tachymetry

Tachymetry is a surveying method used to determine horizontal distances, vertical heights, and elevations of points relative to a station. It is a branch of surveying that utilizes a tachymeter, a surveying instrument that combines a theodolite and an electronic distance measurement (EDM) device, or a total station, which integrates these functionalities with a microprocessor. The fundamental principle behind tachymetry involves taking readings on a graduated staff, known as a stadia rod, held vertically at the points to be surveyed.

The tachymeter is set up at a known station, and its horizontal and vertical angles are carefully measured to the stadia rod. The readings on the stadia rod, along with the known vertical angle and the instrument's internal constants (multiplying and additive constants), are used in trigonometric formulas to calculate the horizontal distance and the difference in elevation between the instrument station and the point where the rod is held. Modern total stations automate these calculations, displaying the results directly.

Tachymetry is highly efficient for topographical surveying, creating contour maps, and establishing control points in construction projects. Its speed and accuracy make it a valuable technique, especially when rapid data collection is required over relatively large areas. The accuracy of tachymetric measurements is influenced by factors such as the quality of the instrument, the distance to the object, atmospheric conditions, and the precision of the staff readings.