Question

The maximum tolerances in overall length of a 20 m and 30 m metric chains should be respectively
1. ± 2 mm,± 8 mm
2. ± 3 mm,± 5 mm
3. ± 5 mm,± 8 mm
4. ± 8 mm,± 5 mm

Answer 1

Correct Answer - Option 3 : ± 5 mm,± 8 mm

Concept:

Chain surveying:

• Chain surveying is the simplest method of surveying.
• Measurements for this survey are taken on the field and other supplementary works like plotting and calculations are carried out in the office.
• The measurements in chain surveying are linear.
• Chain, tape, ranging rod, arrows and, cross-staff, Offset rods, Plumb bob are the tools required for chain surveying.

The maximum tolerance allowable for the metric chain are as follows:

05 m chain ⇒ ± 3 mm

10 m chain ⇒ ± 3 mm

20 m chain ⇒ ± 5 mm

30 m chain ⇒ ± 8 mm

Types of chain:

• Revenue chain - 33 ft long (16 links)
• Gunter's chain - 66 ft long ( 100 links)
• Engineer's chain - 100 ft long (100 links)
•  
• Metric chain -  30 m (150 links) or 20 m (100 links)

The unsuitability of chain:

• As being heavier, it sags considerably when suspended in the air.
• It is suitable for ordinary work only because the length of links gets altered by shortening.
• As per IS specifications every meter length of the chain should individually be accurate to within ±2 mm when measured under a tension of 80 N (8 kgf).