A thin overlay structure for use in imaging based metrology is disclosed. The thin overlay structure may include a first structure and second structure, the first and second structures designed to have a common center of symmetry, both structures being invariant to a 180 degree rotation about the common center of symmetry, wherein a mark region defining the extent of the structures is characterized by a first direction and a second direction orthogonal to the first direction, a length of the mark region along the first direction being greater than a length of the mark region along the second direction.