Elevation comparisons between Edge and eTrex

[mbeardsl]

Went on a ride yesterday with my trusty Garmin Edge 305 and brand new Garmin eTrex 30 to compare the two.  Predicatbly, they roughly recorded the same track over 22 miles.  What I wasn't expecting was the elevation numbers.

When I uploaded to Garmin Connect I got 990 ft (eTrex) and 2427 ft (Edge).  Knowing those numbers never seem to be "correct", I exported to TF and got more confused with 1722 (eTrex) and 4922 (Edge).  I looked here on the forums for awhile and read through the manual, finally found the "Correct elevation using DEM" button and got 1690 ft (eTrex) and 1863 ft (Edge).

So, which is "correct" and why?  Why such different initial numbers from two units?  Playing around with some of my older tracks I see a wide variety of swings from very close to original TF reading to 50% gain and 50% loss using the same process above.

Another anomoly which I have seen before generally when riding in changing weather (I think) is that the start and stop points should have read the same eleveation but instead saw a steady increase in the elevation overall so ended up higher than I started in the exact same spot (disappears after correcting using DEM).  Barometric pressure changes affecting the units?

Thanks for any insight!

[ScottMorris]

Hi,

The 'correct' elevation gain figure is a difficult concept, as you are no doubt noticing.  For what it's worth, the original Edge units (205/305) are notorious for exaggerated elevation gain, simply because their elevation readings have a lot of noise that get added up in the *sum* that elevation gain is.  I wouldn't trust a 305's numbers without some filtering.

The best I hope for is consistency between tracks from the same unit, or by using the correct elevation using DEM button as you found.  But in general Garmin's own units I tend to feel are too low, and TopoFusion's base numbers are a bit too high, unless you correct with DEM, or pull up climbing analysis and look at some of the filtered (e.g. training center preset) values.

Barometric pressure can make a change like that, but the 305 doesn't have a barometer (right?) so it wouldn't be that in this case.  The 30 does, but generally a normal ride isn't long enough to notice that effect, IMO.

Hope that helps a bit.  It's a bit of a sticky issue.

[mbeardsl]

Thanks for the reply Scott.

The Edge 305 does have a barometric altimeter.  It seems correct most days but far off on others.  I've only noticed the net elevation differences at my start and stop points on 6+ hr rides that either start pre-dawn or move into the late night where temps may change 20-30 degs.  Who knows.

I'll continue to use the DEM button and trust those numbers as they seem most accurate after loading past tracks of the same trails and correcting.

[mbeardsl]

If anyone is curious, here are two files from my most recent ride.

Edge shows 1161ft online, but in TF 2240, corrected with DEM to 1093.
eTrex shows 400ft online, 880 in TF, corrected with DEM to 932.

I think Scott is right regarding the Edge accumulating more noise.  I need to set the eTrex to record "Most Often" as the difference can be huge.  In this 10mi sample the Edge recorded 3932 points vs the eTrex's 882 so there is likely a ton of false evelation data (also the reason the eTrex recorded a shorter distance I'm sure after zooming in for detail).  I think elevation data would also be much closer if eTrex was capturing every second instead of how it is set currently.

[aaron w]

I'm surprised by how far off the distance is for the 2 files over just a 10 mile sample.  9.2 miles for the vista vs 10.5 for the edge.  Of course when I zoom in on the 2 tracks I can see where the difference lies...the file for the edge just looks more real. 

If the eTrex were set up to capture points more often, what would the downside be?  Would doing so consume more battery power?