On a rough terrain, what is the achievable accuracy of a chain under conductive conditions?
🔗 Understanding Accuracy in Chain Surveying
Accuracy in chain surveying refers to the degree of perfection obtained in a measurement. It is often expressed as a ratio, like "1 in 250," which signifies a maximum acceptable error of 1 unit for every 250 units of measured distance.
The term "conductive conditions" in this context refers to an environment that is "conducive to errors," such as rough or hilly terrain. On such ground, it's challenging to keep the chain perfectly level, straight, and under constant tension, leading to a lower expected accuracy compared to measurements on flat, smooth ground.
🔬 Detailed Analysis of the Options
The achievable accuracy of chaining heavily depends on the nature of the terrain and the equipment used.
B. 1 in 250
This is the correct answer. For chaining on rough, uneven, or hilly ground, an accuracy of 1 in 250 is the generally accepted standard. The terrain introduces several sources of error, such as sagging of the chain and difficulty in alignment, which limit the achievable precision.
A. 1 in 200
This represents a slightly lower level of accuracy and is not the standard benchmark, although errors could exceed this in extremely difficult conditions.
C. 1 in 500 & D. 1 in 1000
These options represent a much higher degree of accuracy. An accuracy of 1 in 1000 is typically expected for ordinary chain measurement on relatively flat and even ground. Achieving this level of precision on rough terrain is highly unlikely with a standard chain.
📊 Limit of Precision in Chain Measurement
The following table summarizes the expected accuracy for different surveying conditions and equipment:
| Condition / Equipment | Achievable Accuracy |
|---|---|
| Chain measurement on uneven ground and in hilly areas | 1 in 250 |
| Chain measurement for ordinary work (flat ground) | 1 in 1000 |
| Measurement by tested chain/steel tape | 1 in 2000 |
| Measurement by Invar tape | 1 in 5000 |
💡 Study Tips
- Inverse Relationship: Remember that as the terrain becomes more difficult, the achievable accuracy decreases (the denominator in the ratio gets smaller).
- Equipment Matters: More precise instruments, like steel or Invar tapes, yield higher accuracy. Invar tapes are least affected by temperature changes, making them highly accurate.
- Context is Key: Always associate the accuracy figure with the conditions. 1 in 250 for hills, 1 in 1000 for flat ground with a chain, and higher for tapes.