Total station principle, importance and uses

Working principle

A total station combines the angle measuring system of an electronic theodolite with an electromagnetic distance meter (EDM) and a microprocessor.

      Prism <---- signal ----> Total station
        (R)      slope dist S      (T)
                 ^ V angle
                 |  H angle from reference

• Angles: the horizontal and vertical circles are coded glass circles read by an electronic sensor; the angle is displayed digitally.
• Distance: the EDM sends a modulated infrared or laser beam to a reflector, which returns it. The phase difference between the transmitted and returned signals gives the double-path distance $2S$, from which the slope distance $S$ is found.
• Computation: the microprocessor applies corrections and computes

$$H = S\cos\alpha,\quad V = S\sin\alpha,\quad \Delta N = H\cos\theta,\quad \Delta E = H\sin\theta$$

where $\alpha$ is the vertical angle (from horizontal) and $\theta$ is the azimuth of the line (horizontal direction measured from north). From the station coordinates, the coordinates $(N, E, Z)$ of the target are found and stored.

Importance

• A single instrument measures angles and distances, so field work is faster and there are fewer observation and booking errors.
• Gives results with high accuracy; computations are automatic.
• Data can be transferred directly to a computer, saving time in plotting and drawing.

Uses in surveying and mapping

• Topographic and detail surveys; traversing and control densification.
• Setting out of structures, roads and curves.
• Contour and DEM generation; area and volume computations.
• Deformation monitoring, as-built and cadastral surveys; mapping by direct input to CAD/GIS.