From memory** what happened was that the I was on a trail that took a fairly gentle left turn, but noticed that the GPS trace had a kink in it. So far, so not very unusual - but then I noticed that the other traces uploaded to OSM had the same left-right kink, and when I went back a couple of weeks (or months**?) later the same think happened.
It wasn’t the sort of multipath confusion that you get with large buildings where a GPS may be all over the place, it was a specific and reproducible offset. Whether modern receivers would show the same issue I’ve no idea.
I was lucky enough to be at a RSGS talk last week with Ordnance Survey professionals describing how they map with GPS and how they map when GPS is not sufficient. They used the term “canyons” (which could be natural or caused by tall buildings). The GPS signals are more imprecise in “canyons” because of “reflections” and “shadows”. In such cases they fall back on laser line-of-sight trigonometrical techniques, triangulating back to a known datum.
From the discussion it seems there might be at least two problems in this question:
fundamentally: does OSM map what can be verified OTG or some geographical “truth” - let’s ignore continental drifts for the moment.
how to resolve the conflict on the ground for the user, who neither knows exactly where they are because they cannot know whether the map or the GPS are wrong, or even if anything is wrong (except for cliff situations) and hence would carelessly leave their triangulation equipment and lasers at home.
There are probably OSM uses where the true location vs verifiable coordinates is not relevant (can’t think of one though) but that would be just as much a specific use case as the other.
This might be academic in excellently mapped areas but is more of an issue where official maps are not trustworthy to begin with.
We are mapping reality (=truth in this context?) that can be verified OTG.
We are not mapping GPS signals, we are not slavishly mapping reality exactly as presented by GPS receivers.
We are using GPS signals to map reality, we are using GPS data and our brains and other sources if present to improve map data.
We are using GPS data (and other sources) to gather info about reality, but these are our tools.
If we map some rock at specific coordinates from GPS - we do it because GPS is our best source about reality, not because GPS display is sacred.
And yes, in many cases we use on the ground info collected not by ourselves - but also collected by other sensors ranging from GPS to aerial imagery (both visible light spectrum and LIDAR and in some cases more exotic ones), satellite imagery, reports from friends, photo recording of streets, government databases etc.
(obligatory note: some of nice resources cannot be used, for example we cannot use aerial in Google Maps)
OTG does not mean that we are not allowed to map while on a boat or that we are obligated to map GPS signal anomalies as reality or that we cannot use aerials or that we cannot import address databases on a compatible license.
But if imported address database claims that address does not exist while it in fact exists and is in use it can be mapped. Yes, also if such address use without reporting it to government database is illegal.
Similarly, if government database recorded single address 45 times it does not mean it should be mapped the same in OSM, if on the ground truth is that it exists once.
If GPS mistakenly claims that peak is 20 m toward north it does not override reality, if we can get more precise info about OTG situation from elsewhere.
