Author Topic: Altimeters  (Read 3126 times)

Hugh Westacott

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Altimeters
« on: November 12, 2014, 03:13:23 PM »
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

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Re: Altimeters
« Reply #1 on: November 16, 2014, 08:22:09 AM »
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

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Re: Altimeters
« Reply #2 on: November 16, 2014, 10:58:24 AM »
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

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Re: Altimeters
« Reply #3 on: November 16, 2014, 04:24:35 PM »
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 √(1.8+2.1) = 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
« Last Edit: November 16, 2014, 04:33:22 PM by Hugh Westacott »

Locus

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Re: Altimeters
« Reply #4 on: November 16, 2014, 10:12:41 PM »
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

krenaud

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Re: Altimeters
« Reply #5 on: November 17, 2014, 11:02:37 AM »
Hugh,

My only experience from altimeters is a five year old Suunto which had bad GPS-reception and poor altimeter which gave me very varying distance and elevation data on the same route. It didn't instill any confidence for using the altimeter for any purpose except perhaps to detect a change in the weather.

I will be getting an iPhone 6 in a few weeks time which has a built in barometric sensor, I am looking forward to see how well it performs and I will report back when I've tried it.


Lost Soul

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Re: Altimeters
« Reply #6 on: November 17, 2014, 08:49:54 PM »
In my opinion and experience the barometric Altimeters on these navigation gadgets are somewhat cheap and nasty.  Nevertheless they are a lot more accurate that GNSS 3D vertical position information.   Couple of years ago I bought a Suunto Ambit (Sports and Navigation watch) in the sorry belief that the altimeter would be reasonably accurate and position fixing on bland terrain would be enhanced.

Unfortunately I very rapidly had similar experienced to Hugh. So I set up a little test. 

Stable weather conditions.  In the car at the top of a hill take a reading.  Drive in a generally down hill direction stopping a couple of time to take a reading.  Bottom of the hill take another reading turn around a go back up the hill.  Stopping to take readings in the same spots as on the way down as well as at the top.  Surprise, surprise big variations in the order of 20 to 30 feet.  Yet the time to conduct the complete test was only 10 minutes.  Repeated it a couple of times with the same sort of results.

Given that there was only a 10 minute gap between the two sets of readings on the hill top then the differences could not be put down to atmospheric pressure changes,  Remember 1 mbar change in pressure represents a height difference of 30 feet.  And in the worst of storm conditions a 1 mbar change per half hour means you are in the midst of something verging on biblical in its advance and ferocity.
 
I would keep the thing on my desk nailed to a reference altitude (i.e a fixed data point around which other readings and displays are meant to move) and note what it did.  The reference altitude would drift in quite a dramatic way over a period of a few days.  In fact it had a habit of making near step changes. 

As a consequence I had a couple of "meaningful" discussions with Suunto.  In the end they agreed to replace the unit for me.  The replacement is much more stable and the hill test produced much more repeatable results.  Interestingly it had to go back to them by Fed Ex.  The van driver said to me when he picked up the watch.  Oh we get to collect a lot of Suunto watches for warranty return.

For a hand held Sat Nav I use a Sat Map Active 10.  That provides height information which correlates extremely well with the contour date on the displayed OS maps.  Its derives it height information by interpolation of data from its digital mapping data base. 

No reliance on GNSS 3D Vertical plotting or separate barometric altimeter.  Just uses GNSS 2D position fixing.  If that is accurate and the height information in the mapping data base is accurate then what the gadget is telling you is accurate; notwithstanding the inherent errors described by Hugh above.

Brynglas

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Re: Altimeters
« Reply #7 on: November 22, 2014, 10:25:32 PM »
I spend a lot of time in the hills in the UK and find that a gps combined with good map and compass skills negates my need for an altimeter.
I also visit higher mountain areas during the year, Mountaineering and ski touring. Again, the improvements in gps technology over the years has found me needing my altimeter less and less.
Thinking about it, the skill of using a barometric altimeter is probably worthwhile maintaining.

JK

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Re: Altimeters
« Reply #8 on: October 13, 2015, 07:47:09 AM »
[I know it is an old topic, but I thought I'd add my thoughts]

Having raced at the top level in Mountain Marathons for many years (where GPS has never been permitted), altimeters are a great tool for navigation (except for ocean/coastal navigation!).

You need to check the reading before you set off and regularly throughout the day. No need to change it, just remember it is reading 20m high, that sort of thing. Even if you have forgotten to check how it is reading you can use it to contour long slopes by keeping the same height.

I've never used a GPS for navigation but as I understand it if you lose the GPS signal they aren't much use.

If you lose the air pressure for your compass then you probably have worse problems.

Suunto Vector isn't cheap, but it does the job nicely.

Angle of Repose

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Re: Altimeters
« Reply #9 on: October 16, 2015, 01:54:15 PM »
First off, excellent post Hugh.
My watch is a Casio that has a digital compass/baro/altimeter function and have been utilizing the altimeter function quite often on hikes. I recently hiked Mt. Washington in New Hampshire (US) and used it as a guideline to have an idea how much further it was to the top (6,289'). They do come in handy as a nice backup and benchmark. I never did use my GPS altimeter. Then again, I don't use my GPS all that much. I rely on DR, maps and my compass.
"You can't get there from here"