UKC

As far as I can tell, GPS devices with an altimeter option use barometric sensing for the altitude - the same as my Suunto Vector. Are there any GPS devices which use any kind of locational positioning to give altitudes (or is this a stupid question)?

In reply to tony: yes. Megellans and some Etrex.

In reply to tony:
You can use the gps to calibrate the altimeter on my Garmin Geko 301. It's obviously not as accurate as using a known altitude or barometric pressure but better than nowt.

In reply to tony: Don't think my Garmin etrex uses a barometer, but uses the triangulation of the satellites to give an altitude. Depends on how many satellites it can see to determine accuracy, 3 or 4 in the valleys of the Lakes this Xmas, but full 12 when on tops. Accuracy down to 6m at times. I thought the accuracy was like a + or - figure for norths, easts and up and down, a bit like a virtual bubble which you are in somewhere.

In reply to tony: Not a stupid questin, but your statement is not correct. Any GPS-only device can and usually will give you a altitude as well as a position reading. However, the altitude reading will not be as accurate as the position reading. The following explanation is from http://gpsinformation.net/main/altitude.htm -

"Generally, Altitude error is specified to be 1.5 x Horizontal error specification. This means that the user of standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP of 1 for 95% confidence. Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry. If we (simplistically) consider just four satellites, the "optimum" configuration for best overall accuracy is having the four SVs at 40 to 55 degrees above the horizon and one (for instance) in each general direction N, E, W, and S. (Note: You will get a very BAD DOP if the SVs are at the exact same elevation. Luckily, this is a rare occurrence.) ... The similar "best" arrangement for vertical position is with one SV overhead and the others at the horizon and 120 degrees in azimuth apart. Obviously, this arrangement is very poor from a signal standpoint. As a result...the calculated solution for altitude is not as accurate as it is for horizontal position. Almost any calibrated altimeter will be more stable at reading altitude than a GPS."

There's lots more sites out there that discuss GPS accuracy in varying levels of detail. (Ignore anything that talks about Selective Availability or SA, as it has been turned off.)

In reply to Martin W:My Silva MultiNav gives both a altitude from the barometer and the gps system.

In reply to tony:

My yellow e-trex gives very accurate altitude......not barometric...I think it relies on getting 4 sattelite fixes.

My suunto-watchy-thing does altitude on barometric pressure but needs to be continually altered as the weather changes when you find reference points.

Only know how to use the e-trex for altitude and grid ref. just incase my navigation goes tits-up. Can't really beat a map and a compass and a bit of know how.

In reply to Removed User:

Particularly as GPS devices tend to tell you the straight line between points and not that if you follow the straight line you will walk over the top of a 700ft cliff....

To quote from Trail, Jan 2004, p39

"A GPS won't tell you about cliffs, rivers in spate, and avalanche-prone slopes sitting between your two entered waypoints. It will simply guide you on the shortest route between two waypoints."

Which isn't really what the original post was about, so, on that point, how frequently do GPS devices update, and if you walked over a 700ft cliff would you know from your GPS whether you were losing altitude before you hit bottom....

So, what I'm getting at is that a GPS should be used to help you locate where you are when you refer to your position on a map, and not used to say it's only 300m to that ridge in that direction - when you are in limited visilbilty, eg a whiteout....

Most of you probably know this, anyway

In reply to tony: Yes Gps should be used as aid to Navigation not a replacement for, im sure if you were accidentally lost you would be happy to know what hill you were stood ontop of, obviously you never want to get into that situation, but people do get lost.

In reply to Maffoo:


So does the eTrex Summit and the Geko 301. When I said "GPS-only devices" I meant devices which only have Global Positioning System (GPS) functionality, not additional functions such as baromatric altimeters and magnetic compasses. Sorry if that was difficult to understand.


Just bear in mind that your observations of the reported altitude vs the actual altitude do not mean that the device is any more accurate than the limitations of the technology allow it to be, which are as noted previously. Indeed, since the Ordnance Survey now relies heavily upon GPS to produce its maps (albeit much higher precision, survey-quality GPS), perhaps it's not surprising that there should be a close correspondence between them! It is still quite possible for a GPS altitude to be out by 75ft.


I think my GPS-12 will only report an altitude if it has at least four satellites in view. I'm not sure if four is the actual minimum requirement to derive an altitude from GPS data.


But that's not due to inaccuracy in the device itself, it's due to undetectable changes in the environment (ie sea level barometric pressure) which introduce an error in to the calculation of altitude from ambient barometric pressure.

The barometer in the Suunto Vector is stated to "have a resolution of" 1mbar. If that means that the pressure sensor is accurate to +/- 0.5mbar then the derived altitude will be accurate to +/- 12ft, ignoring the effects of sea level pressure changes. That's more accurate than the +/- 75ft accuracy of GPS altitude indicated above.

As I said on another thread, the best advice is to be properly aware of the limits of accuracy of the instruments you are using, and to treat the information they give you appropriately.


