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[mkgmap-dev] Sensible resolutions - (or patch 5)

From Charlie Ferrero charlie at cferrero.net on Sat Mar 26 12:11:08 GMT 2011

On 26/03/2011 15:50, Johann Gail wrote:
> Actually, we do have a mean: if there are multiple parallel tracks (each
>> drawn as a separate way with railway=*), it is a major railway. It
>> should be doable to merge adjacent ways at lower resolutions and sum the
>> "weights" of the ways, to decide what to draw. A style file extension
>> could be useful, to specify e.g., the following:
>> * draw individual railways at resolution 24
>> * merge parallel tracks to one and draw them at resolution 22..23
>> * merge parallel tracks to one and draw if count>1 at resolution 21
>> * merge parallel tracks to one and draw if count>3 at resolution 20
>> For polygons, it could be useful to specify the minimum size that
>> qualifies for inclusion in a given resolution. Of course, small adjacent
>> polygons would have to be merged together first.
>> I am beginning to think that it could be useful to have low-resolution
>> versions of the OpenStreetMap data, generated by an experimental
>> algorithm that attempts to merge polygons and lines as suggested above.
>> If it is not too CPU intensive to merge adjacent areas and lines,
>> perhaps it could be implemented inside mkgmap after all.
> I'm thinking since some time about an algorithm for merging small
> polygons. Up to now I have not found an suitable algorithm. In another
> case I have thought about merging parallel lines, especially the both
> tracks of highways.
> While reading your mail I got the insight, that this are possibly two
> similar problems. In both cases there should be two objects with a small
> space between it be replaced by a merged single one.
> The algorithm in general would be:
> - Increase all polygons/lines with a outline.
> - Check all increased polygons, if some of them overlap.
> - If so, merge the original version of them.
> But I expect this to be a really resource hungry task.
> Regards,
> Johann.
I thought the --merge-lines option did part of what you're saying?  i.e. 
merge lines at low zoom levels.  If it doesn't do that, then what does 
--merge-lines do?


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