ELECTROMAGNETIC DISTANCE MEASUREMENT (EDM)
main instrument for surveyors on site today is the ‘total station’. It is an instrument that combines the angle measurements that could be obtained with a traditional theodolite with electronic distance measurements.
Taping distance, with all its associated problems, has been rendered obsolete for all baseline measurement. Distance can now be measured easily, quickly and with great accuracy, regardless of terrain conditions. Modern total stations as in Figure and Figure contain algorithms for reducing the slope distance to its horizontal and vertical components.
For engineering surveys total stations with automatic data logging are now standard equipment on site. A standard measurement of distance takes between 1.5 and 3 s. Automatic repeated measurements can be used to improve reliability in difficult atmospheric conditions. Tracking modes, for the setting out of distance, repeat the measurement several times a second. Total stations with their inbuilt EDM enable:
(1) Traversing over great distances, with much greater control of swing errors.
(2) The inclusion of many more measured distances into control networks, rendering classical triangulation obsolete. This results in much greater control of scale error.
(3) Setting-out and photogrammetric control, over large areas, by polar coordinates from a single base line.
(4) Deformation monitoring to sub-millimetre accuracies using high-precision EDM, such as the Mekometer ME5000. This instrument has a range of 8 km and an accuracy of ±0.2 mm ±0.2 mm/km of the distance measured ignoring unmodelled refraction effects.
Classification of instruments:
Historically EDM instruments have been classified according to the type and wavelength of the electromagnetic energy generated or according to their operational range. Very often one is a function of the other. For survey work most instruments use infra-red radiation (IR). IR has wavelengths of 0.8–0.9 μm transmitted by gallium arsenide (GaAs) luminescent diodes, at a high, constant frequency.
The accuracies required in distance measurement are such that the measuring wave cannot be used directly due to its poor propagation characteristics. The measuring wave is therefore superimposed on the high-frequency waves generated, called carrier waves. The superimposition is achieved by amplitude (Figure ), frequency (Figure) or impulse modulation (Figure ). In the case of IR instruments, amplitude modulation is used. Thus the carrier wave develops the necessary measuring characteristics whilst maintaining the high-frequency propagation characteristics that can be measured with the requisite accuracy.
In addition to IR, visible light, with extremely small wavelengths, can also be used as a carrier. Many of the instruments using visual light waves have a greater range and a much greater accuracy than that required for more general surveying work. Typical of such instruments are the Kern Mekometer ME5000, accurate to ±0.2 mm ±0.2 mm/km, with a range of 8 km, and the Com-Rad Geomensor CR204.