On 18/11/2020 09:55, James Derrick wrote:
Building UPRN tags appear to be more clear-cut, with the U*SN location
node around the centre of a building way.
As we all learn more about the data, perhaps I (and others?) may have
been to quick to add USRN tags as they first became available?
USRNs are relatively straightforward, for several reasons. Firstly, they
only apply to highways, so they're much less ambiguous than UPRNs. And
they can, generally, be matched with the relevant OSM way (or relation)
just by using a map overlay (although be aware that an OSM way may not
exactly match the geometry of OS MasterMap, which is what the OS
OpenUSRN geometry is based on). And, finally, the OpenUSRN database only
contains current USRNs, so there's no danger of inadvertently tagging a
road with an inactive one.
UPRNs are much harder, partly because the OpenUPRN database includes
inactive ones (there are good operational reasons for this, but it's
unhelpful for mapping purposes when you're trying to map what exists
rather than what might once have been) and partly because UPRNs can
apply to pretty much anything, including subdivisions or different
levels of an entity that may be a single mapped object as well as things
that can't actually be seen (and therefore wouldn't be mapped on OSM).
What I'd suggest, therefore, is that we should add as many USRNs as
possible, based on a best-match between the relevant OSM way and the OS
OpenUSRN geometry. But we should only add UPRNs that are unambiguously
the correct one for a particular building or structure.
Going back to USRNs, there are a few gotchas that mappers need to be
aware of. The first is that a road can have multiple USRNs. The most
common instance of that is a trunk road (that is, one operated by a
national, rather than county, highway authority), which will have a USRN
issued by the national authority for its entire length and individual
USRNs issued by each highway authority that it passes through. But even
smaller roads can have multiple USRNs if it suits the highway authority
to assign them for operational purposes.
The other main gotcha is that USRNs don't have a one-to-one
correspondence with OSM ways that represent mapped streets. For example,
a cross-country road that crosses a highway authority boundary will have
a different USRN in each authority, but will usually be a single OSM
way. On the other hand, an urban street that is one-way for only part of
its length will be separate OSM ways (as those restrictions have to be
applied on a per-way basis), but a single USRN. So when tagging a way
with a USRN, you do have to be sure that you are tagging the right way
with the right USRN.
Mark
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