[postgis-users] WKT polygon and interior ring orientation

[Kevin Neufeld]

If you look at the OGC spec v.1.1.0 and the current v1.2.0, Part 2, the 
only reference to orientation is in a footnote, "Polygon rotation is not 
defined by this specification; actual polygon rotation may be in a 
clockwise or counter-clockwise direction."

Prior versions don't even mention orientation.

http://www.opengeospatial.org/standards/sfs

It's my understanding that polygon orientation doesn't really matter and 
even varies from one database vendor to another.  Martin Davis may be 
able to answer the whys better, but since the specs didn't specify at 
the time, I believe JTS/GEOS chose to follow the somewhat more accepted 
orientation and follow the Right-Hand-Rule - that is, the orientation of 
the rings are such that the thing they are bounding is to the right.  In 
particular, an exterior ring is clockwise and interior rings are counter 
clockwise.  PostGIS even has a forceRHR for polygons ... but like I 
said, I don't think it really matters.

-- Kevin

Tara Athan wrote:
> Noobie question here:
> I am trying to understand WKT so I can build my own polygons from GPS 
> points for entry by SGL commands. In reading the OGC specification
> http://portal.opengeospatial.org/files/?artifact_id=18241
> I thought I understood it to say that interior rings should have a 
> clockwise orientation,
>
> "6.1.11.1 Description
> A Polygon is a planar Surface defined by 1 exterior boundary and 0 or 
> more interior boundaries. Each interior
> boundary defines a hole in the Polygon. A Triangle is a polygon with 3 
> distinct, non-collinear vertices and no
> interior boundary.
> The exterior boundary LinearRing defines the “top” of the surface 
> which is the side of the surface from which the
> exterior boundary appears to traverse the boundary in a counter 
> clockwise direction. The interior LinearRings will
> have the opposite orientation, and appear as clockwise when viewed 
> from the “top”, "
>
> but the example given in the PostGIS documentation
> http://postgis.refractions.net/documentation/manual-1.3/ch04.html#RefObject 
>
> gives an interior ring with counter-clockwise orientation. Am I 
> missing something?
>
> Thanks, Tara
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