Total Station

A total station is the ultimate in survey instruments. A total station combines a digital theodolite and an EDM that work together with a microprocessor to rapidly and accurately perform tasks. With this combination, the total station can measure horizontal and vertical angles, slope, and horizontal and vertical distances. A total station also has a built-in calculator that performs trigonometric calculations, as well as an electronic field notebook used for storing data. The total station can interface with a computer for data transfer.

Total stations are mainly used by land surveyors and civil engineers, either to record features as in topographic surveying or to set out features (such as roads, houses or boundaries). They are also used by archaeologists to record excavations and by police, crime scene investigators, private accident reconstructionists and insurance companies to take measurements of scenes.

EDM (Electronic Distance Measuring Device)

An EDM is a device mounted on the top of the total station that sends out a light wavelength. The EDM measures the time it takes for the light to travel from the EDM to a prism, bounce off the prism, then return to the EDM. This measurement is displayed as distance, but it is really a measurement of time. Very accurate distance measurements are taken using an EDM.

Theodolite (Transit)

A theodolite measures horizontal and vertical angles. Horizontal angles are measured along a level horizontal axis. Vertical angles are measured along a level vertical axis.

Data Collection Procedures

The data are then processed in the microcomputer to produce a coordinate file which contains point number, point code, X-Y-Z coordinate values, and a point descriptor.

Once the data are on the workstation, they are converted into a graphics design file for use in a CAD program such as Micro Station or AutoCAD. Level, label, symbol, and line definitions are assigned to each point based upon point code. The program can transform data into a two dimensional (2D) or three-dimensional (3D) design file. The 3D file is used to create the digital terrain model (DTM) which is used to produce the contours & Sections. The resulting topographic data are then plotted for review. Final editing and addition of notes are completed, yielding topographic data in a digital format or as a plotted map.

Uniform operating procedures are needed to avoid confusion when collecting survey data. The use of proper field procedures is essential to prevent confusion in generating a map. Collection of survey points in a meaningful pattern aids in identifying map features.

Coding Field Data

Whether data are recorded manually or electronically, one of the most time-consuming survey operations is the recording of a code or description to properly identify the point during processing. For example, in a topographic or planimetric survey, identification points which locate the position of curbs, gutters, center lines, manholes, and other similar features are essential for their correct plotting and contour interpolation.

Especially in topographic or planimetric surveying, many surveyors have wished for some way to speed up the process. For the most part, surveyors tolerate the time-consuming coding process, because it is the only way of ensuring an accurate final product.

In spite of this slow coding process when using data collectors available today, the advantages heavily outweigh the disadvantages. These advantages include collection blunder-free numeric data from electronic total stations virtually at the instant they are available and the error-free transfer of these data to an office computer system without the need for manual entry.

Field coding allows the Surveyor to become the drafter and provide a more logical approach, as the field Surveyor can virtually produce the map from the field data and eliminate the need for many field book sketches. They can also eliminate office plotting, editing by connecting the dots, etc., to produce a final product. The coding scheme is designed so the computer can interpret the recorded data without ambiguity to create a virtually finished product.


Coordinate measurement : Coordinates of an unknown point relative to a known coordinate can be determined using the total station as long as a direct line of sight can be established between the two points. Angles and distances are measured from the total station to points under survey, and the coordinates (X, Y, and Z or easting, northing and elevation) of surveyed points relative to the total station position are calculated using trigonometry and triangulation. To determine an absolute location a Total Station requires line of sight observations and must be set up over a known point or with line of sight to 2 or more points with known location.

For this reason, some total stations also have a Global Navigation Satellite System receiver and do not require a direct line of sight to determine coordinates. However, GNSS measurements may require longer occupation periods and offer relatively poor accuracy in the vertical axis.

Angle Measurement : Most modern total station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles to 0.5 arc-second. Inexpensive "construction grade" total stations can generally measure angles to 5 or 10 arc-seconds..

Distance measurement : Measurement of distance is accomplished with a modulated microwave or infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and reflected by a prism reflector or the object under survey. The modulation pattern in the returning signal is read and interpreted by the computer in the total station. The distance is determined by emitting and receiving multiple frequencies, and determining the integer number of wavelengths to the target for each frequency. Most total stations use purpose-built glass corner cube prism reflectors for the EDM signal. A typical total station can measure distances with an accuracy of about 1.5 millimetres (0.0049 ft) + 2 parts per million over a distance of up to 1,500 metres (4,900 ft).

Data processing

Some models include internal electronic data storage to record distance, horizontal angle, and vertical angle measured, while other models are equipped to write these measurements to an external data collector, such as a hand-held computer. When data is downloaded from a total station onto a computer, application software can be used to compute results and generate a map of the surveyed area. The new generation of total stations can also show the map on the touch-screen of the instrument right after measuring the points.




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