What are Difficulties Faced during Leveling in Surveying?

DIFFICULTIES FACED IN LEVELLING

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 O₁and 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.

Then,

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.

Then,

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.