Tacheometry - a procedure to obtain horizontal distances and differences in elevations based on the optical geometry of the instrument employed.
It uses subtended intervals and angles observed with an instrument like transit or theodolite, on a graduated scale or rod as a rapid and indirect way of measurement.
A relative accuracy of 1/300 up to 1/500 can be obtained for most horizontal measurements, and differences in elevation to within plus or minus 3 centimeters. This type of measurement also have a lower order of accuracy as compared to taping and differential leveling.
Usage of subtense bar for tacheometric measurements consists of a 2meter long bar mounted horizontally on a tripod aligned perpendicular to the line by means of a sighting device on top of the bar. The horizontal angle subtended by the two sighting marks on the bar is hence read by a transit or theodolite and by trigonometry the distance is computed.
This method may also employs the sighting of a telescope of an instrument in reading a small angle along a vertical plane and in determining the length which the angle subtends on a graduated rod held vertical on the distant point.
Tachemetric methods are used
- to check the more accurate taped distances to uncover gross errors and mistakes
- to determine differences of elevation between points
- to carry lines of levels where a relatively low order of accuracy is required
- to measure lengths of traverse lines
Its most general use is found in compilation of planimetric and topographic maps, in field completion surveys for photogrammetric mapping and in hydrographic surveys' location of details.