EDM in surveying stands for Electronic Distance Measurement, which is a method of determining the length between two points using electromagnetic waves. EDM is commonly carried out with digital instruments called theodolites. 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.

EDM technology is widely used in land surveying to establish property boundaries, create topographic maps, and perform precise construction layout. The measured distances and coordinates are recorded electronically, making it easy to compile detailed survey data for mapping or construction purposes. EDM instruments are highly reliable and convenient pieces of surveying equipment and can be used to measure distances of up to 100 kilometers. They have accuracy up to 1 in 105. EDM instruments are vital for creating detailed topographic maps, measuring distances between boundary markers or reference points, precisely marking the positions of structures, foundations, and utilities, and mapping underground geological surveys.