Purpose of Backsight Observation in Levelling
In the process of differential levelling, observations are taken using a levelling instrument and a graduated staff. Two primary types of observations are made:
- Backsight (BS): An observation taken on a point of known elevation (typically a benchmark).
- Foresight (FS): An observation taken on a point whose elevation needs to be determined.
Role of the Backsight
The main goal of taking a backsight observation is to determine the elevation of the instrument's line of sight, also known as the Height of Instrument (HIS). This is calculated using the known elevation of the benchmark (BM) and the staff reading obtained during the backsight.
The formula is:
$ \text{HIS} = \text{Elevation of BM} + \text{Backsight Reading (BS)} $
Once the HIS is established, it remains constant for all points sighted from that instrument position. Subsequent foresight readings (FS) are then used to calculate the elevation of unknown points:
$ \text{Elevation of Unknown Point} = \text{HIS} - \text{Foresight Reading (FS)} $
Therefore, the backsight is essential for establishing the reference elevation for the line of sight from the current instrument setup.
Analysis of Options
- 1. To ascertain the elevation of line of sight: This is the primary purpose. The backsight reading on a benchmark allows the calculation of the instrument's line of sight elevation (HIS). (Correct)
- 2. To find out the collimation error of the instrument: Collimation error is checked through specific tests (like the two-peg test) or by ensuring equal BS and FS distances, not by a single backsight observation.
- 3. To find the elevation of the benchmark: The elevation of a benchmark is predetermined and known; it is used as a reference, not determined, by the backsight.
- 4. To find the elevation of the point over which the staff is held during the backsight: This is incorrect. The backsight is taken on a point (benchmark) whose elevation is already known. The purpose is to determine the instrument's line of sight elevation, not the benchmark's elevation.
Important Questions from Levelling Methods
- The observations of a reciprocal levelling operation carried out from two stations A and B are given in table. It is known that the instrument has collimation error. The correct staff reading over station B when the instrument is set very near to station A is ________ m (Rounded off to three decimal places).
Assume all other errors are negligible.Staff readings at station Instrument set very near to station A (m) B (m) A 1.275 2.005 B 1.040 1.660 - A level with the height of the instrument being 2.550 m has been placed at a station having a Reduced Level (RL) of 130.565 m. The instrument reads 3.665 m on a levelling staff held inverted at the bottom of a bridge deck. The RL of the bottom of the bridge deck is __________ m (rounded off to 3 decimal places).
- In levelling between two points P and Q on opposite banks of a river, the level was set up near P, and the staff reading on P and Q were 2.165 m and 3.810 m, respectively. The level was then moved and set up near Q and the respective staff readings on P and Q were 0.910 m and 2.355 m. The true difference of level between P and Q is __________ m (rounded off to 3 decimal places).
- Change Point (CP) in levelling refers to a location where
- At a fixed instrument location in levelling, if the backsight reading at a point P is more than the foresight reading at a point Q, then
