In this method, the reduced levels of points are computed by calculating the reduced levels of the plane of collimation for each set up of the instrument.
The height of collimation is obtained by adding the staff reading, which must be back sight to the known reduced level of the point on which the staff stands. Reduced levels of all the other points are obtained by subtracting the staff reading from the height of collimation. When the instrument is changed to a new station, a new height of collimation is obtained by adding the new back sight with the reduced level of the last point obtained from the previous set up of the instrument. The steps involved in booking and reading the level in the height of collimation method are illustrated with the help of an extract from a field book given below.
In this table, the back sight, intermediate sight and fore sight are the readings taken in the field and in the remarks column bench marks and change points are specified. Then, with the help of this available data the height of the instrument and reduced levels are calculated and the usual checks are carried out.
- To begin with, the elevation of the plane of collimation of the first instrument station (I1) is to be calculated. This is done by adding the back sight to the reduced level of the bench mark. The reduced level of the bench mark, i.e., point A, is taken as 100.00 and back sight is taken by holding a staff at point A (i.e., 0.665).
This is entered as the height of the instrument (HI) as seen in the table.
2. Then, from the same instrument station, readings are taken of the staff held at B and C and entered as the intermediate sight. Then the reduced level of B and C are calculated.
3. Now the instrument is shifted to the next point I2 and the back sight and fore sight are taken. Then, the HI and reduced level at this point are calculated.
4. This procedure is followed till the last point is reached.
5. Now the checks are carried out to ascertain the correctness of the readings.
i.e., ΣBS – ΣFS = Last RL – First RL
ΣBS = 5.430; ΣFS = 4.235; First RL = 100.00; Last RL = 101.195
5.430 – 4.235 = 101.195 – 100 = 1.195
Hence checked.
Problem-1
Complete the levelling table given below.
Solution: The levels are computed as below.
Check: ΣBS – ΣFS = Last RL – First RL
BS =6.44; ΣFS = 7.91; Last RL = 192.15; First RL = 193.62
i.e., 6.44 – 7.91 = –1.47 m
192.15 – 193.62 = –1.47 m
Hence checked.
Problem-2
Compute the levels from the levelling field book using HI method.
Solution. The levels are computed as below.
Check: ΣBS – ΣFS = Last RL – First RL
ΣBS = 6.00; ΣFS = 3.42; Last RL = 25.68; First RL = 23.10
i.e., 6.00 – 3.42 = 2.58 m
25.68 – 23.10 = 2.58 m
Hence checked.
Problem-3
The following are the staff readings taken while making levels of a field. The back sights are underlined. Tabulate the levels in a field book and compute the level difference using HI method. 0.813, 2.170, 2.908, 2.630, 3.133, 3.752, 3.277, 1.899 and 2.390,
Here, in this problem, the readings have to be entered in a book form, entries checked and the reduced levels found.
Check: ΣBS – ΣFS = Last RL – First RL
ΣBS = 4.565; ΣFS = 5.523; Last RL = 38.605; First RL = 39.563
4.565 – 5.523 = –0.958
38.605 – 39.563 = –0.958
Hence checked.
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