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short, measuring the time taken is difficult. The improved techniques
use phase difference method in which the number of completed wave
and incomplete wave is measured. Knowing the length of wave,
distances are calculated.

EDM instruments may be classified into the following three types:

1. Micro Wave Instruments make use of micro waves. Such

instruments were invented as early as 1950 in South Africa by Dr.
T.L. Wadley who named them Tellurometers. The instrument needs
only 12 to 24 V batteries. They are light and highly portable.
Tellurometers can be used in day as well as in night. The range of

these instruments is up to 100 km. It consists of two identical units.
One unit is used as master unit and the other as remote unit. A speech
facility is provided to each operator to interact during measurements.

2. In Infrared Wave Instruments amplitude modulated infrared
waves are used. Prism reflectors are used at the end of line to be
measured. These instruments are light and economical and can be
mounted on a theodolite. With these instruments accuracy achieved is

± 10 mm. The range of these instruments is up to 3 km. These
instruments are useful for most of the civil engineering works.

3. Light Wave Instruments rely on propagation of modulated
light waves. This type of instrument was first developed in Sweden
and was named as Geodimeter. During night its range is up to 2.5 km
while in day its range is up to 3 km. Accuracy of these instruments

varies from 0.5 mm to 5 mm/km distance. These instruments are also
very useful for civil engineering projects.
The advantage of using EDM instruments is the speed and

accuracy in measurement. Several obstacles to chaining are
automatically overcome when these instruments are used.

Task 3. Answer the questions to the text.

1. What is EDMI used for??
2. How do the field-to-finish systems work?
3. What are the ways of EDMI mounting?

4. What waves are useful for distance measurement?
5. What are the three types of EDM instruments?
6. What do you known about microwave instruments?
7. What instruments use the amplitude modulated infrared waves?

8. What is a geodimeter?

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