Modern electronic distance measurement (EDM) measures distance by ______

Question

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The correct answer is Option D: measurement of phase difference between transmitted and reflected signals. Modern Electronic Distance Measurement (EDM) instruments determine distance by measuring the phase difference between the modulated carrier wave transmitted to a reflector and the reflected signal returned to the instrument. This phase shift is proportional to the time taken for the wave to travel the round-trip distance.

Step-by-Step Solution & Concepts:
Modern Electronic Distance Measurement (EDM) instruments determine distance by measuring the phase difference between the modulated carrier wave transmitted to a reflector and the reflected signal returned to the instrument. This phase shift is proportional to the time taken for the wave to travel the round-trip distance.

Working Principle: The instrument emits a light or infrared wave modulated by a frequency $f$. When the wave returns after reflection, the instrument compares its phase to the transmitted wave. The phase shift $\Delta\phi$ is measured, allowing the determination of the fractional wavelength distance, while the number of full wavelengths is determined by varying the modulation frequencies.

Key Formulas: $D = \frac{n \lambda}{2} + \frac{\Delta\phi}{2\pi} \cdot \frac{\lambda}{2}$ — representing total distance $D$ where $n$ is the number of full wavelengths and $\lambda$ is the modulation wavelength; $\Delta\phi = 2\pi \cdot \frac{2D}{\lambda}$ — relating phase shift $\Delta\phi$ to distance $D$

Key Points: EDM instruments utilize electromagnetic waves (visible light, infrared, or microwaves) for measurement.; The phase difference method is highly precise for short to medium range measurements.; Atmospheric conditions (temperature, pressure, humidity) affect the refractive index and hence the speed of the wave.

Advantages: High accuracy over long distances compared to traditional taping.; Capable of measuring across obstacles (e.g., rivers or valleys) where physical chaining is impossible.

Disadvantages: Requires a power source and sophisticated electronic components.; Susceptible to errors due to atmospheric variations if corrections are not applied.

Applications: Topographic surveying and mapping.; Setting out of engineering projects and tunnel alignment.

Additional Info: Option B is the principle of traditional surveying using physical tapes, not electronic.. The speed of light $c$ is typically taken as $299792458 	ext{ m/s}$, and the actual velocity in air depends on the refractive index $n$ calculated as $v = \frac{c}{n}$.

Answer: D is correct — Modern EDM instruments function primarily by detecting the phase difference between the sent and received electromagnetic waves.

Exam Tip: In competitive exams, remember that while microwave EDM often uses time-of-flight (pulse) measurement, the most common optical/infrared EDM instruments used in Total Stations rely on the phase difference principle.

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Option A: measurement of velocity of infra-red signals

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Option B: measurement by invar tape

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Option C: measurement of wave travel time

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