Equipment > General navigational Kit

Altimeters

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Hugh Westacott:
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

Hugh Westacott:
I'm disappointed by the lack of response to my post about the use of altimeters. I've learnt a lot from this forum and I believe that it would be a pity if, now that Lyle's guiding hand has been temporarily withdrawn, it quietly dies through lack of interest.

Hugh

boogyman:
Speaking for myself, there is certainly no lack of interest Hugh. But I observe (in my own behavior) that a decreasing number of posts impacts the number of times I come to visit the forum. It's been a week since my previous visit here -- so I could not have reacted to your post yet.

Speaking about it... what I have often wondered (and I cannot remember a discussion on that topic), is the accuracy of the topomaps themselves. Their contour lines for example, how accurate are they? Isn't their accuracy (in general) at least as bad as the accuracy of the instruments we tend to rely on?

Hugh Westacott:
Now you have opened a can of worms, Chris!

What is meant by the accuracy of topographical maps is extremely complicated and my limited knowledge of the subject is confined to the (British) Ordnance Survey. This is the best summary that I have encountered and is taken from.http://www.hills-database.co.uk/altim.html mentioned in my original post:

Map errors

For contours with a 10m vertical spacing, the OS quote an accuracy of ±5m and an rms* error of 1.8m (ref. 3). Harley's book (ref. 2) gives some results of accuracy tests on contours which, if typical, would imply that bias can be ignored for practical purposes.

There is less information on spot heights or trig points as only estimates of the maximum error are available. Harley gives these as ±3.3m and ±2ft respectively (ref. 2), although the latter figure is subject to the method of measurement and no general figure can be quoted for trig points. (This error is quoted in feet as the OS network of levelled heights was established in the days of imperial measurements.) If bias is negligible for contour heights it seems reasonable to suppose that this will also be the case here. To estimate the standard deviations it is necessary to make an assumption about the distribution of the population of errors within the quoted accuracy ranges. The maximum expected error in any test accepted by the field accuracy testing group of the OS is defined as three times the standard error plus the systematic error. The OS only systematically test their maps for planimetric errors and contour heights so there is no assurance that this yardstick is applicable to the quoted errors for spot heights and trig points; however if we assume this to be the case, and negligible bias, the standard deviation of a spot height would be estimated as 1.1m and a trig point, 0.2m. The question of the additional error in trig points introduced by metrication is not mentioned by Harley. The error due to rounding to the nearest metre will have a uniform distribution on (-0.5, 0.5m) with standard deviation 0.5/√3. This would increase the standard deviation of a trig point to 0.35m.

When a starting height is estimated by interpolation between two 10m contours, an estimate of the standard deviation of interpolation is needed. If the walker had no idea of his vertical position between the contours then the standard deviation of the interpolation error would be 2.9m. In practice he will have some knowledge and we have subjectively estimated the standard deviation as 2.1m. Using equation 1, the standard deviation of a starting height based on contour interpolation is √(1.8²+2.1²) = 2.8m. This assumes that the errors in the adjacent contour lines are highly correlated, for which we have no proof but which seems intuitively reasonable. Multiplying by 3 and rounding gives the maximum error of 8m quoted in table 1 of the main text.
*rms means 'route mean square' which is a method of averaging errors to give amore accurate overall result.

There is another source of error not mentioned in the article. Cartographic draughtpersons occasionally make mistakes. Some years ago, I discovered a spot height in Kent, approx 40 km southeast of London, given as 498 metres, when the greatest elevation in the county is 251 metres. The correct height is 152 metres.

As practical navigators, we set our altimeters to the altitudes given on the map and if, as seems likely, that any deviation from absolute accuracy is reasonably consistent in the contours surrounding the immediate area, any error is unimportant.

The authors state that they found little difference in the accuracy of altimeters with a stated accuracy of ±1 metre and those with a stated accuracy of ±5 metres. I certainly did, and common sense seems to suggest that this would be the case.

Hugh
I grow old...I grow old, I shall wear the bottom of my trousers rolled. T.S. Eliot

Locus:
Overall I agree with Boogyman. Being used to very active forums, it took me a little while to get used to the different pace / activity level of the MN forum but that does subsequently determine how often I visit because the pace tends to mean several days before some posts are responded to, or a topic is added to or created. One recent post of mine (feedback though, not a question) made on the "water myth" thread at the end of the October had no following posts after several days of visiting to see if there were any follow ups to it / alternative opinion. People either have something to add, or they don't. Various posts I see appear, I lack knowledge of the subject to be able to add any meaningfull input but any new posts are still read anyway. In regard to altimeters, I also have the Foretrex 401 but haven't had conditions to truly utilise the altimetre to its potential for navigation. Occasionally I've looked at it out of curiosity while at known heights and also found it was pretty close. Most of my recent trips have been overseas in conditions that were spotless blue skies though, so the altimetre was only a gimmick function  ;D

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