Kevin Neufeld wrote:
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
I vaguely remember reading somewhere that if you are trying to calculate
normals to a plane (or something similar) then you must have the rings
in the same orientation. Now the interesting thing to notice is the
MS-SQL's geometry type enforces ring orientation, so we can only guess
that they need this property for some of their spherical calculations.
ATB,
Mark.
--
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
T: +44 870 608 0063
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