Question
Download Solution PDFIn levelling, backsight is
Answer (Detailed Solution Below)
Detailed Solution
Download Solution PDFExplanation:
Backsight in Levelling
In levelling, backsight refers to the first staff reading taken immediately after the instrument is set up at a new position. This serves as the reference point for subsequent calculations of elevation differences. Let’s analyze the given options:
Analyzing the Given Options
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Option 1: "A fixed point of known elevation." (Incorrect)
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A fixed point of known elevation is referred to as a benchmark, not a backsight.
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Backsight is a staff reading, not a fixed point.
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Option 2: "The first staff reading taken after setting the instrument." (Correct)
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This is the correct definition of a backsight in levelling.
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The backsight reading is used to establish the height of the instrument (HI).
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Option 3: "The last staff reading taken before shifting the instrument." (Incorrect)
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The last staff reading before shifting the instrument is called a foresight, not a backsight.
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Option 4: "Any staff reading taken on a point of unknown elevation." (Incorrect)
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Staff readings on points of unknown elevation are generally intermediate sights or foresights, not backsights.
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