Difference between Height of Instrument (HI) and Rise and Fall method
|Line of Collimation or HI method Rise and fall method
|Rise and fall method
|It is more rapid and saves a considerable time and labour.
|It is laborious as the staff reading of each station is compared, to get a rise or fall.
|It is well adopted for reduction of levels for construction work such as longitudinal or cross-section levelling operations.
|It is well adopted for determining the difference in levels of two points where precision is required.
|There is no check on reduction of R.Ls. of intermediate stations. .
|There is a complete check on the reduction of R.Ls. of intermediate stations.
|There are only two arithmetical checks i.e., the difference between the sum of the back sights and the sum of the fore sights must be equal to the difference in R.L. of the last station and first station.
|There are three arithmetical checks i.e. the difference between the sum of the back sights and the sum of the fore sights must be equal to the difference between the sum of the rises and the sum of the falls as well as it must be equal to the difference in R.Ls. of the last station and first station.
|Errors if any in intermediate sights are not detected.
|Errors in intermediate sights are noticed as these are used for finding out the rises or falls.
Example of H.I. Method: The following staff readings were observed successively with a level, the instrument is moved after fourth and sixth readings- 2.25, 1.50, 0.95, 2.10, 2.90, 1.54, 0.75, 1.95, 2.10 m.
Enter the above readings in a page of a level book and calculate the R.L of points by height of instrument method, if the first readings was taken with a staff held on a bench mark of 100 m.
Every station having reading at B.S will have separate H.I.
Formulaes: R.L= H.I- I.S or F.S
H.I= R.L + B.S
|Σ B.S – Σ F.S = Last R.L -First R.L