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.
Other forms of tacheometry in surveying include the use of stadia rods with theodolites or plane-table alidades. 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.
Another device used in tacheometry is the subtense bar. This is a rigid rod, usually of a material insensitive to changes in temperature and humidity such as invar, that is of fixed length (typically two metres). This 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 allows the bar to be oriented perpendicular to the line of sight to anring station. At the angle measuring station, a theodolite is used to measure the angle between indicators on the two ends of the subtense bar. The distance is determined with simple geometry to be the altitude of a triangle with the theodolite at the upper vertex and the subtense bar length at its base.
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