Electro-optical EDMs use the velocity of light waves to determine the distance between two points. The earliest of these instruments, typified by the Geodimeter, was developed during the same decade as the electromagnetic EDMs. Figure 12-3 shows an example of a Geodimeter. Like the electromagnetic instruments, the first generation of electro-optical instruments were heavy, bulky, and not well suited to the needs of the practicing surveyor; however, through later development, modern electro-optical EDMs are smaller, lighter, easier to use, and require less power. Modern short-range instruments have ranges from 0.3 miles to 3 miles. Longer range instruments, using coherent laser light, have ranges from 50 feet to 36 miles.

To use an electro-optical EDM, you set up the instrument at one end of the line being measured and a 

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).

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.

Figure 12-2.Electromagnetic distance-measuring equipment in use.