Source of ride elevation data

[lnh]

Just curious...

What is the source of the point elevations which become embedded in exported TCX files? If I import into Garmin connect, sometimes there is almost a 3x elevation gain difference between the raw data and letting Garmin Connect correct it using maps. I'm thinking the only valid data you should use would be uncorrected raw numbers because that is what your software uses to determine grades. Does that sound right?

[VeloReality Video]

The exported alt data in the tcx files is a slightly smoothed version of the original data thats used inside v-ride, so the heights of the climbs should be correct. If you upload it to Strava etc many of these sites use landsat data which can not see bridges & tunnels etc; This can cause huge differences in the meters climbed these sites report.