Some difficulties which are commonly faced during levelling are as follows –
1. When the Staff is Too near the Instrument.
2. Levelling Across a Large Pond or Lake.
3. Levelling Across a River.
4. Levelling Across a Solid Wall.
5. When Bench Mark is Above the Line of Collimation.
6. Levelling Along a Steep Slope.
7. Levelling Across a Rising Ground or Depression.
1. When the Staff is Too near the Instrument:
Ø During levelling if the levelling staff id held very near the levelling instrument, the graduations of the staff are not clearly visible.
Ø In such case, apiece of white paper is moved up and down along the staff until the edge of the paper bisected by the line of collimation.
Ø The reading is then noted from the staff with naked eyes and the reading sometimes taken by looking through the object glass.
2. Levelling Across a Large Pond or Lake:
When the levelling has to be done across a very wide pond or lake –
Ø As we know that the water surface of a still lake or pond is considered to be level then all the points above the water surface is considered to be in same level and have the same RL.
Ø Two pegs A and B are fixed on opposite banks of the lake or pond.
Ø The top of the pegs are just flush with the water surface.
Ø The level is set up at O1 and the RL of A is determined by taking an FS on A.
Ø The RL of B is assumed to be equal that of A.
Ø Then the level is shifted and setup at O2 and taking a BS on Peg B and the levelling is then continued.
3. Levelling Across a River:
Ø During the levelling across a river the water surface cannot be considered as level.
Ø The water level on the opposite edges will be different.
Ø In such case the method of reciprocal levelling is adopted.
Ø Two pegs A & B are driven on the opposite banks of the river (not flush with the water surface). The RL of A is determined in the usual way (As in case of Levelling across a Large Pond or Lake).
Ø Then the true difference of level A & B is found out by reciprocal levelling and the RL of B is calculated and the levelling is continued.
4. Levelling Across a Solid Wall:
Ø During levelling across a brick wall, two pegs A & B are driven on either side of the wall, just touching it.
Ø The level is set up at O1 and staff reading is taken on A, let the reading be AC.
Ø Then the height of the wall is measured by staff, let the height be AE.
Ø The H1 is found out by taking a BS on any BM or CP.
RL of A = H1 – AC
RL of E = RL of A + AE = RL of F (Same Level)
Ø The level is shifted and set up at O2 and the staff reading BD is noted and the height BF is measured.
RL of B = RL of F – BF
H1 at O2 = RL of B – BD
Ø The levelling is then continued by working out the H1 of the setting as shown in figure.
5. When Bench Mark is Above the Line of Collimation:
Ø This happen when the BM is at the bottom of a bridge girder or on the bottom surface of a culvert.
Ø It also happens when the RLs of the point above the height of the line of collimation have to be found out.
Ø Considered, if the BM exists on the bottom surface of a culvert, and that it is required to found out the RL of A.
Ø The RL is set up at O1 and the staff is held inverted on the BM.
Ø The staff reading is taken and noted with a negative sign.
Ø The remark “staff held inverted” should be entered in the appropriate column.
Ø Let the BS and FS reading be 1500 and 2500 respectively.
Now the height of the instrument = 100.000 – 1.500 = 98.500
RL of A = 98.500 – 2.250 = 95.250.
6. Levelling Along a Steep Slope:
Ø During levelling along a steep slope specially in hilly areas, it is very difficult to have equal BS & FS distances.
Ø In such cases the level should be set up along a zig-zag path so that the BS and FS distances may be kept equal.
Ø Let AB be the direction of levelling l1, l2, ..... are the positions of level and S1, S2, S3....... the positions of the staff.
Ø The levelling is continued in this way and the RL of the points are calculated.
7. Levelling Across a Rising Ground or Depression:
Ø During levelling across a high ground, the level should not be placed on the top of this high ground, but on one side so that the line of collimation just passes through the apex.
Ø During levelling across a depression the level should be set up on one side and not on at the bottom of the depression.