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Monday, March 25, 2019

What are Difficulties Faced during Leveling in Surveying?


DIFFICULTIES FACED IN LEVELLING

What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

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.
What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

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.
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.

What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey


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.
What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

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.
What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

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.
What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

Ø  During levelling across a depression the level should be set up on one side and not on at the bottom of the depression.
What are Difficulties Faced during Leveling in Surveying?, Major Dificulties faced in survey

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