1. Electromagnetic Energy

The electromagnetic principles of Electronic Distance Measurement (EDM) theory and operation are well covered in most surveying text books and on the internet. The intent here is to give the reader a general understanding of EDM so that error sources are better understood and controlled.

An EDM uses electromagnetic (EM) energy to determine the length of a line. The energy originates at an instrument at one end of a line and is transmitted to a "reflector" at the other end from where it is returned to the originating instrument. The nature of the "reflector" is dependant on the type of EM.

If electro-optical (infrared or laser) EM is used, Figure A-1, then the "reflector" is typically a passive device which bounces the signal back to the transmitter.

If the EM is microwave, Figure, A-2, then the reflector is a second instrument which captures the incoming energy and re-transits it back to the originating instrument.


 a 1

Figure A-1
Electro-optical EDM


 a 2
Figure A-2
Microwave EDM


In either case the measurement is the total distance from the instrument to the reflector and back to the instrument. 

We will limit discussion to electro-optical EM instruments since the majority of EDMs and Total Stations employ that EM type.