[geos-devel] Polygon winding according to OGC

Sandro Santilli strk at keybit.net
Tue Sep 16 00:44:39 PDT 2014

On Mon, Sep 15, 2014 at 11:21:55PM +0300, Mika Heiskanen wrote:
> Hello Sandro,
> I just checked this page:
>   http://en.wikipedia.org/wiki/Well-known_text
> Quote:
>   It's useful to note that the OGC standard definition requires a
>   polygon to be topologically closed. It also states that if the
>   exterior linear ring of a polygon is defined in a counter clockwise
>   direction it will be seen from the "top". Any interior linear rings
>   should be defined in opposite fashion compared to the exterior ring,
>   in this case, clockwise.[1]
> Strange. I did not know the winding rule could be left so arbitrary.
> However, if the statement is true, should not GEOS also enforce a
> winding rule?

Reading the actual document (06-103r4):

While there's a mention of this "opposite direction" for shell and holes
there's no trace of that requirement in the list of assertions that define
polygons as "valid".

That said, there's a "normalize" function in GEOS that ensures exterior
ring is clockwise and holes are counterclockwise (thus, according to the OGC
document, making the polygon be viewed from the "bottom").


 ()  ASCII ribbon campaign  --  Keep it simple !
 /\  http://strk.keybit.net/rants/ascii_mails.txt  

More information about the geos-devel mailing list