My understanding is that a GPS is less accurate than a calibrated barometric altimeter for the followiwng reasons…..

1) The timing error is higher in the vertical direction so by default it is about 1.5x horizontal.
2) The stated elevation is relative to the WGS84 datumn which is a mathematical approximation of the Earth’s ellipsoid and mean sea level as opposed to local sea level.

I am happy with this but need to convince a mate. I can’t seem to find any definite links online so if anybody has any that would be appreciated.

Ta.

Does it really matter, especially if you’re both riding the same trail?

If it does, there’s something here but unless you’re planning to get into single-speed fixed wheel hang-gliding, I’d just not worry about it 🙂

Discourse

The timing error is the same, the issue is the geometry which magnifies the error much more in the vertical plane as the difference in heights between the satelites relative to the user is much smaller than the difference in the horizontal plane.

I expect it depends on how frequently you recalibrate the altimeter (and against what), compared to the rate of change of barometric pressure. Also how accurate the particular GPS and altimeter are. So I think you’ll struggle to find anything that’s ‘definitive’.

Are you measuring absolute altitude or change in altitude (such as a climb or descent)?

On personal experience, the altitudes I get from my standard commercial GPS are not as accurate as my altimeter – which is pretty good traditional ‘dial’ one with 10m graduations.

With the Mikrokopters I built, GPS is used for x/y positioning but barometric for z – and that’s even using differential GPS.

With the MKs, I get vertical accuracy of well less than 1m.

Barometric always more accurate over a short period, though if a pressure system is moving through you might find that it’s 500 feet out by the end of the day.

GPS is likely to be accurate to about 10m if decent signal, so for an all day hike that’s probably more useful.

Most effective is a GPS unit with a barometric altimeter built in, which is accurate in the short term and will use the GPS altitude to continuously update the barometer reference.

Good reference here: http://gpsinformation.net/main/altitude.htm

I am happy with this but need to convince a mate. I can’t seem to find any definite links online so if anybody has any that would be appreciated.

Might also be worth checking that you’re both arguing the same point. So for instance if you record a (ground based) GPS route, then display it on a computer you may get a very accurate altitude track simply because the computer’s accessing a separate height map and getting your height from the location.

Did a road ride on Saturday, my barometric altimeter/computer gave me a total of 4051ft of elevation whereas my friends GPS system returned a total of over 5000ft of elevation. Not sure which is the more accurate to be fair.

With the Mikrokopters I built, GPS is used for x/y positioning but barometric for z – and that’s even using differential GPS.

I thought dGPS died a death when the Americans switched off the error in normal GPS? All it did was broadcast a correction for a base station and the GPS figured out the actual position based on what the satelites told it and what the base station told it.

I’ve an edge 800 (barometric), fiends on Sunday had edge 200’s (GPS), mine says I climbed 360m, their’s say they did 200m. I’ve no idea which is more accurate though, but my old edge305 (GPS) agreed with using OS maps and couting contour lines and dividing by 2.

Thanks footflaps.

I found that one but it is from 2001 so was looking for something more recent.

I thought dGPS died a death when the Americans switched off the error in normal GPS?

Not really, given the best normal accuracy is still only +-15m and you can easily get better than +-1m using dGPS.

Did a road ride on Saturday, my barometric altimeter/computer gave me a total of 4051ft of elevation whereas my friends GPS system returned a total of over 5000ft of elevation. Not sure which is the more accurate to be fair.

I’ve an edge 800 (barometric), fiends on Sunday had edge 200’s (GPS), mine says I climbed 360m, their’s say they did 200m. I’ve no idea which is more accurate though

When it comes to ride/walk/climb stats the higher figure is always the most accurate 😉

Not really, given the best normal accuracy is still only +-15m and you can easily get better than +-1m using dGPS.

Way better than that – you can get mm accuracy over small areas.

Yup, the dGPS in the MKs is accurate to less than 20cms usually – and that’s real-world positioning accuracy, not the accuracy of the receiver.

It uses a dozen base stations around the UK I believe. It’s different from the Doppler-type dGPS that used to use a long sample time to get an accurate position when the error was still turned on.