Originally Posted by
_ForceD_
Strava doesn’t know. The GPS (receiver and software in your phone) resolves your elevation. The Strava app uses that GPS data and applies it to the various aspects of the app. GPS can resolve your vertical position the same way it resolves your horizontal position. It uses timing signals between the GPS satellites and your GPS receiver device down to the nanosecond (One nanosecond is to one second as one second is to 31.71 years.). When you go up or down a hill, that timing signal gets longer or shorter. When it detects that, it goes one step further and does the math to determine the degree of inclination, or percent grade.
Dan
actually, strava doesn’t use GPS elevation if it can be avoided given how very inaccurate it is. if you read the link above you’ll see they first use barometric altimeter data, and if you “correct” it they use an elevation basemap which was made from that same data, crowdsourced, and then they fall back to public elevation maps/models. if the GPS position is off of course the elevation correlation will be off too.