Garmin elevation discrepancy.

huwsparky

Über Member
Location
Llangrannog
Hi all.

I have a Garmin edge 500 and so does a mate of mine. Today we went out for a ride together and I noticed whilst comparing our Strava uploads that my Garmin was over-reading on elevation compared to his. There was almost 100ft difference over the 1600 odd that we climbed.

Is there a way of recalibrating them or is this normal?

Thanks.
 

screenman

Legendary Member
When returning to the same spot in the garage after a ride there is always a difference on mine. Between the amount of uphill downhill I covered, before getting one I thought it was just in the legs that felt that way. Technology has confirmed it.
 

MickeyBlueEyes

Eat, Sleep, Ride, Repeat.
Location
Derbyshire
Hi all.

I have a Garmin edge 500 and so does a mate of mine. Today we went out for a ride together and I noticed whilst comparing our Strava uploads that my Garmin was over-reading on elevation compared to his. There was almost 100ft difference over the 1600 odd that we climbed.

Is there a way of recalibrating them or is this normal?

Thanks.
Do you both use Garmin Connect too? Might be worth having a look to see if your 'elevation correction' is set to the same, enabled or disabled.
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
Strange that. To be over 100ft out to me indicates that there is a problem with one of the units. I may use the phone as well next time to see what that says.
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
Do you both use Garmin Connect too? Might be worth having a look to see if your 'elevation correction' is set to the same, enabled or disabled.
Neither of us use Garmin connect but if Santa brings me an improved laptop at Xmas or soon after I will be. I didn't release that the elevation could be 'corrupted' by things such as Garmin connect. Maybe Strava has settings too. I'll have to have a look. I always thought that what my Garmin said was what got uploaded.
 
6% is not an awfully large discrepancy in /elevation/ change if the device in question is purely using gps (which I assume to be the case, but don't know). GPS altitude values are considerably less precise than lat/lon values (due to the geometry: narrow angle of the intersecting lines from the various satellites in the vertical dimension. As well as that, there's a possible effect of sampling rate: most gps devices can be set to sample position at different rates. Typically, they'll default to one second, but the two devices could be set to different rates, which means you could 'miss' small elevation changes on one device, but not the other (imagine one is set to a minute and you ascend 20 metres in that minute, but also descend 20 metres: the '1 minute device' will record no ascent; the '1 second device' will record 20 metres.

Even if they're both at the same sample rate the variations in exactly when they record, combined with the gps vertical accuracy 'issue' means that the sort of error you're seeing is not at all surprising.

If you want better accuracy you need to combine gps data with a barometric altimeter (if those devices do that then ......disregard the above as an explanation!).

In summary: assuming it's pure gps data you're collecting, then it is normal.

Incidentally, the sample rate thing will affect horizontal accuracy too: if you have a long sample rate and you're on a curved road, it becomes a series of straight lines and the total will be less than you travelled. Again, at the 1 second sample rate that I imagine they default to, this effect will be negligible. The thing to look for is battery saving options: battery usage is typically saved by lowering the sample rate, so all the above effects start to apply unless you're travelling in a remarkably straight line at a constant slope!
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
Thanks for your detailed reply @Sea of vapours. I'll check my device and ask my mate to amend his settings to the same as mine if they are different to see if that helps with the problem.

I thought that the edge has a GPS and barometric sensor, may be wrong though.

I didn't realise that it was as complicated as it is!
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
You're a foot taller than your mate and you cycled for 100miles. Seriously though I wouldn't get hung up about it ;)
I'm not really hung up about it, just was wondering if there was a simple explanation as to why it was happening. I'm fine with being told I'm climbing more in my rides than others!!
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
I may have found a possible cause. Just changed Data recording to 'every second' from 'smart recording'. I presume the smart recording is some kind of power saving mode.

Will need to do a previous route to confirm if it has made a difference at some point.
 
I may have found a possible cause. Just changed Data recording to 'every second' from 'smart recording'. I presume the smart recording is some kind of power saving mode.
That sounds like the explanation, at least in part. The Edge 500 does have a barometric altimeter, which will certainly help accuracy a lot (gps really isn't at all good for altitude - tens of metres out is typical, though it'll get a good answer, averaged over time, if you stay still .... ;-) ). 'Smart recording' will almost certainly have some kind of algorithm which says 'am I in a fairly stable state with respect to speed, direction and altitude? If so, I'll relax a bit and just check on where I am every once in a while for a bit'. Pretty handy and can save lots of energy, but it will pretty inevitably lead to at least single digit percentage errors in altitude recording on non-constant slopes.

If /both/ devices were set that way then the difference would be even greater as they'll both be doing their own thing in subtly different ways! It is, as you say, pretty complex. Then you can add in the effect of changing atmospheric pressure messing with the altitude the barometer is recording over a long period ....So many variables and mechanisms that I'm always surprised how good altitude data are really :-)
 
OP
huwsparky

huwsparky

Über Member
Location
Llangrannog
Set elevation points, no idea where the menu is but it should be in the manual
Found it in the settings.

It gives me my bearings and an elevation which can be alerted. If I were to take the unit down to the beach (1.5 miles so no big deal) I suppose I could alter it to 0ft at sea level? Or at least find out if it's reading correctly.
 
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