Interesting. My gpx files often have "mapsource v6..." as creator, as I trim ends of the file. If I edit it to "HCx with barometer", the Strava elevation on my commute home changes from 22m to 67m, which seems incorrect - as it's mostly downhill/flat. Perhaps it's just that my Etrex barometric altm is not calibrated, and Ordnance survey spot heights show Etrex = 10-15m less.