Modern electronic distance measurement (EDM) measures distance by:
- Ameasurement of wave travel time
- Bmeasurement of phase difference between transmitted and reflected signalsCorrect
- Cmeasurement of velocity of infra-red signals
- Dmeasurement by invar tape
Solution
Concept
Most EDM instruments find distance by comparing the phase of the sent signal and the reflected signal.
Formula
2D = nλ + Δλ, so D = (nλ + Δλ)/2
Step-by-step solution
1. The EDM sends a modulated carrier wave (infra-red, microwave or laser) to a reflector. 2. The reflected wave comes back to the instrument. 3. The instrument measures the phase difference between the sent and returned waves. This gives the fraction of a wavelength, Δλ. 4. Using several modulation frequencies, it finds the whole number of wavelengths, n. 5. Distance D = (nλ + Δλ)/2. Answer (B).
Explanation
Standard EDM and total stations use the phase comparison method. The phase difference between the outgoing and returning signals gives the extra part of a wavelength. The whole number of wavelengths is found by changing the frequency.
Why the other options are wrong
A: The pulse (time-of-flight) method exists, mainly in reflectorless or long-range units, but it is not the standard EDM principle. C: The velocity of the signal is a known constant and is not measured. D: Invar tape is a manual taping method, not EDM.
Shortcut
EDM = phase difference method.
Common mistake
Choosing travel time (A) because light speed is known, but most EDMs measure phase, not direct time.
Correct answer: B) measurement of phase difference between transmitted and reflected signals