[OSM-dev] 0.6

Iván Sánchez Ortega ivan at sanchezortega.es
Tue May 6 23:05:24 BST 2008

El Martes, 6 de Mayo de 2008, Frederik Ramm escribió:
> That's a problem with renderers and not a problem with the model.
> Whether an area is drawn clockwise or counterclockwise *should* not
> make a difference. It is true that you might achive better results *at
> the moment* by drawing your areas in a certain direction but it is
> certainly not necessary from a data model perspective.

OGC actually takes clockwiseness* into account:

"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”, [...] "

(See Simple Features Specification, the bit about polygons)

A mathematician/topologist would also agree with the fact that a polygon must 
be counter-clockwise to refer to the "inside" isntead of the "outside".

* If "clockwiseness" wasn't a word, I guess that, by now, it is one.

> (Unless you want to start discussing that a way loop drawn around a
> forest does not exactly specify whether the little bit inside is the
> forest or the whole rest of the sphere of the Earth is the forest.
> That, indeed, is left open by our way of dealing with things.)

It's not left open: it's *defined* by the clockwiseness :-)

That old joke about a topologist building a spherical cage and 
getting "inside" in order to catch a lion comes into my mind...

Iván Sánchez Ortega <ivan at sanchezortega.es>

MSN:i_eat_s_p_a_m_for_breakfast at hotmail.com
Jabber:ivansanchez at jabber.org ; ivansanchez at kdetalk.net
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