The 'EDM' (Electronic Distance Measurement) instrument measures distances using:
Choose the correct answer
Chain and tape stretched between two points
Phase difference (or time-of-flight) of modulated electromagnetic wave — accuracy ±3 mm + 2 ppm
Acoustic/ultrasonic signals under water
Magnetic field strength variation
Correct Answer
B. Phase difference (or time-of-flight) of modulated electromagnetic wave — accuracy ±3 mm + 2 ppm
Why is this the answer?
EDM: measures distance by timing phase shift of modulated electromagnetic wave (carrier: infrared/laser for short range; microwave for long range through rain/fog). Distance = phase difference × modulation wavelength / (4π). Accuracy: ± (3 mm + 2 ppm × D). At D=1 km: accuracy ≈ ± 5 mm. Total station integrates EDM + theodolite. GNSS more accurate for longer distances but EDM used in precise construction layout.
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