Tachymetry Surveying 

Is mean a tacheometric surveying), a method of determining the horizontal distance, direction, and relative  elevation of  a  point  with   respect   to   the   instrument   station by  a  single   sighting  with  a tachymeter telescope on a graduated rod.

The   terms   “tachymetric   surveying”   and   “tachymetry”   (or   “tacheometry”)   are   often   used synonymously. In Soviet usage, however, the term “tachymetry” (takheometriia) is applied to the branch of geodesy that deals with measurement methods and the organization of measurements in the plotting of tachymetric traverses and in the carrying out of a tachymetric survey as a form of topographic survey of an area. The stadia method of surveying used in the USA is an example of a tachymetric method.

In tachymetric surveying, the telescope of the tachymeter is sighted on a rod located at the point being determined, and three coordinates of the point are obtained. These coordinates are the polar coordinates of the point—that is, its direction and its distance s—and the difference h between the elevation of the point and the elevation of the instrument station. The coordinates may be obtained directly, or, with some tachymeters, the coordinates may be computed from the measurement data obtained. In the latter case, the following equations are used:

A tachymeter or tacheometer is a kind 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. A tachymeter scale is also used as the name of the

scale sometimes inscribed around the rim of an analog watch, useful in converting time intervals to

speed or other rates of events.

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

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.

Subtense bars

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 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 an angle measuring 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.