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

Mon Sep 27 15:49:12 BST 2010

Torsten Leistikow (de_muur at gmx.de) wrote:

> 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

OK, but in practical terms if mkgmap generated a nature reserve  
polygon, a wood multipolygon and an inner water polygon, wouldn't the  
visible results be undefined?  In other words, you could end up with  
either:
a) Wood multipolygon & water polygon hidden underneath a nature  
reserve polygon, or
b) A nature reserve polygon hidden underneath the wood mp and water polygon
depending on draw order of the polygons (which afaik you can't control).

-- 
Charlie