That’s a known issue with any sort of mapping or GPS based technology. Here is the best short explaination of it I’ve found. RIDE WITH GPS FAQ Ride with GPS produces 1 value, MapMyRide.com produces a second value (Lower), and if I take the GPX Track file from the ride, run it through Excel and calculate elevation gain from point to point, I get a third value (And it’s the highest, I think in part because the gps has a natural margin of error, and in part because the small little 10 foot rise in the road gets counted in).
Here’s my view on it. All of them are of reletive value, in terms of knowing what the day has in store, especially if you know how those numbers relate you your legs. I know what a 20 mile ride with 1000 feet climbing feels like in RideWithGPS, I don’t know how that ride feels in MMR, or other systems.
I do like the fact that the GeoBike shows the elevation gain over the course of the day, but his totals are about 50% higher than what RWG shows. (I suspect that his numbers don’t filter out or smooth small variations).
The truth is, as long as you’re using map data or raw GPS Data, what you get is going to be an approximation. It’s good data, it helps you plan, but probably the only way to get the truly dead nuts on value would to be to actually survey the route with equipment designed for that purpose.