My apologies for not stating my question clearly. I am not questioning
whether polygon orientation must be counter-clockwise or clockwise- it
is clear from the specs, v1.2.0 at least, that either is acceptable,
although the result would be a change in the normal to the surface.
However OGC v1.2.0 part 1 does make it clear that the interior rings
should be oriented in the "opposite" direction as the the exterior
rings. This would be consistent with a RHR convention because when
traversing a hole the RHR would cause the switch from clockwise to
counter-clockwise, or v.v.
The question them becomes- does the software implement this
convention? If there is no error in the PostGIS documentation, then
the example of a polygon with a hole
POLYGON((0 0,4 0,4 4,0 4,0 0),(1 1, 2 1, 2 2, 1 2,1 1))
that is given in the PostGIS documentation would appear to violate the
OGC v1.2.0 as well as the RHR (or the LHR).
Or perhaps it is a typo.
But surely it matters?
Tara
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
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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Tara Athan
Principal, Alternatives to Invasive Species
tara_athan [AT] alt2is.com
707-485-1198
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