoring for any inidicated change in altitude. For specific gain or loss of height over a given distance (traversing) it is possible to combine pacing the distance with the altimeter reading and to set a specific angle of travel route across any sloped ground. Upper and Lower limit alarms are useful for providing an indication that you have arrived at a predetermined altitude. This could perhaps be on the ascent to prevent over-ascending and suffering from altitude sickness or to locate camp or key navigational location or as a warning on the descent to avoid a cliff or crevasse noticed during the climb up. Combining map and traversing skills allows for direction finding on sloped ground. This technique is also useful for aiming at a target location althogh as with good compass use, it is usually better not to aim directly at your target but slightly away from it so that on arrival at a easier to identify nearby feature ( stream , wall, crag etc) it is a simple matter to turn toward the actual target rather than wonder whether you are slightly high or slightly low. The generally accepted wisdom of aiming-off with an altimeter is to aim low when ascending and aim high when descending which in both cases avoids overshooting the target and having to backtrack. Additional accuracy can be obtained using the altimeter in conjunction with a compass to locate a contour line along a given bearing and this does not require pacing regardless of how difficult the ground or conditions underfoot become. The compass can be used to take a bearing of the direction of the slope (known as the Aspect of the Slope) of the ground, this can then be transferred to the map. Now using the Altimeter to read the absolute altitude, where the nearest contour line crosses the Aspect of the Slope is your current location. This technique requires enough visibility to take an accurate reading of the Aspect and on shallow ground this is very hard to be accurate about but as with all navigational skills selecting the most appropriate method to use is a) down to having a wide range of options available and b) knowing how and when to use them.
 

WEATHER AND ALTITUDE

As mentioned earlier, barometric pressure is converted into altitude readings by an altimeter. So be definition an altimeter is also a barometer and therefore capable of making some weather observations. Generally a falling pressure of 2mbar or more per hour means poorer weather is on its way. Lower pressure is normally also associated with greater altitude so if your own physical altitude is constant but the altimeter is indicating that you have increased height by 30m or so per hour then barometric pressure is falling. The reverse is also true - better weather is normally associated with an increase in air pressure which will be shown as a drop in altitude. Because these changes are relatively small it may not be possible to accurately predict weather while moving. The fastest changing air pressure, particularly in the hills, is the wind. Bear in mind wind causes high pressure on the windward side and low pressure on the leeward side of any obstruction (boulder / ridge etc.) which in turn may change the measured air pressure by up to +/- 5mbar or so.