Tachymeter (survey)

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A tachymeter or tacheometer is a type of theodolite used for rapid measurements and determines, electronically or electro-optically, the distance to target, and is highly automated in its operations. Such tachymeters are often used in surveying.

Tachymetry or tacheometry is the process of measuring distance indirectly. This can be done by measuring time and speed in a moving vehicle or by sighting through small angle a distant scale transverse to the line of sight.

Stadia measurements[edit]

Other forms of tacheometry in surveying include the use of stadia rods with theodolites or plane-table alidades.[1] These use stadia marks on the instrument's reticle to measure the distance between two points on the stadia rod (the stadia interval). This is converted to distance from the instrument to the stadia rod by multiplying the stadia interval by the stadia interval factor. If the stadia rod is not at the same elevation as the instrument, the value must be corrected for the angle of elevation between the instrument and the rod.

d = k × s + c is the formula most widely used for finding the distances. k and c are additive and multiplicative constants. s is the stadia interval. Generally, the instrument is made so that k = 100 and c = 0 exactly, to simplify calculations.

Subtense bars[edit]

Wild brand subtense bar

Another device used in tacheometry is the subtense bar.[1] This is a rigid rod, usually of a material insensitive to change in temperature such as invar, of fixed length (typically two metres). The subtense bar is mounted on a tripod over the station to which the distance is desired. It is brought to level and a small telescope on the bar enables the bar to be oriented perpendicular to the line of sight to the angle measuring station.

A theodolite is used to measure the angle between indicators on the two ends of the subtense bar. The distance from the telescope to the subtense bar is the height of an isosceles triangle formed with the theodolite at the upper vertex and the subtense bar length at its base, determined by trigonometry.


  1. ^ a b Raymond Davis, Francis Foote, Joe Kelly, Surveying, Theory and Practice, McGraw-Hill Book Company, 1966 LC 64-66263