[OSM-talk] JOSM Tools to fix coastlines

Robert (Jamie) Munro rjmunro at arjam.net
Wed Aug 8 12:37:09 BST 2007


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The quoting on this message has all messed up - sorry!

Francisco R. Santos wrote:
> 
> Yesterday I began coding this check. The  opposite directions check is
> done, and the "meet other coastline" is also easy, I think.

Cool.

> > >> Could validator detect when two coastline ways run parallel without
> > >> other ways in between running opposite?
> > >
> > > That is more complicated, and right now I don't know how to do it.
> > > I'll need to think about it.
> 
> > If it helps, it occurs to me that any line drawn through a map should
> > cross coastlines in alternate directions. You could test the  lines
> > making the borders of the downloaded area, then draw lines that pass
> > through any ways that haven't been tested already.
> 
> 
> I've thinking about it, and maybe this test is not necessary at all. If
> two coastlines run in parallel, it is because at some point the lines
> meet pointing in opposite directions, right?

No, it's because they could be separate islands, or lakes and one of
them is the wrong way round. Also the part of the map where they join
might not have been downloaded because it is off the screen.

See an area like:
http://openstreetmap.org/?lat=62.72517471851086&lon=7.237492307228715&zoom=12&layers=B0F
for a complicated example. In the mapnik layer, because the coasts
aren't filled, you can see what is supposed to happen by the fact there
is a town at the top. If you switch to the osmarender layer, you can see
that it needs sorting, but it's hard to tell what is supposed to be land
and what is supposed to be sea.

Robert (Jamie) Munro
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