The Rise and Fall Method

On the other hand of finding the instrument height at a setup station, the difference between consecutive points is obtained from their staff readings with that immediately preceding it. The difference denotes a rise or a fall. The decrease level of each point is then acquired by adding the rise to or subtracting the fall from the RL of the preceding point. The arithmetic check in this method is as follows:

∑ BS - ∑ FS = ∑ Rise - ∑ Fall

= Last RL - First RL

It can be noted that the first method of collimation is simpler and faster than the rise and fall method. Therefore, there is no check in reduction of levels at intermediate stations in collimation method although the second method gives arithmetic check on all the level reductions. We can conclude that the collimation method can be preferred for profile levelling or setting out construction levels, although rise and fall method is preferred for differential levelling, check levelling and other important applications.

Some precautions in recording the measurements in field books should be taken to avoid error in recording and subsequent computations. Care should be taken to make entries strictly in the respective columns prescribed for them in order of their observation. The first entry on the fresh page in field book shall always be a backsight while the last entry is a foresight. If the last entry happens to be a staff position at intermediate point, instead of a change point, it shall be made both in foresight and backsight columns at the end of the preceding page and as the first entry into the succeeding page. In the remark column, bench marks, change points and other important information shall be briefly but accurately recorded, preferably explained with the help of sketches by free hand drawn on the left side of the page.