Torsten Leistikow (de_muur@gmx.de) wrote:
charlie@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