User:Navaneeth S Raj/sandbox

Electronic Distance Meter(EDM) The electromagnetic principles of 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 then the "reflector" is typically a passive medium which bounces the signal back. If the EM is microwave, then the reflector is a second instrument which captures the incoming energy and re-transits it back to the originating instrument. http://jerrymahun.com/library/Distance/graphics/img1.gif

Fig 1 - Electro-optical System

http://jerrymahun.com/library/Distance/graphics/img2.gif

Fig 2 - Microwave System

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

Comparing the two EM types:

EM Type	Advantages	Disadvantages Electro-optical	Less susceptible to atmospheric conditions. Less expensive: only a single transmitter needed. Shorter range. Microwave	Can penetrate fog and rain. Longer range. Transmitter at both ends allows voice communication. Atmospheric affects are greater. Susceptible to ground reflected signals. More expensive: requires two transmitters. The rest of this chapter will limit discussion to electro-optical EM instruments since the majority of EDMs (and Total Stations) employ that EM type.

An EDM does not determine distance by measuring the travel time of the EM signal. Instead, an EDM uses the signal structure and determines the phase shift. The EM signal has a sinusoidal wave form. Remember from trigonometry that the sine curve looks like: http://jerrymahun.com/library/Distance/graphics/img4.gif An EDM measures the line of sight distance between the instrument and reflector. This is a slope distance and not horizontal unless the EDM and reflector are at the same elevation. http://jerrymahun.com/library/Distance/graphics/img11.gif Fig 4 - Slope Distance

In order to determine a horizontal or vertical distance additional information is needed. Combining an EDM with a digital theodolite results in a Total Station Instrument (TSI). When distance measurement is made, the TSI measures the slope distance and a zenith angle. http://jerrymahun.com/library/Distance/graphics/img12.gif

Fig 5 - Total Station Instrument From these two measurements, the Horizontal and Vertical distances are computed by the instrument:

H=S×Sin(Z) Eqn (III-2) V=S×Cos(Z) Eqn (III-3)

It's a little more complex than this and we'll discuss a refinement in the section on Errors.