[OSM-dev] Algorithm help

[SteveC]

if the polygon is p1,p2,p3,p4...

c = centre point of polygon

v1 = the vector from c to p1 (p1 - c)
v2 = the vector from c to p2 (p2 - c)

take the cross product of v1 and v2

	http://en.wikipedia.org/wiki/Cross_Product

if its positive in the z axis its one way, if its negative its the other


On 28 May 2008, at 21:26, Richard Fairhurst wrote:

> Can anybody point me in the direction of an algorithm that will
> determine whether a closed way (polyline) is clockwise or anti-
> clockwise?
>
> cheers
> Richard
>
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Best

Steve