1. A method of determining a position of a detection plane relative to a detectable structure, the detectable structure including first and second parallel members having predefined locations in a first coordinate system and a third member connected between the first and second members in a predefined position and not perpendicular thereto, the method comprising: impinging the detection plane on the detection structure at a plurality of locations; collecting position information of intersections of the first, second and third members with the detection plane in a second coordinate system at each of the plurality of locations; determining positions of the third member relative to the first and second members in the second coordinate system for each of the plurality of locations; and determining, for the determined positions of the third members, a position of the third member in the first coordinate system at the intersection at each of the plurality of locations.