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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