Figure 12-1.—An electromagnetic distance-measuring instrument.
reflector at the other end of the line. As with the
electromagnetic EDM, the line must be free of
obstacles; however, unlike using the electromagnetic
device, the stations at both ends of the line must also
be intervisible. After setup, the EDM sends a
modulated beam of light to the reflector that, acting
like a mirror, returns the light pulse back to the
EDM. When the instrument receives the reflected
light flash, it registers readings that are converted
into linear distance between the EDM and the
reflector (with corrections made for atmospheric
conditions).
DIRECTION OF EDM MEASURED LINES
As you can see from the above discussion, an
EDM transmitter, by itself, is useful for determining
only the length of a line. So, how is both the length
and direction of a line determined when EDM
equipment is used? With some of the older models of
EDMs, distance and direction are determined by
separate setups of an EDM and a theodolite over the
same station. In more recent EDM systems, the EDM
transmitter is mounted on the theodolite or is built
into the theodolite.
12-2
