[OSM-dev] JOSM: Several tags with same key

David Earl david at frankieandshadow.com
Fri Apr 3 14:50:47 BST 2009


On 03/04/2009 14:05, Ed Loach wrote:
> Currently we seem to have nodes, ways and relations. A way or node
> can be part of any number of relations (I believe), and there are
> people using relations for more and more things. Group all the ways
> that make a given road into a road relation; all the ways that make
> a junction into an interchange relation (or so I've noticed on the
> A14 near Bury St Edmunds this morning); and more.

Relations were specifically introduced to allow for groupings, and 
especially those which *utilise* other ways and nodes or things which 
aren't physical - for example a bus route or NCN route runs along 
multiple ways of different types. We could use them to link the way 
running under a bridge with the bridge passing over, and so on.

> In some ways this would allow you to add more than one tag on the
> same way with the same key. I could have a relation which represents
> a bridleway, which for part of its length happens to run along a
> lane (highway=unclassified, perhaps with it's own road relation)
> that itself is the boundary between two parishes (boundary
> relation). In this case the way is really nothing more than a line
> and the relations dictate the three separate uses.

Hmm. Notwithstanding the other discussion about bridleways, a bridleway 
is a kind of road or path and has a physical presence, so that's not 
really what a relation was intended for. A "horse route" would be and 
might utilise a road here, a path there and a bridleway somewhere else 
along it.

> I think relations are going to have to be given some serious
> consideration as they evolve. If a way is in a relation do the tags
> on the way or the relation take precedence - say in a road relation
> do you put the access restrictions that apply to the road in the
> relation, but if there is a short way in the middle of the road with
> different restrictions do we tag that way and hence give the way
> tags precedence. Or if a road relation has the ref for the whole
> road but another exists on a way, and it differs, do we assume that
> the way tag takes precedence, or a mistake.

Grouping streets that share a ref seems like a good use for a relation, 
though I think it would be a shame if this became the standard way of 
doing it as it is a rather more complicated way of doing it. But in some 
cases, roads do have more than one ref (European + national, NCN + B 
Road, or even just two roads come together A11 + A14 north of newmarket, 
though I think strictly speaking the A14 is the road number and the A11 
breaks in two here).

So always a mistake is probably too strong, but I'd imagine it generally 
would be a mistake. We'd have to look at each problem case on its merits 
I'd have thought.

David





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