When calculating the gain/loss for a route planned using our tool, we generate a rough estimate using imperfect data derived from satellites, which is explained below in more detail. After you actually ride that route and record the activity using your GPS unit, you'll have a more accurate estimation of the true gain and loss for that route. This is because most Garmin GPS units have a barometric pressure based altimeter, which is much more accurate than the values provided when planning, using the datasets on our server. In general, a nice GPS unit with a barometric altimeter has the most accurate elevation numbers. Our datasets are second. The worst elevation values come from low end GPS units without a barometric based altimeter.
The elevation gain/loss calculated by our service after you upload may be slightly different than the value provided by your GPS unit. This is because every service/device uses a different method for dealing with the errors in the elevation numbers provided. In general, we strive for values that are within 10% of what a nice barometric pressure based altimeter would report for the same ride.
When planning a ride, we use several different elevation datasets. The most accurate dataset has one point every 10 meters on the ground, and is only available inside the United States. The second most accurate dataset, called ASTER, has one point every 30 meters on the ground. ASTER covers the entire world, from a latitude of -83 to 83. The third dataset, SRTM, also covers the entire world, though it only spans as far as -60 to 60 degrees South/North and has only 1 point every 90 meters. There are several problems with these datasets - first, they only cover one point every so many meters, so the points that fall in between are unknown. For example, this is an issue when you are planning over a bridge: the elevation dataset didn't capture the bridge's elevation, rather it captured the elevation of the water below. You can see this in action when planning a route over a tall bridge, in that the elevation actually dips in the spot of the bridge.