[OSM-talk] [OSM-dev] Polygons in OSM don't necessarily comply to simple feature specification

Iván Sánchez Ortega ivan at sanchezortega.es
Wed Apr 16 23:37:26 BST 2008


El Jueves, 17 de Abril de 2008, Frederik Ramm escribió:
> Hi,
>
> > The only exceptions apparently
> > being that self-intersecting polygons are allowed in OSM.
>
> The wiki page you cited says "In general, a 2D geometry is simple if
> it contains no self-intersection." - what do they mean by "2D
> geometry"? Specifically, would a self-intersecting *line* be allowed?

Oh, why, the answer is clearly in document OGC 05-126, section 6.1.6.1. I 
don't know how you missed that ;-)

A Curve is a 1-dimensional geometric object that is the homeomorphic image of 
a real, closed, interval D = [a,b] = {x ∈ ℜ. | a u x u b} under a mapping f:
[a,b] → ℜ 2.

A Curve is simple if it does not pass through the same Point twice (Reference 
[1], section 3.12.7.3):
    ∀ c ∈ Curve, [a, b] = c.Domain,
    c.IsSimple⇔ (∀ x1, x2 ∈ (a, b] x1 ≠ x2 ⇒ f(x1) ≠ f (x2)) ∧ (∀ x1, x2 ∈ [a, 
b) x1 ≠ x2 ⇒ f(x1) ≠ f(x2))


-- 
----------------------------------
Iván Sánchez Ortega <ivan at sanchezortega.es>

Someone whom you reject today, will reject you tomorrow.
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