Oppolzer is located within one crater diameter of the origin of the selenographic coordinate system at 0° N, 0° W. Attached to the surviving remnants of the southeast rim is the crater Réaumur. This formation consists of little more than an irregular arc of mounts along the south, and a ring of scattered ridges along the northern rim. There is a particularly wide gap in the rim to the northeast where no ridges survive to mark the crater perimeter. (wikipedia entry)
