[postgis-users] intersection direction

[Stephen Woodbridge]

David,

I think you can determine that by looking at the z value of a cross 
product  of the lone segments that intersect. It will be negative or 
positive based on the direction of travel.

-Steve

David Bitner wrote:
> Is there a way to get at directionality in an intersection result?
> 
> Internally, intersection must be finding the from/to pair of each line
> segment that are crossing before it interpolates, if I could calculate
> the azimuth of each of those pairs, I could compare those and then
> (assuming from/to equates to left/right) determine if the intersecting
> line came from above or below.  Any other ideas of how to get at this
> information?
> 
> Basically, I have a line that I need to test other lines against and
> if they come through from one side I don't care, but from the other
> side I need to know about it.
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