Survey markers vary considerably from one country to another. In the 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, 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.