Hi Rockwell,
I looked at a recent GPX recorded on my phone. I usually pause recording to the GPX whilst doing the activation, so as not to leave a mess of track points at the summit, but this time I forgot. So here’s about an hour and a half with my Moto E4 sat in the same spot, right next to a summit trig point.
Mean 556.0 m
Std 7.2 m
I’ve quoted the standard deviation there as an illustration of how much a phone gps can drift about. But I hadn’t anticipated the mean would be so far off (it’s low by 69m). I might wave my hands and say it’s an ellipsoidal vs. geoidal height thing, but I suspect it’s wrong in the wrong direction. Will have to look more closely at that. I think GPS will typically be quoting ellipsoidal height (I stand to be corrected), which mostly won’t be the same thing as a local mean sea level height.
The drifting about I suppose might be a lot less once we’re using Galileo. And of course a phone GPS is a poor cousin to dedicated units.