bench

A bench in surveying is a fixed bracket or stone platform set into a building wall, cliff face, or other permanent structure to hold a theodolite, level, or other instrument at a stable height. Unlike a tripod setup, which can be moved and adjusted on site, a bench is meant to stay in place, providing a permanent or semi-permanent observation point for repeated measurements over months or years.

The bench itself is typically a small granite block, roughly 10 to 15 centimeters in depth, with a flat upper surface or a recessed socket. The socket often contains a brass or steel pin, called a bench mark or reference mark, that marks a precise point. The instrument mounting bracket bolts to the bench, and the pin acts as a known elevation reference that surveyors can return to and rely on across multiple survey campaigns.

Common installations and variants

Benches are set into walls of lighthouses, church towers, fortifications, and other buildings that are unlikely to move. They are also cut directly into rock faces on hillsides or mountain survey routes. The depth and strength of the bench depend on local geology and the weight of the instrument; a rough-country bench for a level may be lighter than one used for a theodolite in precise geodetic work. Modern survey benches often come with stainless steel hardware to resist weathering.

The term bench originates from the physical resemblance to a small seat or shelf. In older surveying practice, the word also referred to the row of marks that surveyors would chip or carve into stone to mark a chain of elevations along a route. Over time, the individual mounting points became simply called benches, and the mark itself is now usually called a benchmark or control point.

Benches are essential in geodetic surveying, where networks of control points must be measured and remeasured at intervals to detect crustal movement, subsidence, or settlement of structures. Because the bench is fixed, two surveyors a year or a decade apart can set up at the same point, measure to the same mark, and compare results with confidence that any difference reflects actual change in the landscape, not drift in their setup.