I posted the msg below over at the Garmin forums, but figured I'd get a wider response here at bikeforums.net
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I have an Edge 305 (2 years old).

I upload my data to GTC, Garmin Connect, and SportTracks.

I have always had issues determining (with any degree of accuracy) what sort of climbing data I experience and is being reported.

Here's an example of a ride I did yesterday: 63 miles.

GTC reports 2789 total ascent.
Garmin Connect reports 3234 elevation gain
Bikeroutetoaster reports 1430 ft ascent
RidewithGPS reports 2638 ft elevation.

Of course, SportTracks reports different values depending on the amount of data smoothing applied, so I left that out.

I'm not sure why I keep bashing my head against the wall on the elevation issue, except that maybe I hope that by doing the same thing over and over, I'll get a different result one day.

Have you guys found any particular mapping program to report reliable elevation for the route? My numbers above are ridiculous.

Edit:

I mapped the same ride on the following sites:
MayMyRide: 1132 ascent
Veloroutes: no elevation data that I could find
bikely: no elevation data
GPSies: 1988 ft
bikemap.net: no elevation

Training Peaks: 4593 elevation gain

More data... just all over the place.

That's very odd, I guess I can't rely on elevation tracking either...This and this agree, and this is 10% off (100 feet difference over 1000 feet). I think I'll stick with mapmyride since I've already used it a bit. I just mapped the same route with bikeroutetoaster and it agrees with the first two, so me thinks those three are the most accurate.

It's not a simple matter to know what the "right" number is. This Excel table https://www.caltriplecrown.com/ClimbingDoubles.xls of altitude gain readings from California double centuries over many years from many cyclists using a wide variety of devices shows just how much variability there is in raw readings from barometric altimeters, let alone how GPS mapping algorithms decide how big/small an increase must be to qualify as a sufficient change to incorporate into the total gain.

When I have a few spare minutes, I'll map that same ride into some other mapping sites and update the original post with the results.

I have a VDO 1.0+ computer, does altitude by pressure changes. It's usually within 10% on the same ride repeated.

bikeroutetoaster gives me readings very near the VDO.