[postgis-users] Checking the point location against a line

[Eric Randall]

from comp.graphics.algorithms:

if your polyline goes from A to B and your point is P, you can use this
formula:

v = (Ay-By)*Px+(Bx-Ax)*Py+(Ax*By-Bx*Ay)

v>0: "Left"
v<0: "Right"
v=0: "On the line" (or in front of B / behind A)


using this and  x(),y(), startpoint(), and endpoint() functions you can 
do it in your SQL statement i think.

-Eric