When it is required to find the difference between two points which are visible from a single position of level. Then the method used is simple levelling.
Suppose A and B are two points and the level is set up at approximately midway between them, suppose at O. After the instrument is correctly levelled, the telescope is directed toward the staff held vertically on A and focused. Reading at which Horizontal hair of the diaphragm appears to cut the staff is taken. Ensure that bubble of the level is in center. Then the staff is held vertically on B. Again reading is taken at point B from point O. Let the respective readings on A and B be 2.350 and 3.315 . The difference between these readings give the difference of level between A and B which is equal to 3.315 m – 2.350 m = 0.965 m
If the reduced level of A is 100 m, then we can find the reduced level of B.
Height of instrument at O = R.L of A + 2.350 m
Height of instrument at O = 100 m + 2.350 m = 102.350 m
R.L of B = 102.350 m – 3.315 m = 99.035 m
- When the point is lower, the staff reading is greater. When the point is higher, the staff reading is smaller.
- The bubble must be in center while the readings are being taken.
- If the true difference of level between two points is required, then level must be set up exactly midway between them.
This method is used in order to find the difference between two points which are far apart or if the difference in elevation between them is too great or if there are obstacles in between them.
In such case, it is necessary to set up level in several positions and to work in series of stages. The method of simple levelling is applied on every stage. It is also known as compound or continuous levelling.