[mkgmap-dev] Handling of boundary multipolygons

Sat Apr 17 16:00:14 BST 2010

I am thinking about how to handle boundary multipolygons and want get 
some feedback and ideas from you.

There are two possible ways to use boundary information:
1. Draw the boundaries as lines
2. Draw the boundaries as polygons with different colours for different 
administrative levels

I think the 2nd option cannot be realized (at the moment). I see no way 
how to define a style that differently colours adjoining polygons with 
rather identical tags (e.g. boundary=adminstrative, admin_level=2).

So let's concentrate on the 1st option.
One way may be part of different boundary relations. (admin_level=2 && 
admin_level=3 && admin_level=4 ....). To be able to process all of them 
with the style system the way must be duplicated for each relation.

The multipolygon handling of boundaries might work as follows:
For all ways (role=outer and inner) part of the boundary relation
- Clone the original way
- Tag the cloned way with the boundary information of the mp
- Remove all tags from the original way that are also contained in the 
mp (with the same value)
- Split all closed ways (polygons) in two ways to ensure that only ways 
are tagged with the boundary information

What do you think? If no one really complains I will start implementing 
this (and expect that it will be accepted when I am ready :-).
In case other multipolygon type also require such a processing we might 
add a config file in future which types are handled in this way. So this 
is ready to be more generic.

WanMil