Question

Which one of the following is the purpose of taking a backsight observation in levelling?

• A. To ascertain the elevation of line of sight
• B. To find out the collimation error of the instrument
• C. To find the elevation of the benchmark
• D. To find the elevation of the point over which the staff is held during the backsight

Hint:

Think about what quantity a backsight reading on a known point lets you calculate for the instrument itself.

Answer 1

The Correct Option is A

Step 1: Recall the sequence of readings in differential levelling.
In levelling, the surveyor sets up the instrument at a station and takes a series of staff readings. A backsight (BS) is the very first reading taken after the instrument is set up, always on a point of already known elevation, such as a benchmark or a previously established change point.
Step 2: Recall how the Height of Instrument (HI) is computed.
Once the backsight reading is taken on the point of known elevation, the height of the line of sight of the instrument (also called the Height of Collimation or Height of Instrument) is obtained as \( HI = \text{Elevation of the known point} + \text{Backsight reading} \). This HI represents the elevation of the horizontal line of sight of the level, which is the reference from which all other readings at that instrument setup (intersights and the final foresight) are subtracted to get elevations of unknown points: \( \text{Elevation of unknown point} = HI - \text{staff reading at that point} \).
Step 3: State the purpose of the backsight explicitly.
So the entire reason a backsight is taken is to transfer the already known elevation of a benchmark or change point up to the level of the instrument's line of sight, that is, to ascertain the elevation of the line of sight for that instrument setup. Every subsequent reading at that setup depends on this HI value being correctly established first.
Step 4: Eliminate the other options.
Collimation error (option B) is a separate instrumental error found by a two peg test, not by an ordinary backsight in a levelling run. The elevation of the benchmark (option C) is already known before the backsight is taken, it is used to find the HI, not found from the backsight. Option D describes the point over which the staff is held during the backsight, but that point's elevation is already known input data, not something the backsight itself determines. Hence the correct purpose is to ascertain the elevation of the line of sight.
\[ \boxed{\text{Option (A): To ascertain the elevation of line of sight}} \]