Levelling in which staff and readings and the distance between the points is required is called:
profile levelling
Understanding Levelling Methods in Surveying
Levelling is a surveying procedure used to determine the relative heights of points on the earth's surface. Different methods of levelling are employed depending on the purpose of the survey and the required accuracy. The question asks about a specific type of levelling that requires the use of a levelling staff, taking readings on this staff, and measuring the distance between the points being levelled.
Analyzing the Given Options for Levelling
Let's examine each option to see which one best matches the description provided in the question:
- Profile Levelling: This method is used to determine the elevations of points along a specific line, such as the centerline of a proposed road, railway, or canal. It involves setting up the level, taking readings on a staff placed at various points along the line (known as stations or chainages), and recording the distances of these points from a starting reference point or from each other. Intermediate sights (IS) are taken at regular intervals or critical points along the line. Therefore, staff, staff readings (BS, IS, FS), and the distance between points are essential for profile levelling.
- Fly Levelling: This is a rapid method used to determine the approximate difference in elevation between two points, usually located far apart. It involves taking a series of setups, with only backsight (BS) and foresight (FS) readings taken at each setup. Intermediate sights and detailed measurements of distances between intermediate points are generally not required in this method, which focuses on quickly transferring a reduced level over a long distance.
- Trigonometric Levelling: This method determines the elevation of a point by measuring the vertical angle to the point and the horizontal or slope distance from the instrument to the point. It does not directly use a levelling staff to measure vertical differences from a horizontal line of sight of a level instrument. Instead, it uses trigonometric principles based on angles and distances.
- Auto Levelling: This term refers to a type of levelling instrument (an automatic level) that uses a compensator to automatically maintain a horizontal line of sight once the instrument is roughly levelled. It is a type of instrument used in various levelling methods, including differential, profile, and fly levelling, but it is not a distinct levelling method itself based on the requirements mentioned (staff, readings, distance).
Conclusion on Required Levelling Method
Based on the characteristics of each levelling method, the process that specifically requires staff, staff readings (including intermediate sights), and the distance between points to determine the elevation along a defined line is profile levelling.
In profile levelling, the distances are measured along the line (often using a tape or electronic distance measurement) to locate the points where staff readings are taken. These distances are crucial for plotting the profile, which is a graph showing the elevation variation along the line.
| Levelling Method | Requires Staff? | Requires Staff Readings? | Requires Distance Between Points for Detailed Profile? |
|---|---|---|---|
| Profile Levelling | Yes | Yes (BS, IS, FS) | Yes |
| Fly Levelling | Yes | Yes (BS, FS) | Generally No (not detailed intermediate distances) |
| Trigonometric Levelling | No (uses vertical angles & distances) | No | Yes (horizontal/slope distance) |
| Auto Levelling | Yes (instrument type, used in methods) | Yes (instrument type) | Depends on the method used with it |
Therefore, the levelling method described, which requires staff, staff readings, and the distance between points, is profile levelling.
Revision Table: Key Levelling Terms
| Term | Definition | Relevance to Question |
|---|---|---|
| Levelling Staff | A graduated rod used to measure vertical distances from the line of sight of a levelling instrument. | Required in methods like profile and fly levelling. |
| Backsight (BS) | The first staff reading taken after setting up the level, usually on a point of known or assumed elevation. | Part of staff readings required in profile and fly levelling. |
| Foresight (FS) | The last staff reading taken before moving the level, usually on a point whose elevation is to be determined or a turning point. | Part of staff readings required in profile and fly levelling. |
| Intermediate Sight (IS) | A staff reading taken on any point whose elevation is required, other than a backsight or foresight. | Crucial for determining elevations at various points along the line in profile levelling. |
| Reduced Level (RL) | The elevation of a point with respect to a datum (reference surface). | The ultimate goal of levelling methods is to determine RLs. |
| Height of Instrument (HI) | The elevation of the line of sight of the level instrument. Calculated as RL of BS point + BS reading. | Used in the 'Height of Instrument' method of calculating levels. |
Additional Information on Profile Levelling Applications
Profile levelling is a fundamental technique in civil engineering and surveying for planning and designing linear infrastructure projects. Its applications include:
- Designing roads, railways, and canals to ensure appropriate gradients for drainage and traffic flow.
- Planning pipeline and sewer layouts, considering gravity flow requirements.
- Estimating earthwork volumes for cutting and filling along the alignment.
- Determining ground clearances for overhead structures or obstructions.
The detailed information about elevations and distances collected during profile levelling allows engineers to create a vertical cross-section of the ground along the proposed route, which is essential for subsequent design stages.
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:
Which level has internal compensator mechanism to automatically adjust the line of sight?
Bench mark is established by -
Which of the following is a correct statement with reference to determining reduced level of a point using rise and fall method?
