TYPES
OF LEVELLING
OF LEVELLING
1. Continuous leveling
2. Reciprocal leveling
METHODS
USED IN CONTINUOUS LEVELLING
USED IN CONTINUOUS LEVELLING
• Collimation
method or height of instrument
method or height of instrument
• Rise
and fall method.
and fall method.
In collimation method height of
instrument
instrument
HI = BS + RL
And reduced level
RL = HI – FS
At the end computation, the
difference of the sum of BS and FS must be equal to the difference of first RL
and last RL data recorded. Meanwhile in Rise and Fall method, a rise will occur
when the staff reading is less than the reading on the proceeding station. The
data recorded are also checked for correctness as above, but this time, rise
and fall are used instead of BS and FS.
difference of the sum of BS and FS must be equal to the difference of first RL
and last RL data recorded. Meanwhile in Rise and Fall method, a rise will occur
when the staff reading is less than the reading on the proceeding station. The
data recorded are also checked for correctness as above, but this time, rise
and fall are used instead of BS and FS.
TRAVERSE
DURING LEVELLING:
DURING LEVELLING:
Definition:
A traverse is a continuous frame
work of line connecting a number of points, the lengths of the lines and their
angular relationship to each other being measured. The lines are known as legs
and the points as stations.
work of line connecting a number of points, the lengths of the lines and their
angular relationship to each other being measured. The lines are known as legs
and the points as stations.
USE
Traverse surveys are used where site
conditions make the chain triangulation method impossible e.g where the survey
is of as large area and details are not required.
conditions make the chain triangulation method impossible e.g where the survey
is of as large area and details are not required.
TYPES:
1) Closed traverse – when the frame
work form a closed figure (or when the traverse connects two stations whose
position are known). This type of traverse is used for surveying lakes or other
areas across which known check line can be run. Such traverse is easily
checked, as the survey starts and finishes at the fixed points.
work form a closed figure (or when the traverse connects two stations whose
position are known). This type of traverse is used for surveying lakes or other
areas across which known check line can be run. Such traverse is easily
checked, as the survey starts and finishes at the fixed points.
2) Open traverse – a traverse whose
starting and finishing stations do not coined or are not both fixed or known
points. This type of traverse is used to survey rivers e.t.c. To enable the
work to be checked, sights are taken on to some reference objects, such as well
defined landmarks
starting and finishing stations do not coined or are not both fixed or known
points. This type of traverse is used to survey rivers e.t.c. To enable the
work to be checked, sights are taken on to some reference objects, such as well
defined landmarks
Note:
a) Chain
traverse √
traverse √
b) Compass
traverse √
traverse √
c) Theodolite
traverse √
traverse √
Terms:
a) Bearing – the term bearing refers
to the angle between the line and the north -south line or median.
to the angle between the line and the north -south line or median.
b) Whole – circle bearing – the
bearing from north to the leg is measured in a clockwise direction and the
angle from the north line right round to the leg is known as a whole – circle
bearing ( as W.C.B)
bearing from north to the leg is measured in a clockwise direction and the
angle from the north line right round to the leg is known as a whole – circle
bearing ( as W.C.B)
A N
N N
A
WCB
O Of
WCB OA
O Of WCB
OA O of OA
A
Forward and Back Bearing: with A as
the starting station of traverse and the direction of the survey being forward
B, the angle between north and AB at A is known as the forward bearing of AB.
The angle between north and AB at station B is known as the back bearing of AB,
this should differ from the forward bearing by exactly 1800.
the starting station of traverse and the direction of the survey being forward
B, the angle between north and AB at A is known as the forward bearing of AB.
The angle between north and AB at station B is known as the back bearing of AB,
this should differ from the forward bearing by exactly 1800.
The forward and back bearings of a
leg will differ by 1800 except
there is local attraction. The presence of metal, metallic ores or electric
currents will divert the compass needle from the north– south line, and thus
cause the readings taken to be inaccurate. Stations should therefore be chosen
so that they are beyond the influence of this attraction.
leg will differ by 1800 except
there is local attraction. The presence of metal, metallic ores or electric
currents will divert the compass needle from the north– south line, and thus
cause the readings taken to be inaccurate. Stations should therefore be chosen
so that they are beyond the influence of this attraction.
Correction for Effects of Local
Attraction:
Attraction:
Line Bearing Diff of FB and BB
AB 600
1800
1800
BA 2400
BC 1200
1800
1800
CB 3000
CD 2100
1780
(-20)
1780
(-20)
DC 320
DA 3170
1820
(-20)
1820
(-20)
AD 135
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