Rise and fall method
The rise and fall method is a technique used in surveying to determine the difference in elevation between two points. It is commonly used in design and construction to create accurate site plans.
A levelling instrument such as a dumpy level or a theodolite is set up at one point (the benchmark) and a staff is placed at a point whose elevation is to be determined. The instrument is then sighted on the staff and the vertical distance between the instrument and the staff is measured. This is known as the backsight.
The staff is then moved to the next point and the instrument is sighted on the staff again. The vertical distance between the instrument and the staff is measured once more, and this is known as the foresight.
If the foresight is higher than the backsight, it means that the point being surveyed is higher than the benchmark. The difference between the foresight and the backsight is known as the rise. If the foresight is lower than the backsight, it means that the point being surveyed is lower than the benchmark. The difference between the backsight and the foresight is known as the fall.
The process is repeated for multiple points, with the rise and fall measurements being added and subtracted as appropriate.
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