Having purchased an altimeter a while back, and carried it with me in the hills ever since, I do find myself wondering what use it is - beyond being a fun toy. If I have a map then I can find my altitude from that, using a GPS position fix if necessary. If I don't have a map then a GPS position fix and an altitude (GPS or barometric) tell me...nothing of any great use.

One possible use I can see for an altimeter is if you are climbing a prominent ridge in poor visibility. Assume that route finding is easy - you follow the ridge and there are no side ridges to confuse things - but you can't see how close to the top you are. If you know the altitude you started from and the altitude you're aiming for then an intermediate altitude reading can tell you where you are on the ridge and how far you are towards your goal, which could be useful eg if you wanted to know whether you have enough daylight left to complete the route. On the other hand, if you had a GPS you could get the same information directly from that. Why carry both? Especially in one unit, when if the batteries fail then both devices are unusable!


Amen to that.

In reply to Martin W:

It doesn't. There's no guarantee that the accuracy of any instrument is better than (or as good as) its resolution.

In reply to Dominion:
Pretty much like a compass then.

In reply to tony:

went to france this summer, didnt know which grid option to choose on my e-trex to suit the maps for the alps.....had to do without GPS. Surved OK

Any advice?

In reply to Li'l Zé:


You're right - but I did say "if", and I used it to derive an example, not make a definitive statement about its accuracy. I couldn't find a statement about the accuracy of the Suunto pressure sensor. (The resolution of the Vector in altimeter mode is actually "better" than in barometric mode since +/- 0.5mbar equates to +/- 3.4m approx whereas the resolution in altimeter mode is +/- 2.5m).

My GPS-12 gives position information as a ten-figure Ordnance Survey grid reference. That's a resolution of 1m, compared to the 15m generally recognised accuracy of hand held GPS. I reckon a six-figure grid reference is the best anyone could usefully resolve using a 1:25,000 map in the wind and rain, so in fact 15m accuracy is fine for most recreational purposes in the hills.

In reply to Li'l Zé:


The difference being that, if you set a bearing using a map and a compass then the map will show you the potential hazards en route. If you just plug in the bearing to the next waypoint from your GPS in to your compass then you may have no idea what lies in wait for you.

In reply to Martin W:
I suppose the argument really is that GPS makes it easier to believe (wrongly) you could get by without a map.
More a limitation of the user than the GPS though.

In reply to Li'l Zé:

Yep, but a gps could allow you to (fairly) accurately work out where you are on by referring to a map, whereas just a compasss reading (either from a compass equipped gps or a magnetic compass)won't tell you where you are.... but you do need to be able to read a map and work out where you are from the GPS co-ordinates...

In reply to Dominion: None of GPS, compass or altimeter is likely to be much use on its own. If all you have is a map then, provided you can see enough of the landscape to orient yourself and locate your position, you should be able to find your way about safely. A compass can help with that eg by taking bearings on visible landmarks. When visibility starts to drop then a compass makes it possible to follow a bearing, given that you have been able to determine your position in some way using map and compass together. When the visibility closes right down a GPS may be the only way to get a sufficiently accurate fix on your position to start from.

The big advantage of map and compass is that they don't require batteries and they weigh so little that it's no burden to carry spares in case of loss or damage.

A good helping of gumption is useful as well.

In reply to Removed User:
[...]

Don't go again?

It is mathematically possible to determine the altitude from three satellites (some GPS units even allow this to be done). However, since the altitude error is approximately twice the horizontal error for GPS readings, and the significance of a certain error is greater in altitude measurements than horizontal measurements (e.g. 50m difference in altitude is quite something, but 50m horizontally isn't usually that important) it is important to have a fourth satellite for error-checking.

For small *differences* in altitude, assuming unchanging weather, barometric measurements are more accurate than GPS - they have, in the past, been used to level geophysical survey grids - but the determination of absolute altitude using GPS is superior to barometric means, due to the huge uncertainties from the weather.

Altimeters have another use in the mountains - keeping an eye on pressure changes at a fixed position. If the altitude of your camp goes up over night, the pressure has dropped, and you are about to get dumped on from on high.

In reply to Goose Fat:On my multinav you can select the map grid ref system. If I remember correct on the map it tells you what the grid system it uses (in french though). We got ours working after playing about for a while.

In reply to Maffoo: Another bodge is to obtain the GPS coords of a known point on the map (e.g. your hut) as a way-point, and, while out hiking, use your GPS to tell you direction and distance from that way-point. You can then estimate your position on the map. Crude, not user-friendly, but on a par with knowing how to use a classic abseil - it could help out when things go pear-shaped.

In reply to everyone:

Many thanks for your contributions. The original thinking behind the question was that a GPS altimeter that used the positional locational thingy might be more accurate than a barometric altimeter, which would appeal to my gadget-anorak tendencies. Since it seems that this is not the case, I'll find something else to do with the money with which I might have bought a GPS.