A. Lietz Company San Francisco Precision Alidade No 31, c1925

This is a gorgeous A. Lietz Company (S.F.Ca.) Precision Alidade No. 31, c1925. Founded in San Francisco in 1882 by German instrument makert Adolph Lietz, the A. Lietz Company grew to become one of the premier scientific instrument manufacturers on the American West Coast. A. Lietz built heavy-duty, highly reliable transits, levels, and alidades that were widely adopted by the U.S. Geological Survey (USGS), military engineers, and forestry departments throughout the 20th century. This instrument is fully functional, optically excellent, all original and fully intact.  Mostly brass, it weighs ~8 lbs, is ~18in long and ~8in high. On the base are a box compass and a long bubble level, both working and accurate.

In traditional field surveying, an alidade is a sighting mechanism mounted onto a flat, straightedge ruler base. It is designed to be used on a plane table (a portable drafting board mounted to a leveled tripod). The surveyor looks through the alidade to sight a distant landmark or target rod. Because the telescope or sight is perfectly aligned parallel to the heavy metal base, the surveyor can immediately draw a precise line of sight directly onto a sheet of drawing paper pinned to the table. This allowed mapping crews to create topographically accurate maps live in the field. This instrument includes a specialized vertical gradient scale called a Beaman Arc. This attachment allows surveyors to rapidly compute complex elevation differences mathematically in their heads rather than relying on laborious trigonometric lookup tables in the field.