[postgis-users] Looking for function to analyze linestring and points

[Eric Randall]

You could see if your points are left or right of the line

 If your polyline goes from A to B and your point is P,
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)

(hope that's right, I used it to check address points and road centerlines)

and then use the line orientation to determine direction  from the line.