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 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.
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.
8.
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