[postgis-users] Intersection between circles

Maxime van Noppen maxime at altribe.org
Wed Aug 12 08:17:24 PDT 2009

On 08/12/09 16:33, Yves Moisan wrote:
> So from adbea to (acba + adebca)
> I know the distances from both a and b and I need to draw segments bc
> and ac, so I need to find the intesection of the two circles centered at
> a and b with the known radii I have.  Is there a function that would
> return the two intersection points of the circles and preferably only
> the one contained in my polygon ?

I don't think there is such a function but I see at least two other

1 - Approximate the circle by constructing a linestring with many of
segments (I use 50 to 200 segments for the whole cirlce depending on the
precesion I need). Using ST_Intersect and ST_Within will let you
determine C quite easily.

2 - Use some trigonometry to determine c by other means (like working
with the two right-angled triangles contained in acb).


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