Explain the principle of levelling.

Levelling is a process used in surveying and civil engineering to determine and measure the height differences between points on the Earth's surface. This is achieved by using an optical instrument called a level, which consists of a telescope mounted on a tripod at a specific height. The principle of levelling involves measuring vertical angles and distances from a known reference point, called the benchmark, to the point of interest. This information allows you to calculate the elevation (height) of the specified point relative to the reference point.

The levelling process can be broken down into the following steps: 1. Setting up the level: The level, which is an instrument consisting of a telescope mounted on a tripod, needs to be set up at a location that has a clear view of both the benchmark and the point of interest. The telescope is adjusted until it is perfectly horizontal. 2. Reading the benchmark: With the level set up, a surveying staff (also called a leveling rod) is placed vertically at the benchmark. The staff is divided into divisions (similar to a ruler) and has a sliding target. The levelling rod is read through the telescope. The height at which the telescope intersects the staff is noted as the back-sight reading. 3. Reading the point of interest: Without moving the level, the surveying staff is moved to the point of interest, and another reading is taken from the staff through the telescope. This reading is called the fore-sight reading. 4. Calculating the height difference: To find the height difference between the benchmark and the point of interest, subtract the fore-sight reading from the back-sight reading. This difference is known as the rise or fall. 5. Determining the elevation: If the calculated difference is positive, it means the point of interest is higher than the benchmark. If the difference is negative, the point of interest is lower than the benchmark. By adding or subtracting the rise or fall value from the elevation of the benchmark, we can determine the elevation of the point of interest.

Levelling requires a clear line of sight between the level and the points of interest. This can be difficult in certain terrains or when there are obstructions such as buildings or trees. Additionally, levelling is most accurate within short distances. Over longer distances, errors can accumulate, resulting in less accurate elevation measurements. By following these steps, you should now understand the principle of levelling and how it is used to determine the height differences between points on the Earth's surface.