Which of the following statements are correct? In chain/tape surveying, the 3-4-5 method can be used: i. to set out contour line, ii. to draw a perpendicular line to the chain line, iii. to draw a perpendicular offset, iv. to remove the obstacle to ranging.
- Aii and iv
- Bii and iiiCorrect
- Ciii and iv
- Di and iv
Solution
Concept
A triangle with sides 3, 4 and 5 units has a right angle, so a chain or tape can set out a perpendicular.
Formula
32 + 42 = 52
Step-by-step solution
1. A triangle with sides 3, 4 and 5 has a 90° angle (Pythagoras). 2. So the 3-4-5 method sets out a right angle on the ground. 3. It can draw a perpendicular line to the chain line (ii is correct). 4. It can set out a perpendicular offset to an object (iii is correct). 5. A contour line needs levelling, not a right angle (i is wrong). 6. Ranging needs ranging rods and line of sight. The 3-4-5 method does not remove an obstacle (iv is not the main use). 7. Answer: ii and iii, option B.
Explanation
The 3-4-5 method gives a right angle using only a chain or tape. So it is used to draw a perpendicular to the chain line and to set out a perpendicular offset. Contours need levelling, and ranging needs ranging rods.
Why the other options are wrong
A: iv is not a direct use of the 3-4-5 method. C: iv is not a direct use, and ii is also correct. D: i is wrong because contours need levelling, and iv is not a direct use.
Shortcut
3-4-5 means right angle, so pick the options that talk about a perpendicular.
Common mistake
Choosing iv because perpendiculars are used to go around obstacles. The obstacle is crossed by geometry, not removed by the 3-4-5 method.
Correct answer: B) ii and iii