Establishment of Benchmark by Spirit Levelling

Spirit levelling is a type of levelling technique that is used to establish the elevation of a particular point with respect to a reference point called a benchmark. The benchmark is usually a fixed point that has a known elevation, and it is used as a reference for all the other points in the survey.

Process of Establishing a Benchmark by Spirit Levelling

The process of establishing a benchmark by spirit levelling involves the following steps:

1. Selection of Benchmark: The first step is to select a suitable benchmark that is easily accessible and has a known elevation. The benchmark should be stable and should not be affected by any external factors such as weather or construction activities.

2. Setting up the Levelling Instrument: Once the benchmark is selected, the levelling instrument is set up at the benchmark. The instrument consists of a telescope that is mounted on a tripod, and it is used to measure the elevation of the other points in the survey.

3. Levelling the Instrument: The instrument is levelled using a bubble level, and it is adjusted until the bubble is centred. This ensures that the telescope is horizontal, and the measurements are accurate.

4. Measuring the Elevation of Other Points: Once the instrument is levelled, the elevation of the other points in the survey is measured by sighting through the telescope and reading the measurement on the levelling staff. The staff is a rod that is held vertically at the point where the elevation is to be measured.

5. Calculation of Elevations: The elevations of the other points are calculated by subtracting the reading on the staff from the height of the instrument at the benchmark. This gives the relative elevation of each point with respect to the benchmark.

6. Recording of Results: The elevations of the other points are recorded in a levelling book, and a benchmark is established for the survey. The benchmark is used as a reference for all the other points in the survey, and it is used to calculate the elevations of other points in the future.

Conclusion

In conclusion, spirit levelling is a reliable and accurate technique for establishing a benchmark. It is commonly used in civil engineering projects such as road construction, building construction, and land surveying. By using this technique, engineers and surveyors can ensure that their measurements are accurate and reliable, and they can use the benchmark as a reference point for future surveys.