Reciprocal levelling is the type of levelling that is necessary across a river ravine or any obstacle requiring a long sight between two points. This technique is commonly used in surveying to determine the difference in elevation between two points that are not accessible by a straight line.

Reciprocal levelling is a method of levelling that involves measuring the vertical distance between two points by taking a series of readings with a level instrument. The instrument is set up at one point and a series of level runs are made to the other point and back, with the instrument being moved each time. This ensures that any errors in the instrument or the level line are eliminated, as any errors in the forward run will be compensated for in the backward run.

The following steps are involved in conducting reciprocal levelling:

1. Set up the level instrument: The level instrument is set up at a convenient location where both the starting and ending points are visible. The instrument should be properly leveled and secured.

2. Take readings: Start by taking a reading at the starting point. This reading is known as the backsight or the reading on a staff held at a known point of elevation. Then, move the level instrument to the ending point and take a reading on a staff held at that point. This reading is known as the foresight.

3. Calculate the difference in elevation: The difference in elevation between the two points can be calculated by subtracting the backsight reading from the foresight reading. This gives the rise or fall in elevation between the two points.

4. Repeat the process: To ensure accuracy, the process is repeated in the reverse direction. The instrument is moved back to the starting point and a reading is taken on the staff at the ending point (backsight). Then, the instrument is moved to the ending point and a reading is taken on the staff at the starting point (foresight). This allows for error-checking and ensures that any errors are compensated for.

5. Calculate the average difference: The average difference in elevation between the two points is calculated by taking the average of the differences obtained from the forward and backward runs. This helps to minimize any errors and provides a more accurate measurement.

Reciprocal levelling is particularly useful when there is an obstacle, such as a river ravine, that prevents a direct line of sight between the two points. It allows for accurate measurements to be taken across long distances and ensures that any errors are eliminated or minimized.