AlpineQuest & OfflineMaps

A weird thing happened a few weeks ago.

I went for a walk, and I was recording the track (AQ 2.2.7). Suddenly the recorded position was displaced some 200m to the East. As I went on with my walk, the recorded position remained shifted by the same vector for more than one hour, then slowly started drifting back to the correct location.

This is not the first time it happens, I have already seen similar shifts in the past. I don't think this is an AQ bug. Is it a known possible glitch of GPS technology? Or is it because I have a poorly designed device?

Does AQ provide some "consistency checks" to point out caution if the recorded position might not be accurate, whenever possible?

Another problem I have constantly: Measured altitude is constantly underestimated some 30m-40m compared to what the map shows. My device does not have barometric sensor, so I rely on the DTM data downloaded by AQ. Is there a cause, and a remedy for that?

Hi Pascal, sorry for the delay.

Concerning the GPS "glitch", I have also seen this one or two times on my S9 (not that much distance though, something like a 20/30 meters drift, but enough to be on the wrong side of a river). The only solution is to wait and the GPS will be back on track after some time (did take up to 30 min for me!). This is a problem that is known to occurs when you're surrounded by reflecting objects, like tall glass buildings, but in my cases it was not the case.

For the application, there is absolutely no way to know if the GPS is reporting a wrong location, or if the user is actually on the other side of the river (in my example).

Concerning the elevations, is you're talking about the elevations displayed when you move the map around, the DEM data is used (when you display a recorded track details, then the GPS elevations are used).
Most of the data comes from the SRTM dataset (some countries use the ASTER dataset), the vertical accuracy objective for SRTM data is 16 m. You can find various articles on the accuracy in practice on the internet).
Of course, on areas with high slopes, the accuracy will be less good, and on flat areas the accuracy is usually very good.