1. A method for determining a pose of an elongate object having a tip contacting a plane surface having an invariant feature, said method comprising: a) illuminating said plane surface and said invariant feature with a probe radiation; b) detecting a scattered portion of said probe radiation returning to said elongate object at an angle τ to an axis of said elongate object; c) deriving said pose from a response of said scattered portion to said plane surface and to said invariant feature; and d) obtaining an absolute position of said tip on said surface from said pose and said invariant feature.