[mkgmap-dev] General tagging problem in multipolygons

Thu Jan 28 20:56:23 GMT 2010

My last patch shows a conflict that the mulitpolygon code cannot win.

Example: ====
A mp can consist of several ways:
Way 1: man_made=pier; natural=water
    (yeah the pier is part of the border of a lake)
Way 2: natural=water
Way 3: natural=water; boundary=administrative
   (this way will be part of another boundary mulitpolygon)
Way 4: without any tags

Way 1 to 4 build up the complete lake.

After constructing the multipolygon I have different options:
1. Remove all polygon tags from the original ways
- This removes the man_made=pier information from way1 although we would 
like to have it
- This removes the boundary tag from way3 although we might need it for 
processing the boundary multipolygon

2. Leave all tags as they are
- This possibly creates flooding for all three ways. Some parts of 
mkgmap seem to autoclose ways if they contain polygon tags. I don't 
understand where it happens but I have seen it several times in 
finnland. (e.g. the big lake near Ikaalinen, multipolygon 303327)

3. Leave all tags as they are but mark them as mp-used. All mp-used tags 
will be removed after all multipolygons have been processed.
- This makes the boundary multipolygon work.
- The first way is still a problem. How should I know if man_made=pier 
should be a tag for the multipolygon?

=========

The solution is that all multipolygons (at least all mps with multiple 
unclosed outer ways) are tagged themselves. But the reality is 
different. Most multipolygons get their tags from the ways.

Another solution could be that ways get an "used by mp" tag so that 
other steps in mkgmap don't try to autoclose these ways. This would give 
a chance to option 2.

What do you think? Any ideas about good solutions for the tagging problem?

WanMil