Comment by phoronixrly 2 days ago
OSM is a community of predominantly amateurs/enthusiasts who trace uncorrected satellite photos and store the data as WGS-84 coordinates with 7-digit precision.
The point I'm trying to make is that there are more important sources of error before you get to tectonic movement and GPS drift... And OSM is plenty useful even without outstanding precision.
Oh yeah, I forgot about the enthusiasts that use uncorrected GPS positioning with low-cost hardware. One meter precision at the very best of conditions...
OSM uses a hundred nanodegrees as the grid resolution ( https://github.com/openstreetmap/OSM-binary/blob/32c3e921665... ). It gives a bit more than 1cm precision.
> The point I'm trying to make is that there are more important sources of error before you get to tectonic movement and GPS drift...
You can absolutely measure tectonic drift on the OSM maps! They've existed long enough for it to be actually significant in a lot of places if you download the old data.
This also comes up all the time when trying to overlap data from local agencies onto the OSM maps. You end up with parcel boundaries visibly off.
> OSM uses a hundred nanodegrees as the grid resolution
As I said, 7-digit precision.
> You can absolutely measure tectonic drift on the OSM maps! They've existed long enough for it to be actually significant in a lot of places if you download the old data. > > This also comes up all the time when trying to overlap data from local agencies onto the OSM maps. You end up with parcel boundaries visibly off.
Yeah, assuming the person who added the features you're observing as 'visibly off' did not use 1-meter Bing imagery with a 10-meter offset...
This is not the correct way to count digit precision, which should be independent of the units used. If there were 360,000 degrees in a circle would this same precision suddenly be 4 digits? If we measured in radians would the digit precision become 9? Of course not…
Given the 100ndeg precision is across the full earth, this would be 1 part per 3.6 billion or 9.5ish digits of precision. The location of the decimal point when displaying it is irrelevant.
What also needs to be taken into account is that real world longitudinal distance changes by the cosine of latitude. 1 degree of longitude is 111km at the equator and 19km at 80 degrees.
In GIS the gold standard for positional accuracy is the Root Mean Square Error, measured in real world distance units.
Whenever I’ve done things on OSM it’s based on gps recorded tracks - no doubt not particularly accurate but there are many overlaid ones which average out to be fairly precise (1mish) for the purpose.