Garmin altitude figures

Dippydog2

Did a ride today. Whilst I am happy with the elevation gained, it seemed to me that the starting elevation must be wrong as at times I was 50m below sea level!

So, I read the manual and just want to make sure I have interpreted everything correctly.

It's a Garmin 800 so it has a built in barometer, but does not correct for temperature.

So, the best thing to do is to manually set your altitude at the start of each ride.

Of course this only matters if you want the raw data to be correct. The metres climbed is not effected as that works on the differential.

After the ride today the unit thought I was at -30m. So, I reset it manually to 50m which is where I live. Two hours later I turned it on and it thought I was at -17m! I know the tempersture in my house is different to outside, but temperature changes about 2degrees C per 300m of elevation so the difference cannot be explained by temperature change alone.

I really don't care if I have to reset it every ride, but am puzzled by what is happening. Too much time on my hands I guess.

Any thoughts?

gloomyandy

No idea what is happening but you should not need to set it manually every time. You should be able to set an elevation point for your house and the device will then automatically calibrate itself when you are at that location (you can set several points if you often start a ride from other places).

ElectronShepherd

Garmin altitude figures are notoriously inaccurate - the use of barometric pressure as a proxy for altitude is fine provided that the weather doesn't change at all during a ride. I've done 25 mile rides before now (a loop starting and finishing at home) and the "total ascent" and "total descent" figures were 2300 and 2500 ft respectively. I'm sure I'd have noticed that much subsidence...

hangeron

there was a thread a few months back about garmins and altitude..I think it was for a 500 though.

Deriving altitude from a GPS is not as easy or accurate as deriving your horizontal location on the surface of the earth (lets pretend the earth is flat for a bit). This is a problem all GPS have..not just garmin cycling computers..it applies to top megabucks surveying GPS, and the stuff they fly airliners with. There are some workarounds though, firstly Grmin allow you to set Elevation points for the start of your ride..ie at your home/workplace/dogging carpark....take the altitude of these places from a 1/25000 OS map by interpolating between the contours and set it...now next time you press start on the garmin the unit nows precisely your attitude.

The barometer inside the garmin also helps to track air pressure as a proxy for altitude, but the accuracy for this will drift if the weather (especially the air pressure) is changing during the course of your ride....If the air pressure (as seen on a weather chart) is pretty stable during your ride then the combination of elevation point and barometer and gps will give a reasonable representation of your height along your route. there is a final hack to derive a accurate height trace, and that's to clamp the horizontal GPS trace to a very high resolution Digital Terrain Map of the countryside you just rode through...that will likley give you the most accurate trace of all...to do that with a Garmin click the Elevation Correction setting in Garmin Express when you upload your route...although I can't vouch for the accuracy of the Digital Terrain Map that Garmin Express uses - chances are though it's not bad for locations in most western (heavily surveyed) nations..I think Strava also have a DTM elevation correction on their interface too..but again I don't know how good it is.

Dippydog2

Thanks for all the answers.

As I said I am not too fussed with how accurately it measures height gained, but it is nice to see the correct starting elevation.

I have just followed these steps.

https://support.garmin.com/support/sear ... 0000000%7D