[Qgis-developer] ftools geometry check

[a.furieri at lqt.it]

On Sun, 31 Oct 2010 14:00:46 +0100, Borys Jurgiel wrote
> Hi,
> 
> In Spatialite layers, if I digitize a polygon counter-clockwise, the 
> ftools' geometry validity check says:
> 
> Feature $n has incorrect node ordering
> 
> Is it intentional, or a bug? It doesn't affect shapefiles.
> 

Hi Boris,

in OGC-SFS specs there is absolutely no 
indication at all concerning the RING's 
node ordering (clockwise / counterclockwise).

and this makes full sense, because for OGC
POLYGONs the first RING always is the EXTERIOR
ring, and any other subsequent RING (if present) 
has to be interpreted as an INTERIOR ring.

this rule is absolutely clear and unambiguous:
so there is no need at all to enforce a preferred
node-ordering.

AFAIK the nowadays obsolescent SHP format used
a completely different rule: 
- the exterior ring has to be clockwise
- any interior ring has to be counterclockwise
- rings relative ordering is not relevant

So I suppose that ftools (incorrectly) checks any
polygon geometry for validity following the 
superseded SHP-like rules.

But from the SpatiaLite's own perspective this
makes no sense at all, because node ordering
is absolutely irrelevant in this case.

bye Sandro