> -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On > Behalf Of Mark Cave-Ayland > Sent: Monday, November 03, 2008 5:21 AM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] WKT polygon and interior ring orientation > > > 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. >
I assume you mean GEOGRAPHY type. The SQL 2008 GEOMETRY type does not suport spheroid projections/calculations. Does ring orientation matter when you are calculating point in poly? When you calc a point in poly, do you take the sum of vertex angles from point to poly vertex and if sum of angles is >= 360 then point is in poly? > > ATB, > > Mark. > > -- > Mark Cave-Ayland > Sirius Corporation - The Open Source Experts http://www.siriusit.co.uk > T: +44 870 608 0063 _______________________________________________ > postgis-users mailing list [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
