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EDMI can be mounted a variety of ways, either on its own tripod,
or more commonly on a theodolite. All will measure slope distances
but depending on the model they could be made to compute other
setting out data, e.g. horizontal distances, differences in height, co-

ordinate differences, etc., by inputting vertical and/or horizontal
angles.
Further, some EDMI will permit corrections to be made

automatically for scale factor or prevailing atmospheric conditions
whilst with others it is necessary to correct measured distances
manually after the measuring process.
Depending on the type of EDMI used, electronic calculators could

be attached to compute additional setting out data, or electronic data
loggers for automatic recording of survey data.
Sun light or artificially generated electromagnetic wave consists

of waves of different lengths. The spectrum of an electromagnetic
wave (figure 11.1.) is as shown below:

Figure 11.1. Types of electromagnetic waves

Among these waves microwaves, infrared waves and visible light
waves are useful for the distance measurement. In EDMI these waves
are generated, modulated and then propagated. They are reflected at

the point up to which distance is to be measured from the instrument
station and again received by the instrument. The time taken by the
wave to travel this 2x distance may be measured and knowing the

velocity of wave, the distance may be calculated. However time is too

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