[mkgmap-dev] type=multipolygon relations

Sun Jan 19 21:44:24 GMT 2014

Hi WanMil, Gerd,

On Sun, Jan 19, 2014 at 10:47:15AM -0800, GerdP wrote:
>Hi WanMil,
>
>I think your example is not valid. One rule in that wiki says:
>"If an endpoint is shared by more than two unclosed ways, it's ill formed
>and a closed polygon can't be reconstructed unambiguously."
>I think both end points of 3 are shared by three unclosed ways.

I agree with Gerd that the example is not valid:

>> 11111111111111111111111111
>> 1                        1
>> 1      22234444          1
>> 1      2  3   4          1
>> 1      2  3   4          1
>> 1      22234444          1
>> 1                        1
>> 11111111111111111111111111
>>
>> MP: tags natural=forest
>> Way 1: role=outer
>> Way 2: role=inner, natrual=scrub
>> Way 3: role=inner, natrual=scrub
>> Way 4: role=inner, natrual=scrub

I think that this should be mapped with 3 multipolygon relations:

MP1: natural=forest: role=outer: way1, role=inner: way2, way4
MP2: natural=scrub: role=outer: way2, way3
MP3: natural=scrub: role=outer: way3, way4

If you really meant natural=scrub for both 2+3 and 3+4, I think that 
this could be simplified by removing way 3 and merging ways 2 and 4.  
The example would make more sense if MP3 is tagged as natural=water, for 
instance.

In my opinion, mkgmap should issue a warning for the duplicate way 3 for 
your example. It does not seem much different from a situation where two 
inner polygons are touching or intersecting each other. I think that the 
unambiguous way of mapping is to draw a bigger hole around the adjacent 
areas, like MP1 in the above example does.

	Marko