In a levelling work, seven readings were taken with a level as - 1.42, 1.57, 0.765, 1.11, 1.95, 2.9, and 1.825 m. If the instrument was shifted after the third and fifth readings, the readings recorded under the fore sight column will be:
In levelling work, understanding how to classify different types of staff readings is crucial. The readings are generally categorized into Back Sight (BS), Intermediate Sight (IS), and Fore Sight (FS). Each type serves a specific purpose in determining the relative elevations of points.
Levelling Reading Types Explained
Let's first define the key types of readings encountered in a levelling survey:
- Back Sight (BS): This is the first reading taken on a staff held at a point of known elevation (like a benchmark) or a previously established point, immediately after setting up the levelling instrument at a new position. It is used to determine the Height of Instrument (HI).
- Intermediate Sight (IS): These are readings taken on any point between the Back Sight and the Fore Sight from a single instrument setup. They are usually taken on points whose elevations need to be determined, but these points are not used as change points.
- Fore Sight (FS): This is the last reading taken from an instrument setup before the instrument is moved to a new position. The point where a Fore Sight reading is taken is typically a Change Point (CP). A Change Point is a specific station where both a Fore Sight reading and a subsequent Back Sight reading (after shifting the instrument) are taken. The Fore Sight is critical for transferring the line of sight to the next setup. The final reading of a levelling run is also generally considered a Fore Sight.
Identifying Fore Sight Readings
The question provides seven levelling readings: $\text{1.42 m, 1.57 m, 0.765 m, 1.11 m, 1.95 m, 2.9 m, and 1.825 m}$. It also states that the instrument was shifted after the third and fifth readings. Let's classify each reading based on the provided information and standard levelling practices.
| Reading Number | Reading (m) | Classification | Reasoning |
|---|---|---|---|
| 1 | $\text{1.42}$ | Back Sight (BS) | First reading from the initial instrument setup. |
| 2 | $\text{1.57}$ | Intermediate Sight (IS) | Taken from the first setup, not the first or the last reading before a shift. |
| 3 | $\text{0.765}$ | Fore Sight (FS) | The instrument was shifted after this reading. This point acts as a Change Point (CP). |
| 4 | $\text{1.11}$ | Back Sight (BS) | First reading from the second instrument setup, taken on the previous Change Point. |
| 5 | $\text{1.95}$ | Fore Sight (FS) | The instrument was shifted after this reading. This point acts as a Change Point (CP). |
| 6 | $\text{2.9}$ | Back Sight (BS) | First reading from the third instrument setup, taken on the previous Change Point. |
| 7 | $\text{1.825}$ | Fore Sight (FS) | This is the last reading in the given sequence. In levelling, the final reading of a series is typically considered a Fore Sight for closure or completion of the survey segment. |
Based on this detailed classification, the readings that fall under the Fore Sight column are $\text{0.765 m}$, $\text{1.95 m}$, and $\text{1.825 m}$.
Important Questions from Levelling
Calculate the true reduced level (m) of a point A after correcting the refraction and curvature. The staff reading at the point taken from an instrument set at a distance of 2 km from the point A is 2.56 m. The staff reading from the same station on a bench mark of reduced level is 100 m is 1.34 m.
The dumpy level is most suitable for levelling survey:
Levelling in which staff and readings and the distance between the points is required is called:
In permanent adjustment of levels, two peg test is done to correct:
A sewer is laid from a manhole A to a manhole B, 250 m apart along a downward grade of 1 in 125. If the R.L. of the invert at A is 205.75 m and the length of boning rod is 3 m, then R.L. of the sight rail at B is
