An Electronic Distance Measurer (EDM) can be used to place objects or points in three dimensions in relation to the unit. The EDM emits a beam of infrared light that is modulated at a controlled rate. During use, the light beam is emitted from the EDM, reflected off a prism or target held at a point to be mapped, and bounced back to the EDM. The phase of the returning beam is shifted from that of the emitted beam, and the shift is directly related to the amount of time it takes for the light to return to the EDM. This shift is what allows the unit to determine the distance traveled by the light. The comparison of the returning signal to that emitted by the EDM allows the unit to determine the distance between the unit and the target surface with an accuracy of approximately 1/8 inch in ¼ mile. Additionally, the EDM also measures the azimuth, or angle from north, and elevation of a point.
The data obtained from the EDM is stored in a data collector and later downloaded for processing using computer drawing software. Below, the image on the top left shows the EDM set up to take measurements, and the top right shows the EDM being used to place the corner of a vehicle at the scene. An example of the type of diagram that can be produced using data collected by an EDM is shown below.