[OSM-dev] osm2pgsql and only-named multipolygons

Jochen Topf jochen at remote.org
Tue Oct 4 11:15:59 BST 2011

On Tue, Oct 04, 2011 at 10:41:10AM +0100, Andy Allan wrote:
> I stumbled across an issue this week and I don't have a great solution
> to it, so I thought I'd share it here.
> In osm2pgsql we handle two types of multipolygons - those with the
> "useful" tags on the relation, and those with no "useful" tags on the
> relation where we use the outer ways instead. Note that we also
> explicitly allow the relations to have a name. See
> http://trac.openstreetmap.org/browser/applications/utils/export/osm2pgsql/output-pgsql.c?rev=26664#L1001
> I've found this very common, especially with buildings. In fact, I
> used it as an example in my talk at SOTM-EU, where another application
> was showing buildings incorrectly - in some cases filling in the whole
> thing (tags-on-outer-ways) and missing the building entirely
> (tags-on-buildings). So both situations are established and we need to
> deal with them.
> However, one relation has caused me lots of issues, namely
> http://www.openstreetmap.org/api/0.6/relation/1715069 . It's a giant
> relation with a name, type, and nothing else, and so osm2pgsql creates
> a polygon using one of the outer ways to find the useful tags.
> Unfortunately it picked up "waterway=stream" from one of them, and I
> supported drawing them as polygons, so I ended up with several hundred
> square km of stream. Argh! But since no other map supports this
> particular level of micro-mapping, it appears that it's my map that's
> broken, so I've changed my styles to explicitly not support
> streams-as-polygons.
> But that's not fixing the general issue - people have a (strange)
> fascination with creating relations for all kinds of (strange)
> reasons, and that will only continue. Can anyone see a better way of
> handling this situation? Should I have drawn a line in the sand and
> demanded the relation in question was changed? Or is it an obligation
> on map makers to tread only in the footsteps of the main layer? I
> guess if osm2pgsql had picked a different outer way for the "useful"
> tags - say one with highway=pedestrian, and it showed up on the main
> style, then the relation would have been changed pronto.

I'd say the algorithm should be: If a relation is tagged type=multipolygon or
type=boundary, create a multipolygon from it. Look at the tags from outer
ways and from relation to come up with the tags this multipolygon supposedly

Its a bit dodgy to assemble those tags when outer way and relation are both
tagged and/or when there are multiple outer ways with different tags. I think
the algorithm implemented in Osmium uses the tags from the relation and if it
doesn't have any tags it uses the union of the tags from the outer ways.
Certain tags (such as "source" or "created_by" are ignored when doing this).

You then have a multipolygon with a set of tags, handle it like you would
for nodes and ways, ie. look at the tags and decide where and how to draw
it. Discard any object that only has tags you don't understand.

Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298

More information about the dev mailing list