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
Why talking so long? Implementation was quite easy so please try this patch and post what you think about this patch. I will post v2 with some more comments in the source code if it should be committed. WanMil