METHODS OF LEVELING
اس کو اردو یا (اپنے زبان ) میں پڑھنے کیلئے ٹرانسلٹ پر کلک کریں اور آپنا زبان متخاب کریں۔
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1. Height of Collimation Method OR Height of instrument (H.I)
2. Rise and Fall Method
1. Collimation Method or Height of instrument:
It consist of finding the
elevation of the plane of collimation (H.I.)
for every set up of the instrument, and then obtaining the reduced level
of point with reference to the respective plane of collimation.
1. Elevation of plane of
collimation for the first set of the level determined by adding back side to
R.L. of B.M.
2. The R.L. of intermediate point
and first change point are then obtained by starching the staff reading taken
on respective point (IS & FS) from the elation of the plane collimation. [H.I.]
3. When the instrument is shifted
to the second position a new plane collimation is set up. The elevation of this
plane is obtained by adding B.S. taken on the C.P. from the second position of
the level to the R.L. C.P The R.L. of
successive point and second C.P. are found by subtract these staff reading from
the elevation of second plane of collimation Arithmetical check
Sum of B.S. – sum of
F.S. = last R.L. – First R.L.
This method is simple and easy.
Reduction of levels is easy.
Visualization is not necessary regarding the nature of the
ground.
There is no check for intermediate sight readings
This method is generally used where more number of readings
can be taken with less number of change points for constructional work and
profile leveling.
2. Rise and Fall Method:
It consists of determining the
difference of elevation between consecutive points by comparing each point
after the first that immediately preceding it. The difference between there
staff reading indicates a rise fall according to the staff reading at the
point. The R.L is then found adding the rise to, or subtracting the fall from
the reduced level of preceding point.
Arithmetic check
Sum of B.S. – sum of F. S. = sum of
rise – sum of fall = last R. L. – first R.L.
This method is complicated and is not easy to carry out.
Reduction of levels takes more time.
Visualization is necessary regarding the nature of the
ground.
Complete check is there for all readings.
This method is preferable for check leveling where numbers
of change points are more.
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Basic definitions:
Bench Mark and Reference Datum In order to
calculate the heights of points a datum is required, i.e. a reference level.
This is usually the mean sea level. For this purpose, the use of Bench Marks is
necessary, and these are classified as follows: Bench Mark (BM) – a point with
known height above mean sea level (or other reference datum). These are
permanent points (e.g. unchanged by weather conditions) and are provided by the
Department of Lands and Surveys.
Reduced Level the height of any target point is referred to
as Reduced Level (RL), because it is reduced to a known datum.
Back sight (BS) First staff reading taken immediately after
setting up the instrument.
Foresight (FS) last staff reading taken before moving the
instrument to another location.
Intermediate sight (IS) all readings taken between a BS and
a FS.
[ Read more
[ Read more
Common sources of errors in leveling
1. Instrument not correctly leveled.
2. Telescope not correctly focused.
3. The wrong cross-hair reading recorded (e.g. top instead
of middle).
4. Staff incorrectly read or not held vertical.
5. Staff incorrectly booked.
All the above are mistakes (blunders) and cannot be
corrected unless the work is repeated.
Leveling operations
Now consider Figure 2 below. The
level is set up as shown, and using the staff at points A and B, height
readings are recorded. This is just the height read through the telescope
horizontal line of sight (known as line of collimation). If no reduced level is
known only the difference in height can be found between A and B, not their
absolute levels. Staff Reading at A is 1.135m Staff Reading at B is 1.875m If
we know that RLA = +120.000m (above datum), then RLB = 120.00 – 0.740 =
+119.260m i.e. a fall from A. If RLB was known we would calculate a rise in
level. Hence, the following can be defined:
Rise – staff reading is less than
previous reading.
Fall
– staff reading is greater than previous reading.
Experiment (Rise & Fall method):
Aim:
To determine the
required level of given points by Rise and fall method
Apparatus Required:
Automatic level, Tripod and Leveling staff
Procedure:
1. Set up the leveling instrument at Level position 1.
2. Hold the staff on
the Datum (RL+50 m) and take a reading. This will be a back sight, because it
is the first staff reading after the leveling instrument has been set up.
3. Move the staff to A and take a reading. This will be an
intermediate sight.
4. Move the staff to B and take a reading. This also will be
an intermediate sight.
5. Move the staff to C and take a reading. This will be
another intermediate sight.
6. Move the staff to D and take a reading. This will be a
foresight; because after this reading the level will be moved. (A change plate
should be placed on the ground to maintain the same level.)
7. The distance between the stations should be measured and
recorded in the field book (see Table 1)
8. Set up the level
at Level position 2 and leave the staff at D on the change plate. Turn the
staff so that it faces the level and take a reading. This will be a back sight.
9. Move the staff to E and take a reading. This will be an
intermediate sight.
10. Move the staff to F and take a reading. This will be a
foresight; because after taking this reading the level will be moved.
11. Now move the level to Leveling position 3 and leave the
staff at F on the change plate.
Now repeat the steps describe 8 to 10 until you finished at
point J.
Field procedures for leveling for vertical control the level
survey should start and close on points of known height (same point or
different). If the survey starts and closes on the same point (e.g. a TBM), as
below, this is termed as a closed level survey.
Arithmetic checks (necessary for checking the reduction)
Σ (BS) - Σ (FS) = Σ (RISES) – Σ (FALLS) = LAST (RL) – FIRST (RL) = Allowable misclosure = ±5 √n mm
; where n = no. of instrument positions.
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