There are two methods of working out the reduced levels of points from the staff readings taken in the field :
· The collimation or instrument height method.
· The rise and fall method.
The first method, i.e. collimation or instrument height method, is less tedious, more rapid and involves less calculations.
Collimation method consists of finding the elevation of the plane of collimation (H.I.) for every set up of the instrument and then obtaining the reduced levels of points with reference to the plane of collimation.
To begin with, determine the elevation of the plane of collimation for the first set up of the level by adding the backsight to the reduced level of the benchmark.
Then calculate the reduced levels of the intermediate points and the first change point, by subtracting the staff readings taken on these points (i.e. I.S. and F.S.) from the elevation of the plane of collimation (H.I.).
Now, when the instrument is shifted to the second position, a new plane of collimation is set up. The levels of the two planes of collimation are correlated by means of the backsight and foresight taken on the change point.
Now, work out the reduced levels of the successive points and second change point, by subtracting their staff readings from the elevation of this new plane of collimation.
After taking the readings calculate the reduced level of different staff stations.
Staff Readings :
Station
|
Readings
|
R.L. of Plane of Collimation (H.I.)
|
Reduced Level
|
Remarks
| ||
Backsignt
|
Intersight
|
Foresight
| ||||
A
|
0.535
|
100.000
|
B.M. on the plinth of the building considered
| |||
B
|
2.145
| |||||
C
|
2.435
| |||||
D
|
1.585
| |||||
E
|
1.535
|
0.630
|
Change Point
| |||
F
|
2.015
| |||||
G
|
3.340
| |||||
Assume the reduced level of benchmark ‘A’ as 100.000.
Height of collimation for the first instrument set up.
R.L. of B.M. + Reading on B.M. = 100.000 + 0.535 = 100.535
How to calculate reduced levels of different stations.
R.L. of station B = 100.535 – 2.145 = 98.390
R.L. of station C = 100.535 – 2.435 = 98.100
R.L. of station D = 100.535 – 1.585 = 98.950
R.L. of station E = 100.535 – 0.630 = 99.905
Now, at station E, collimation will be changed for new instrument set up.
R.L. at point E + backsight reading. = 99.905 + 1.535 = 101.440 (new H.I.)
So, now calculate the R.L. of other stations.
R.L. of station F = 101.440 – 2.015 = 99.425
R.L. of station G = 101.440 – 3.340 = 98.100
To check the leveling work, take the reading on B.M. ‘A’ and calculate the R.L. It should be 100.00 If the error exceeds the permissible limits, the work must be repeated.
Arithmetical check
The difference between the sum of the backsights and the sum of the foresights should be equal to the difference between the first and the last reduced levels. This check verifies the calculation of the reduced levels of the planes of collimation and of the change points.
Sum of the backsights = 2.070
Sum of the foresights = 3.970
Difference = - 1.900
First R.L. = 100.000
Last R.L. = 98.100
Difference = - 1.900
Result :
Station
|
Readings
|
R.L. of Plane of Collimation (H.I.)
|
Reduced Level
|
Remarks
| ||
Backsignt
|
Intersight
|
Foresight
| ||||
A
|
0.535
|
100.535
|
100.000
|
B.M. on the plinth of the building considered
| ||
B
|
2.145
|
98.390
| ||||
C
|
2.435
|
98.100
| ||||
D
|
1.585
|
98.950
| ||||
E
|
1.535
|
0.630
|
101.440
|
99.905
|
Change Point
| |
F
|
2.015
|
99.425
| ||||
G
|
3.340
|
98.100
| ||||
Arithmetic check
|
2.070
|
3.970
| ||||
-1.900
|
-1.900
| |||||
