I’ve had the same or similar problems you seem to experience. I’ve included elevation in my speedometer applications, of which I’ve always felt was quite inaccurate, sometimes double what I’m expecting or think I should have. Either other apps have been developed incorrectly, or you’re not getting a reliable reading from comparable sources.
I use whatismyelevation.com. Here you can put your full address or use your device location (GPS), gps is best
As a result of the data from the above website using the devices GPS, I note that one of my speedometer apps, is very close to the reading given by whatsmyelevation.com (warning, this site is awkward to use on sailfish browser, but it does yield eventually).
