I've been considering the practicalities of employing altimeters when navigating in mountainous areas. Yes, I know that handheld satnavs will do the job much better, but there must still be a few walkers who rely on them.
I have two altimeters which I used for my experiments in the Chiltern Hills. My Garmin Foretrex 401 incorporates a barometric altimeter with a claimed accuracy of ±1 metre. The barometric altimeter in my Suunto Vector wrist computer has a barometric altitude mode with a claimed accuracy of ±5 metres.
Over the past few weeks, I've kept a record of the elevation data recorded on both instruments with some surprising results. In both cases, I never walked more than a kilometre before recalibrating against a known height. The Foretrex was often spot on and never more than 3 metres out, but the Suunto was often as much as 15 metres out and I should not care to rely on it in poor visibility in the mountains.
Factors that can affect the accuracy of altimeters include the calibration interval of the instrument; fluctuations in air pressure (which can be considerable in the UK); the humidity of the atmosphere (air with a high water content is heavier than dry air); the temperature. Also, spot heights and contours are subject to minor inaccuracies. Further information can be found at
www.hills-database.co.uk/altim.html.
Altimeters can
sometimes be used to establish your position. This technique can only be used when climbing or descending steeply otherwise the distance on the ground between contours is too great to be of much use. If the altimeter is accurate to ±5 metres on a 10° slope angle (18% gradient) your position could be off by up to 29 metres in either direction. On a 20° slope angle (36% gradient) you could be off by up to 15 metres in either direction. On a 22° slope angle (40% gradient) you could be off by up to 13 metres in either direction.
If the altimeter is accurate to ±1 metre, on a 10° slope angle (18% gradient) your position could be off by up to 6 metres in either direction; on a 20° slope angle (36% gradient) you could be off by up to 2.9 metres in either direction; on a 22° slope angle (40% gradient) you could be off by up to 2.7 metres in either direction.
When relying on an altimeter as a navigational aid in poor visibility it is essential to fix your position against known heights as frequently as possible. This can be done at the contour nearest to any feature that can readily be identified on a map such as trig points (these are few and far between); summits; bodies of water (lakes, lochs, tarns and loughs); footbridges; junctions of paths, walls when following a path; junctions of walls when in pathless terrain.
When relying on pacing and/or timing as an aid to navigtion, you have to take into account the forshortening effect of depicting steep slopes on maps. On a map showing a slope angle of 27°, for every 100 metres measured on the map, the distance on the ground is actually 112 metres. Over a distance of 500 metres this discrepancy is significant and has to be taken into consideration. A credit card-sized calculator for measuring the slope angle on 1:25,000 and 1:50,000 maps, and calculating the extra distance, can be obtained from
www.shavenraspberry.com.
Hugh
I grow old...I grow old, I shall wear the bottom of my trousers rolled. T.S.Eliot