What is EDM?
To simplify the procedures to obtain accuracy in surveying, the electronic distance measuring method was introduced. Here the distance is measured with the help of electro-magnetic waves such as micro wave, infra red wave and other similar waves. Here the electronic distance measuring equipment and a reflector are necessary to carry out the survey. The wave emitted from the electronic distance measuring equipment reaches the reflector and return back to the electronic distance measuring equipment. Then the distance is measured with the help of time taken for the above process – time taken by the wave for the emission and return. Then the following equation used to measure the distance,
Distance= Velocity X time.
The present EDM instruments have the following features, absolute circle reading, laser plummet, endless drives, 30 x magnifications, high resolution LCD display, Upload and transfer data, data editing and exchange and connectivity to 3rd party devices. The electronic distance measuring equipment is differentiated into three types as, microwave instrument, infra red equipment and light wave equipment.
Micro wave instrument
The surveying equipment functions with the help of micro waves come under this category. Normally these used to measure long distances within 100km with their frequency range. The very first instrument was discovered in South Africa by Col. Harry and developed by Dr. T. L. Wadley in 1950’s and the equipment named as Tellurometer. Tellurometer uses the above mentioned technique, the phase comparison of wave to determine the distance. In this method two equipment and two operators needed to carry out the procedure. Micro wave signals are radiated from primary source equipment to the reflector and then the reflector amplifies the micro wave signals to send them back to the primary source equipment in exact phase as they emitted from. B this method a maximum distance of 80 to 100km can be measured with an error correction of + or – 5 to 15 mm per km.
Infra-red wave instrument
The surveying equipment functions with the help of modulated infra-red waves come under this category. The use of infrared instrument is high in civil engineering surveying as they are inexpensive and more accurate when compare to the rest. Although these type of instrument used to measure up to a maximum distance of 3 – 5km, many civil engineering works limited to this distance. Distomats and electronic tacheometer or total station instruments falls under this category. In distomat, the instrument should be focused to the reflector distomat to receive, monitor and reflect back the wave signal. With a single touch the instrument displays the distance on the screen after measures the five modulation frequencies of infrared wave signal. The total station is the combination of electronic theodolite and electronic distant measurement equipment which can show the angles, elevation and distance. In these instrument an accuracy of + or – 10mm per km can be obtained.
Light wave instrument
The light wave instrument use visible or simply light which means wave lengths that are visible to human eyes to measure the distance. The measuring range of light wave instruments is lesser that that of microwave instruments as because of the less transmitting power of light wave instruments. Here primary equipment is used as light wave emitting instrument and corner cube prisms are used as reflectors. When compare to the rest this method is more cheap when compare the value of instruments. Geodimeter falls under this type of instrument. The one and only main consideration in this type is the alignment of equipment as the reflected wave travel parallel to the emitted wave which means the divergence of emitted wave is less than that of one degree. The plus points of this type instruments are, no signal loss, cheap instrument, one main instrument is enough and a very high accuracy of = or – 0.2mm to 1mm per km.
