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Electromagnetic Distance Measurement

For many years, accurate distance measurement was the most challenging part of surveying. The introduction of electromagnetic distance measurement (EDM) has made the task of distance measuring a simple operation. Nowadays most Texas land surveyors and civil engineers will work with electronic microprocessor. These are controlled instruments that can measure long distances within a few millimetres at the press of a button. Two groups of instrument can be identified, namely the electronic, or microwave, types and the electro-optical velocity of the electromagnetic wave in air must be known precisely for the accurate determination of distance.

In 1926 Michelson determined the velocity of light as 299 796 km/s by measuring the time taken for light to travel between two concave mirror systems. An eight-sided drum was positioned at the principal focus of one system, each side being a plane mirror, and the drum was rotated until a steady image of a source of light was seen in an eyepiece. This occurred at 528 revolutions per second, thereby implying a travel time of 1/4224 s. The US Coast and Geodetic Survey had established the length of line over which the light travelled, and they appreciated that the method could be used in reverse to measure distance. However, it was never adopted directly for that purpose because rotational methods are limited by mechanical considerations.

electromagnetic distance measurement

Microwave systems

The original instruments were developed in South Africa. These systems held an important position in land surveying because they were used to measure distances from say, 50m to at least 50km. However, the introduction of satellite positioning systems (GPS) has diminished that importance.

One instrument, the master, is set up at one end of the distance to be measured, and a second, the remote, is established at the other end. An operator is required at each, intercommunication being possible by means of built-in radio telephones. A modulated signal is transmitted from the master, received by the remote and transmitted back to the master, where the phase difference between the transmitted and returned modulation signal is measured and displayed. The display is usually calibrated to read out directly in metres.

Electro-optical instruments

These are the instruments most likely to be used by the civil engineer. They measure lengths from a few metres to 1km or more and some in fact can measure up to 60km.

The main components of instruments in this group are: a light source, visible light, produced by a tungsten lamp, Xenon flash tube or laser light, or infra-red light; photomultiplier and phase meter; and a readout unit.  In addition a passive reflector system is needed, usually a retrodirective prism at the remote station. The daylight range of instruments having tungsten light sources is much reduced when compared with the microwave instruments, because their radiation has to compete with that of the sun.

The most common types of electronic instrument now available are termed total station instruments.  These incorporate a theodolite with electronic circles and an EDM. The EDM normally works concentric with the telescope eyepiece, and is generally housed in a casing that forms part of the telescope.

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