[mkgmap-dev] Question about warning

Mon May 3 20:28:48 BST 2010

> On Mon, May 03, 2010 at 12:25:06PM +0100, Toby Speight wrote:
>> I've found that I got roundabout errors when people created
>> multipolygons with junction=roundabout - that tag was transferred to
>> the synthetic ways of the multipolygon, which then overlapped with the
>> real roundabout.  I generally fixed that by deleting the multipolygon -
>> a roundabout is a joined line, not an area.
>
> What if there is forest inside a large maxspeed=100 roundabout and a
> lake inside the forest? :-) In my opinion, the right solution would be
> never to share a line for highways and areas. The shared lines are a
> nuisance when you add footways, cycleways, bus stops and the like.
>
> On a somewhat related note, I noticed that some have started tagging
> overlapping lines for landuse multipolygons: one tagless line for the
> role=inner and another line (with tags) for the inner area. That is kind
> logical, but overlapping lines can be confusing when editing the map.
>
> 	Marko

Here's the algorithm of mkgmap how to handle the tags in multipolygons.

Basic rule: All polygons are duplicated by the mp algorithm.

1. Check if the mp is tagged with one of [boundary, natural, landuse, 
land_area, building, waterway].

1.1 Yes
   1.1.1 Copy the mp tags to the new artificial polygons.
   1.1.2 Remove all tags from the original ways identical to the mp tags.

1.2 No (mp has no useful tags - tags must be retrieved from the members)
   1.2.1 Build the true intersection of all outer way/polygon tags.
   1.2.2 Tag all new artifical polygons with this intersection
   1.2.3 Remove all tags from the original ways identical to the true 
intersection

Some explanations:
1.1 means that the tags are taken from the multipolygon if this is at 
least tagged with one polygon tag. This list is non complete! This might 
be fixed in future.

1.1.2 is necessary because lots of multipolygons are tagged twice - 
first the multipolygon and second all ways. If the identical (which 
means tag AND value must match) are not removed here the map will 
contain each polygon twice.

1.2.1 True intersection means: only the tag/value combinations that are 
used on all ways are copied to the resulting artificial polygons.
1.2.3 Similar to 1.1.2 these tag/value combinations must be removed from 
the original ways/polygons.

 From my point of view sharing lines between multipolygons and lines 
should work if the tags for the mp are set in the multipolygon.

WanMil