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

Ed Loach ed at loach.me.uk
Fri Apr 3 14:05:59 BST 2009


I've been noticing recently how things seem to be heading. 

Before I was involved with the project there were nodes, segments
and ways. Whether there were also relations at that time I don't
know. Perhaps the same key could be used more than once on a way -
again I don't know.

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.

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.

I don't believe the renderers handle all such relations at present
(I know they don't handle the interchange relation to display the
junction_ref, as it was when I was going to tag nodes as
highway=motorway_junction that I discovered the relations), but it
does seem that relations are becoming more and more important for
many things that we map. Whether in the above case you'd have the
dotted line for bridleway and the dotted line for parish boundary
rendered on top of the road or not I don't know. I suspect that
would be the most accurate rendering.

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.

Just some idle thoughts.

Ed






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