Which one of the following is not correct regarding measurement of horizontal distances ?
- AAbsolute correction of chain or tape = True length – Nominal length.
- BSteel Tape will give better result than invar tape.Correct
- CTape or chain supported at the two ends will always sag.
- DThe correction due to ‘chain or tape not horizontal’ is always subtractive.
Solution
Concept
Steel tape versus invar tape
Step-by-step solution
1. Invar (nickel–steel alloy) has a very low coefficient of thermal expansion, so invar tapes are far more accurate than steel tapes. 2. Hence the statement that a steel tape gives better results than an invar tape is incorrect. 3. The other statements are correct: sag always makes the tape shorter in horizontal distance (correction subtractive), and slope correction is always subtractive.
Explanation
Invar tapes are used for precise base-line measurement because temperature has almost no effect on them. Steel tapes are less accurate.
Why the other options are wrong
Statements (a), (c) and (d) are true.
Shortcut
Invar = precision; steel = ordinary work.
Common mistake
Marking a true statement in a "not correct" question.
Correct answer: B) Steel Tape will give better result than invar tape.