[OSM-dev] Algorithm help

Dirk-Lüder Kreie osm-list at deelkar.net
Thu May 29 01:11:08 BST 2008


Nathan Vander Wilt schrieb:
> 
> On May 28, 2008, at 1:35 PM, Richard Duivenvoorde wrote:
>> 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?
>> Richard,
>> there is a good set of (explanations of) algorithms here:
>>
>> http://www.faqs.org/faqs/graphics/algorithms-faq/
> 
> 
> If you can treat the earth as a cartesian plane 360 units wide by 180 
> tall these equations could be used. But be careful here. Lat/lon 
> coordinates actually are based on a more complicated topology. A 
> spherical polygon always encloses a finite area less than the surface 
> area, so the usual method of "checking for negative area" doesn't work 
> out so well.

Our data-model does not support earth as a sphere.

-- 

Dirk-Lüder "Deelkar" Kreie
Bremen - 53.0952°N 8.8652°E


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