On Fri, Oct 18, 2019 at 11:29:04AM +0900, Joseph Eisenberg wrote:
> Is anyone aware of a way to import dual-carriageway roads as a single
> linestring?
> There's a request at Openstreetmap-carto to render dual-carriageway
> roads differently at mid-zoom levels:
> https://github.com/gravitystorm/openstreetmap-carto/issues/3934
> It doesn't appear that there are any tags in the database which hold
> this information. Ideally one would import a linestring midway between
> two highway ways, if they were tagged with the same highway class,
> name, and oneway=yes, but it could be difficult to get this right at
> locations where the two carriageways separate.
> Has anyone implemented a rendering that handles dual-carriageway
> roads, or designed a way to import such ways into a (rendering)
> database as a single linestring midway between the two OSM ways?

I have experimented with this and never got far enough to make this
actually useful. It is incredibly difficult to find the center line
between two carriage ways. The problem is not the lines, but the
junctions. All the algorithms I have tried do the right thing in many
cases but totally jumble up some complex junctions. I think that with
some more effort it should be possible to do this for some cases, like
only showing motorways in low-zoom level maps. But aggregating the lines
for mid to high zoom levels in urban areas is really hard, especially
when you expect the generalized center lines to still connect to the
rest of the road network.

Jochen Topf  joc...@remote.org  https://www.jochentopf.com/  +49-351-31778688

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