[Tagging] Data redundancy with "ref" tag on ways vs relations

[Peter Wendorff]

Am 30.07.2012 18:22, schrieb Paweł Paprota:
> Hi all,
>
> As part of the Poland remapping effort I have implemented a reporting
> system called OSMonitor which analyzes road network in Poland in OSM
> data and produces reports. Recently one user requested additional
> validation - checking if ways in a relation for a specific road contain
> proper "ref" tag values (where "proper" means that "ref" on ways
> includes "ref" from the relation).
>
> This is what came out of OSMonitor:
>
> https://wiki.openstreetmap.org/w/index.php?title=OSMonitor/Poland_Major_Roads&oldid=791535
>
> Note the error named "relation contains ways with wrong ref". So for
> some roads the ways contain multiple variants of "ref" value. More -
> "ref" tag for ways is out of sync with relation membership, see
> http://www.openstreetmap.org/browse/way/172192711 (I am referring to the
> version 2 of this way in case it has been fixed in the meantime) for
> example.
>
> So the question is - why does "ref" on way level make sense at all when
> there is another (better and more flexible) way (pun intended) of doing
> things?
On the one hand it's easier to add for users than to maintain route 
relations.
That in mind "allowing" this as one option enables even beginners to add 
refs, too.

The other thing is that it's not more difficult to handle refs on single 
ways for software than to pull these from relations as the relations 
often are broken, too, so unconnected routes have to be handled with 
both options - from single ways as well as from relations.

What makes relations easier to use for data consumers (not mappers) is 
that it's defined which ways belong to the relations and therefore it 
may be easier to "guess" missing links between unconnected parts.

I think, ref makes sense on both: relations and ways, as this allows 
mappers to easily add a tag where it belongs to, even if it's not 
possible to edit the relation - due to the usage situation online and a 
restricted editor used, due to missing knowledge about route relations 
or whatever.

On the other hand it allows to find possible errors by checking if 
there's a conflict - like you do now.
A conflict may be, where a ref is on a way directly and on the relation 
the way belongs to, and these refs mutually exclude each other.
This isn't always the case: a cycleway-ref may be correct in parallel to 
a county street ref and so on; but sometimes it may in fact be an error, 
and at least it's "not complete" in a sense that on the way a ref might 
be missing, when it's on a relation where the way is a member.

regards
Peter