Hi!
I think that should be 5 decimals for commercial GPS, per that link? It also suggests that "The sixth decimal place is worth up to 0.11 m: you can use this for laying out structures in detail, for designing landscapes, building roads. It should be more than good enough for tracking movements of glaciers and rivers. This can be achieved by taking painstaking measures with GPS, such as differentially corrected GPS."
This does not seem to be typical (or recommended) use case for Wikidata. If you need to build a road, you better have some GIS database beyond Wikidata I think :)
Do we hope to store datasets around glacier movement? It seems possible. (We don't seem to currently https://www.wikidata.org/wiki/Q770424 )
I skimmed a few search results, and found 7 (or 15) decimals given in one standard, but the details are beyond my understanding:
Note that there's a difference between what general GIS standard would require (which has much more use cases), what we want to store on Wikidata and what we want to use for RDF export and querying. The latter is of more concern to me - as overprecision there might actually make things a bit harder to work with (such as - "are these two things actually the same thing?" or "are they located in the same place?") Of course, all those problems are solvable, but why not make it easier?