Figure 12-1.—An electromagnetic distance-measuring instrument.reflector at the other end of the line. As with theelectromagnetic EDM, the line must be free ofobstacles; however, unlike using the electromagneticdevice, the stations at both ends of the line must alsobe intervisible. After setup, the EDM sends amodulated beam of light to the reflector that, actinglike a mirror, returns the light pulse back to theEDM. When the instrument receives the reflectedlight flash, it registers readings that are convertedinto linear distance between the EDM and thereflector (with corrections made for atmosphericconditions).DIRECTION OF EDM MEASURED LINESAs you can see from the above discussion, anEDM transmitter, by itself, is useful for determiningonly the length of a line. So, how is both the lengthand direction of a line determined when EDMequipment is used? With some of the older models ofEDMs, distance and direction are determined byseparate setups of an EDM and a theodolite over thesame station. In more recent EDM systems, the EDMtransmitter is mounted on the theodolite or is builtinto the theodolite.12-2
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