Q4. In surveying, which of the following instrument is used for indirect measurement?
📚 Detailed Explanation: Why This Question Was Canceled
This question was officially canceled and declared faulty by the examining commission because every instrument listed — passometer, pedometer, metric chain, and speedometer — is used for direct linear measurement. The question asked for an indirect measurement instrument, but none of the four options qualifies. Since no valid correct answer existed, the question was withdrawn.
Direct measurement: distance is physically measured on the ground step by step, using a chain, wheel rotation, or body movement.
Indirect measurement: distance is computed mathematically from angles, optical readings, or electromagnetic wave travel — without physical contact with the ground over the full length.
Direct vs. Indirect Linear Measurement in Surveying
| Method | How Distance Is Found | Examples |
|---|---|---|
| Direct | Physically measured or counted on the ground | Metric chain, steel tape, pacing, pedometer, passometer, perambulator (odometer), speedometer |
| Indirect | Calculated mathematically from observed angles or signals | Tacheometry (stadia method), EDM instruments, subtense bar |
Why Each Option Is a Direct Method
A. Passometer: A small mechanical counter worn on the body that registers each footfall. Distance is estimated as: steps counted × average step length. The distance is physically walked — direct measurement.
B. Pedometer: Similar to a passometer but typically electronic. It counts steps (or detects body movement) and estimates distance from a calibrated step length. Still a direct, ground-contact method.
C. Metric chain: A chain of standard length (20 m or 30 m) physically laid along the ground to measure horizontal distance. The most direct of all methods — the chain literally spans the distance being measured.
D. Speedometer (Odometer/Perambulator in surveying context): A wheel of known circumference rolled along the ground surface. Each revolution records one circumference length, and total revolutions give total distance. This is mechanical direct measurement along the ground.
Indirect Measurement Methods You Should Know
| Instrument / Method | Principle | Accuracy |
|---|---|---|
| Tacheometer (stadia) | Staff intercept between stadia hairs × constant gives horizontal distance | ±0.3–0.5 m over 100 m |
| Subtense bar | Fixed 2 m bar observed; angle subtended gives distance trigonometrically | ±1:1000 to 1:5000 |
| EDM / Total station | Phase shift or travel time of IR/laser beam to reflector and back | ±1–5 mm + ppm |
Key Concepts for Students
- Tacheometry — the most exam-tested indirect method: A theodolite or tacheometer is set up at one point; a graduated staff is held at the other. The instrument reads the staff between its upper and lower stadia hairs; the intercept multiplied by 100 (the stadia constant) gives the horizontal distance without chaining.
- EDM — the modern standard for indirect measurement: Electronic Distance Measurement uses modulated electromagnetic waves. The instrument emits a signal that reflects off a prism and returns; the measured phase difference or travel time gives the slope distance. EDM is inherently indirect because the signal travels the path, not a chain or person.
- Exam strategy for canceled questions: If a past paper question has no valid answer among the choices, check whether it appears on a published answer key with a cancellation note. Do not guess or memorize a wrong answer. Instead, use the question as a concept check: can you identify what a valid answer would have been?