Originally Posted by
DeanB
If you're using Strava with a smartphone, then it has to rely on map data to figure out your elevation, but if you go on overpasses or something that often doesn't show up. Also if there are elevation changes or unknown elevations between the mapped elevation levels you'll lose that gain in your summary as well. If you use a Garmin with an altimeter you'll get a more accurate reading.
I'm not sure ridewithgps works though.
This doesn't make any sense. Both RWGPS and map data are using DEM data for elevation, probably the 30-ft resolution version if you're in the USA.
The answer to OP's question is that Strava selectively filters out small elevation changes to smooth the data (to avoid the "Richardson Effect" jmX mentioned). DEM will often NOT have accurate data for roads but rather simple low-res topographic data which doesn't account for roads cutting through hills, bridges, etc. Also, RWGPS uses very little smoothing in their calculation.