AbstractThe plane mirror, used in early Geodimeter work, has been replaced as a reflector in EDM by the corner cube. The reasons for this are well known and need no elaboration here. However, if the plane mirror is mounted on the objective of a theodolite telescope normal to the line of sight, a very effective reflection system is obtained which has several advantages over the corner cube, if used directly, or if used in conjunction with the corner cube. In this paper we discuss two applications of the plane mirror mounted on the theodolite.