[mkgmap-dev] Proposal: Multipolygon handling and line based multipolygons

Sat Sep 11 12:52:48 BST 2010

The latest question about the boundary handling of multipolygons 
encouraged me to start another try to fix the problem in mkgmap (please 
read http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2010q3/008976.html).

Now I have another idea.
The problem is:
The resulting multipolygons usually are used as polygons. But some 
should only be used for the line processing (country borders). The 
multipolygon algorithm creates artificial polygons which are annoying if 
you want to use them as lines: you get artificial borders inside your 
country.

My solution:
The mp algorithm adds a tag mkgmap:styleusage=polygon to all 
artificially created polygons. Additionally it duplicates all input ways 
of the multipolygon, tag them with the mp tags and add 
mkgmap:styleusage=line.
The style handling now applies line rules only if the tag 
mkgmap:styleusage=line is set or if it is completely unset. The same 
with polygons.
With this solution the style writer can decide on its own if he wants to 
use the polygon information or the line information of the multipolygon.
2nd advantage: the style writer need not to change any existing styles 
as this new feature is handled internally in mkgmap.
3rd advantage: the line information of properly tagged multipolygons 
could also be used if the multipolygon is not completely contained in 
the tile. This if often the case for country boundaries.

The --process-boundary-relations option of mkgmap would become needless.

I am not an expert in styles so maybe I have missed something?!?
What do you think about my idea?

WanMil