Tacheometry

Tacheometry (/ˌtækiˈɒmɪtri/; from Greek for "quick measure") is a system of rapid surveying, by which the horizontal and vertical positions of points on the Earth's surface relative to one another are determined using a tacheometer (a form of theodolite).

[1] The ordinary methods of surveying with a theodolite, chain, and levelling instrument are fairly satisfactory when the ground is relatively clear of obstructions and not very precipitous, but it becomes extremely cumbersome when the ground is covered with bush, or broken up by ravines.

Chain measurements then become slow and liable to considerable error; the levelling, too, is carried on at great disadvantage in point of speed, though without serious loss of accuracy.

[1] In western countries, tacheometry is primarily of historical interest in surveying, as professional measurement nowadays is usually carried out using total stations and recorded using data collectors.

Alternatively, also by readings of the staff indicated by two fixed stadia wires in the diaphragm (reticle) of the telescope.

[1] Other forms of tacheometry in surveying include the use of a level staff known as a stadia rod with a theodolite or plane-table alidade.

[2] This is a rigid rod, usually of a material insensitive to change in temperature such as invar, of fixed length (typically 2 metres (6.6 ft)).

A tachymeter or tacheometer is a type of theodolite used for rapid measurements and in modern form determines, electronically or electro-optically, the distance to target.