Recently, I picked up a garmin 500. previously, I used mytracks and uploaded that info to ridewithgps.com. i decided i wanted something with a bit better data tracking, and to get HR, cadence, etc, so i picked this up.

one of the other things i was looking forward to, was accurate elevation data...oh, was i a fool for expecting this.

I frequently do group rides and start/stop at the exact same point, yet i get different elevation data. sometimes its not A LOT, but sometimes, i've looked and just said "what the hell?". I've also compared with others who i do the rides with, who also have garmin units, and they get different info from me...heres one - starting elevation, 835', ending (same spot), 916' - http://connect.garmin.com/activity/198310306

some examples -
tonights ride http://connect.garmin.com/activity/203160886

garmin connect elevation correction disabled - 1025' at start, 1034' at end. Elevation Gain: 738 ft, Min Elevation: 1,025 ft Max Elevation: 1,154 ft
garmin connect elevation correction enabled - 1025' at start, 1034' at end. Elevation Gain: 898 ft Min Elevation: 862 ft Max Elevation: 1,127 ft

ridewithgps - http://ridewithgps.com/trips/803219 1025' at start, 1041' at end, Elevation: + 714 / - 705 ft ...and if i go to "edit this route", it gives me an option to refetch elevation, and says "Using the elevation data from your GPS, your trip has +714 / -705 feet gain/loss. Our elevation data shows this route has +1378 / 1379 feet"

strava - http://app.strava.com/rides/14611377 1025' at start, 1034' at end, 704' elevation gain

NOW...before you say "the majority of it, you're looking at a 40' difference total, thats not what im concerned with.

when i check with the USGS website, which, i think i trust them more than anyone, when i check certain spots, theres a huge difference. on tonights ride, about 3/4 mile or so in when we turn left (north) onto cubbage road, right there getting onto the road, USGS has that spot listed at 965.24', ridewithgps has it listed at 1073', strava 1073', garmin connect 1075'. thats a 100+ foot difference. im sure strava/GC/RWGPS are pulling data from a similar source, but where are they getting their elevation data, and should i trust the USGS or the source for these sites?

when i use ridewithgps' "our elevation shows XXX feet", i check that spot, its 965' USGS vs 969' RWGPS corrected. a second spot, USGS 1012' vs 1013' RWGPS corrected. a third, 910' USGS vs. 905' RWGPS corrected.

so it looks like ridewithgps' elevation data most closely matches what the USGS has.

and i did see on my garmin that i could set controlled elevation points, i went out on a portion of this route, stopped at about a dozen points along a few points in this route where i had found elevations on USGS site, high spots at hilltops and low spots where they bottom out, as well as my starting spot which is from the same parking spot weekly, but i guess my GPS didn't care that i set an elevation point there

so who do i use to get the most accurate information, or who do you use/trust?