Total Station

Total station

A total station is an electronic theodolite combined with an electronic distance meter (EDM) and a microprocessor, so that horizontal angle, vertical angle and slope distance are measured together, and horizontal distance, elevation difference and coordinates are computed and stored.

Typical features

• Electronic angle measurement (accuracy 1″–5″) with digital display; automatic compensator for the vertical axis error.
• Built-in EDM using an infrared or laser beam to a prism, or reflectorless measurement on surfaces; range of several kilometres with a prism.
• Microprocessor that computes horizontal distance, vertical distance, coordinates (N, E, Z), and also remote elevation, missing line, area and offset.
• Internal memory or memory card for data storage; data can be sent to a computer by cable or Bluetooth.
• Laser plummet, dual-axis tilt sensor, keyboard and graphic display, and built-in programs for resection, setting out and traverse.
• Corrections for temperature, pressure, curvature and refraction, and prism constant.
• Motorised models: automatic target recognition, tracking and robotic control.

Applications in civil engineering

• Topographic and detail surveys; contour and DEM mapping.
• Traversing and control surveys.
• Setting out of buildings, roads, bridges, tunnels and curves.
• Measurement of areas, volumes and cross-sections quickly.
• Deformation and settlement monitoring of dams and structures.
• As-built surveys and land (cadastral) surveys with direct transfer of data to CAD.

Features of a total station

• Electronic theodolite, EDM and microprocessor in one instrument; measures horizontal angle, vertical angle and slope distance together.
• Angle accuracy of 1″–5″; EDM range of a few km with a prism and reflectorless modes.
• Automatic computation of horizontal distance, elevation difference and coordinates with corrections for curvature, refraction, temperature and pressure.
• Dual-axis compensator, laser plummet and onboard programs (resection, setting out, area, remote height).
• Internal memory or memory card, and data transfer by cable, USB or Bluetooth.

Steps of data recording

1. Set up and level the instrument over the station; measure the instrument height.
2. Create a new job or file; enter the station name, instrument height, prism height, and the atmospheric parameters.
3. Orient the instrument by backsighting a known point (set the azimuth or coordinates).
4. Sight the target prism and press the measure key to record the horizontal angle, vertical angle and distance.
5. Enter the point number and code (feature description) for every shot, and store it in memory.
6. Repeat for all points and check the closing readings.
7. Download the data to a computer and process it (editing, adjustment and plotting).

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 of the transmitted and returned signals gives the double path, so 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 $\theta$ the horizontal direction. 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 the observation and booking errors are less.
• 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.

Principle of a total station

A total station is an electronic theodolite with a built-in EDM and a microprocessor. The electronic circle measures the horizontal and vertical angles; the EDM sends an infrared or laser beam to a prism and calculates the slope distance from the phase shift of the returned signal. The microprocessor corrects the readings and computes horizontal distance $H = S\cos\alpha$, vertical difference $V = S\sin\alpha$ and the coordinates of the target.

Digital data recording fundamentals

1. Job set-up: a job file is created and the station name, coordinates, instrument and prism heights and the atmospheric data are entered.
2. Orientation: backsight is taken on a reference station to set the azimuth.
3. Measurement: each sighted point is stored in the memory (or card) automatically as point number, horizontal angle, vertical angle, slope distance and code (feature).
4. Storage and transfer: data are kept in internal memory or on a card and transferred to a computer through cable, USB or Bluetooth in standard formats.
5. Processing: the raw data are edited and adjusted with software, and coordinates, maps and sections are produced.

Digital recording removes manual booking errors and speeds up the survey.

A total station is an instrument that combines an electronic theodolite, an electronic distance meter (EDM) and a microprocessor, and measures horizontal angle, vertical angle and slope distance together, then computes coordinates and stores them.

Typical features

• Electronic angle measurement (1″–5″) and EDM range up to several km with prism.
• Reflectorless (laser) distance measurement on surfaces.
• Dual-axis compensator, laser plummet and built-in programs (resection, setting out, area, remote elevation).
• Memory card or internal memory, USB/Bluetooth data transfer.
• Automatic corrections for curvature, refraction, temperature and pressure.
• Robotic models with automatic target tracking.

Three points to keep in mind when recording data

1. Correct set-up data: enter the correct station name/coordinates, instrument height, prism height and prism constant, and the atmospheric (temperature, pressure) data.
2. Backsight/orientation: the instrument must be oriented by sighting a known point, and the orientation must be checked.
3. Unique point number and code: each point is stored with a unique point number and a feature code so that data can be processed correctly; also check the storage capacity and battery before starting, and transfer the data safely.