In levelling back, sight is also called as ______.
plus sight
Understanding Back Sight in Levelling
In the process of levelling, which is used to determine the relative heights of different points on the ground surface, specific terms are used for readings taken with a levelling instrument and a staff. One such term is 'back sight'.
A back sight (BS) is the first staff reading taken after setting up the levelling instrument (or changing its position). This reading is always taken on a point of known elevation, such as a benchmark (BM), a turning point (TP), or any point whose reduced level (RL) has already been determined.
Why Back Sight is Called Plus Sight
The main purpose of the back sight reading is to determine the Height of the Instrument (HI). The HI is the elevation of the line of sight of the instrument above a reference datum. The formula to calculate HI is:
\( \text{HI} = \text{Reduced Level of the point} + \text{Back Sight Reading} \)
Since the back sight reading is added to the known reduced level of a point to get the Height of the Instrument, it is also commonly referred to as a plus sight.
Comparing Back Sight and Fore Sight
To further clarify, let's compare back sight with fore sight:
- Back Sight (BS) / Plus Sight: The first reading taken after setting up the instrument, always on a point of known or previously determined RL. It is added to the RL to find the HI.
- Fore Sight (FS) / Minus Sight: The last reading taken before moving the instrument to a new position, typically on a turning point or the final point whose RL is to be determined. It is subtracted from the HI to find the RL of the point where the FS is taken.
Detailed Explanation of Levelling Terms
Let's look at some key terms used in levelling:
- Back Sight (BS): The initial reading on a point of known elevation. It's a "plus" reading.
- Fore Sight (FS): The final reading before moving the instrument, or on the target point. It's a "minus" reading.
- Intermediate Sight (IS): Readings taken on points whose elevations are required, taken between the BS and FS from the same instrument position. Like FS, IS readings are subtracted from the HI to find the RL of the point.
- Height of Instrument (HI): The elevation of the line of sight of the instrument. Calculated as \( \text{RL of Back Sight Point} + \text{BS} \).
- Reduced Level (RL): The elevation of a point above a chosen datum. Calculated as \( \text{HI} - \text{FS} \) or \( \text{HI} - \text{IS} \).
- Benchmark (BM): A fixed reference point of known elevation, used as a starting or checking point in levelling.
- Turning Point (TP): A point on which both a fore sight and a back sight are taken when the instrument position is changed.
Therefore, in levelling, a back sight is also known as a plus sight because its value is added to the reduced level of the point to determine the height of the instrument.
| Term | Also Known As | Purpose | Calculation Relation |
|---|---|---|---|
| Back Sight (BS) | Plus Sight | To calculate Height of Instrument (HI) | Added to RL: \( \text{HI} = \text{RL} + \text{BS} \) |
| Fore Sight (FS) | Minus Sight | To calculate RL of a new point | Subtracted from HI: \( \text{RL} = \text{HI} - \text{FS} \) |
| Intermediate Sight (IS) | Minus Sight | To calculate RL of an intermediate point | Subtracted from HI: \( \text{RL} = \text{HI} - \text{IS} \) |
Revision Table: Key Levelling Concepts
| Concept | Description |
|---|---|
| Levelling | Process of determining elevations of points relative to a datum. |
| Back Sight (BS) | First reading from a setup, on known point. |
| Plus Sight | Another name for Back Sight. |
| Fore Sight (FS) | Last reading from a setup, on a point whose RL is sought or a TP. |
| Minus Sight | Another name for Fore Sight and Intermediate Sight. |
| Height of Instrument (HI) | Elevation of the instrument's line of sight. |
| Reduced Level (RL) | Elevation of a point above datum. |
Additional Information on Levelling Techniques
Levelling is a fundamental surveying operation. Different methods exist, such as differential levelling, profile levelling, and cross-section levelling. All these methods rely on accurately taking back sights, fore sights, and intermediate sights using a levelling instrument (like a dumpy level or auto level) and a levelling staff. Proper recording of these readings in a level book and applying the correct arithmetic checks (like the arithmetic check: \( \sum \text{BS} - \sum \text{FS} = \text{Last RL} - \text{First RL} \)) are crucial for accurate results.
Important Questions from Levelling
The expression for sensitivity of the bubble tube (α) can be taken as, ______
Where n = No. of divisions, s = Net staff reading, D = Distance, R = Radius of curvature, l = Length of one division
A vertical line which is perpendicular to the level line is called:
In levelling between two points A and B on the opposite sides of a river, the level was first set up near A and the staff readings on A and B were 2.645 m and 2.30 m respectively. The level was then moved near B and set up; the respective staff readings then were 1.085 m and 1.665 m on A and B respectively. What is the true difference of level between A and B?
A level, when set up $20$ m from peg A and $70$ m from peg B, reads $0.750$ m on a staff held on A and $2.065$ m on a staff held on B, keeping the bubble at its centre while reading. If the reduced levels of A and B are $100.500$ m and $101.800$ m respectively, what is the collimation error per $100.0$ m?
A 'level line' is a-
