Reference Vector Transformations
• A reference vector is a unit vector associated with a point P in a spatial reference frame.
• If the point P and the reference vector are in a PCS the inverse transformation of the PCS is used to find the origin of the canonical LTP system on the surface of the ERM. That is: Locate the LTP origin at T-1(P(x,y)), where T is the projection transformation.
• The z component of a reference vector in a CLTP is always perpendicular to the LTP plane.
• A reference vector in a PCS or APCS is referenced to grid north (the y axis) and generally not to true north. By definition the CLTP has its y axis pointing to true north. This means that the vector must be rotated with respect to the CLTP y axis.
• The required rotation angle is the convergence of the meridian (COM). This is an angle whose value depends on P.