[mkgmap-dev] Trouble getting some multipolygons to render inmkgmap

Wed Oct 13 21:09:53 BST 2010

> charlie at cferrero.net schrieb am 27.09.2010 15:20:
>>> As an example take a nature reserve consisting of a wood with a lake inside.
>>> This migth be mapped with two polygons and a relation:
>>> polygon A: leisure=nature_reserve (the complete area)
>>> polygon B: natural=water (only the inner area)
>>> multipolygon relation: natural=wood and outer=polygon A and inner=polygon B
>>> (only the surrounding area)
>>>
>>> Right now polygon A seems to be missing in the resulting map.
>>>
>> But how would mkgmap know which of the two outer polygon types to use
>> (ie nature reserve or wood)?
>
> It should use both:
>
> The nature reserve should cover the complete area.
>
> The wood should cover only the area defined by the multipolygon.
>
> This is (one of) the intended tagging of the multipolygons. Allowed alternatives
> (with the same logical interpretation) would be:
>
> 1. You could use an additional polygon for the outer limit of the multipolygon
> (polygon C) which would have the same nodes as polygon A. Polygon A and B would
> stay unchanged.
> multipolygon relation: natural=wood and outer=polygon C and inner=polygon B
>
> 2. You could put all tags from the relation on polygon C, polygon A and B would
> stay unchanged.
> polygon B: natural=wood
> multipolygon relation: outer=polygon A and inner=polygon B
>
> 3. You could move the nature reserve tag into the multipolygon area and the
> inner area.
> polygon A:
> polygon B: natural=water and leisure=nature_reserve
> multipolygon relation: natural=wood and leisure=nature_reserve and outer=polygon
> A and inner=polygon B
>
> 4. And you could move the tags from the relation of variant 3 to the outer polygon.
> polygon A: natural=wood and leisure=nature_reserve
> polygon B: natural=water and leisure=nature_reserve
> multipolygon relation: outer=polygon A and inner=polygon B
>
> I think these five possibilities are all allowed under the actual accepted
> multipolygon scheme and they should all result in nearly the same garmin map.
> (Alternative 3 and 4 split the nature reserve into to areas, but in the end it
> covers teh same area).
>
> Gruss
> Torsten

Hi Torsten,

I am back and I have thought about your request. It would be possible to 
implement a patch which keeps the original polygons in case they are 
tagged differently to the multipolygon.

I have also read Charlies answers and before digging in the code I would 
know if this feature is really needed and useful? Can you estimate how 
many multipolygons are affected?

WanMil