A subtense bar is a bar of known length, with targets at either end. It is usually made of a stable material such as invar. When used with a theodolite, it serves as a quick and convenient method of measuring distances indirectly. The subtense technique is similar to that involving a
tachymeter and graduated rod. Subtense bars were in use in Europe by 1930.
Ref: Wild Heerbrugg, Wild GBL 2m Subtense Bar (Heerbrugg, 1972).
F. Ackerl, "Entfernungsmessungen mit der Wildschen Invar-Basislatte," Zeitschrift für
Instrumentenkunde 9 (1932): 393-400.