U.S they are usually bronze discs about 3.5 inches in diameter. A typical disk is slightly inset into the top of a concrete pillar set vertically with its surface flush with the ground or projecting slightly. Others are metal rods driven into the ground and possibly surrounded by a capped metal or plastic pipe that can be engraved and stamped like disks. Disks can also be set in rock ledges or boulders, and in the concrete of a large structure such as a building, bridge abutment, or the base of a tower. In the UK, the mark is usually carved into a wall, or on a metal plate set into a wall. In the U.S., a survey disk is usually engraved with the name of the agency that placed it. The name of the mark (or the "station" it locates) is usually stamped into its surface, along with the date on which it was set (or re-set, since markers that have been destroyed, can be replaced). Disks marked with a triangle are known as "triangulation station disks", since they mark the position of the primary point used for triangulation (or map-making). Disks marked with an arrow are called reference marks (RMs), since they point (or "refer") to the principal station that may be located many feet away. A triangulation station often had two or three reference marks. Reference marks were set to enable the primary station to be re-established (or re-set) if needed. Sometimes a survey mark is made much easier to find by the presence nearby of a witness post, a stake (or a small sign) driven into the ground, and used to draw attention to (and to warn against disturbing) the mark. Care must be taken to assure that a found disk is the one described on the data sheet, so as to avoid false reports. A reference mark may be mistakenly reported as the station mark. A disk set in 1945 may be confused with a similarly named disk that was set in 1946 by a different agency, and so on. A RESET elevation benchmark should not be reported as the original. Sorting out these differences and reporting them correctly is an important part of the hobby. Typical locations In the U.S., markers are often found at the summits of mountains, along ridge lines, or on bare rock ledges with commanding views, because such sites provided good