site leveling
TRANSCRIPT
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LAND SURVEYING
LEVELLING(Lecture 4)
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Objective of Levelling
After studying this chapter, thestudent shall be able to:
1. Carry out levelling survey, &
2. Calculate the results relative to
some chosen datum.
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How to achieve theObjective?
Understand the background theoryof levelling, &
Carry out satisfactory levellingworks on site.
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How to achieve theObjective?
You have to ask & answer yourself
the following questions:
1. What is levelling?2. What are the required surveying
instruments?
3. What are the methods of recording?
4. What are the possible errors in
levelling?
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What is LEVELLING?
The operationrequired in the
determination ofheights of points onthe surface of theearth
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Figure 4.1
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Levelling from 1st Basic Principle
1st refer to Fig. 4.1 which shows a table & chairstanding on a level floor.
The difference in height between them could
be easily found using a rule in any of the threeways:
1. By measuring upwards from a horizontalplane, i.e. the floor;
2. By measuring downwards from a horizontalplane, i.e. the ceiling; &
3. By measuring downwards from an imaginary
horizontal plane established by a spirit level.
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Levelling from 1st Basic Principle
If the spirit level was held at eyelevel, say 1.500m, & the
horizontal plane extended overthe table & chair, the planewould cut the rule held on the
table at 0.850m & on the chairat 1.050m.
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Levelling from 1st Basic Principle
In practical levelling, the simplerule is replaced by a levelling staff
while the spirit level is replaced bya surveying instrument called alevel.
The level is, in fact, only a spiritlevel attached to a telescope
which is mounted on a tripod.
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Levelling from 1st Basic Principle
If the table & chair are replaced by2 points on the surface of the earth,the simple illustration in Fig. 4.1
becomes an actual levellingexercise as in Fig. 4.2.
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Fig. 4.2
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Levelling from 1st Basic Principle
How to find out:1. The difference in height between the
table (pt. A) & the floor (datum)?
2. The difference in height between the
chair (pt. C) & the floor (datum)?
The datum, in this case, is animaginary horizontal plane throughthe top of peg B.
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LEVELLING
Datum
The plane of reference where
the heights of the points arereferred to.
Ordnance Datum
The mean sea level recordedat Newlyn Harbour, Cornwall,over the period of 1915 to 1921
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Level Line
A curve line at a
constant height relativeto mean sea level
Horizontal line
A tangential to the levelline at any particularpoint
h
h
horizontal line
correction for effect
of curvature
mean sea level
level line
LEVELLING
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Levelling Instrument
Automatic Level
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Levelling Instrument
Digital Level
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Levelling Instrument
Digital Level
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Levelling Instrument
Wooden Tripod Aluminum Tripod
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Levelling Instrument
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Levelling Instrument
Levelling Staff
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Levelling Instrument
Levelling Staff
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Levelling Terms used:
1. BS 10. BM
2. IS 11. TBM
3. FS 12. CP4. Rise 13. OS
5. Fall 14. Close Back
6. RL7. Distance
8. Remark
9. HPC
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Back sight (BS)
Fore sight (FS)
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Intermediate Sight (IS)
Change point (CP)
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Important Points
Levelling should always start and finish atpoints ofknown reduced level so that
misclosures can be detected. When only onebench markis available, levelling lines mustbe run in loops starting and finishing at thebench mark
Where possible, all sights lengths should bebelow 50m
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The staff must bee held vertically by suitable
use of a circular bubble
BS and FS lengths should be kept equal for
each instrument position
Readings should be booked immediately after
they are observed and important readings,particularly at change points should bechecked
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Arithmetic checking must always be applied
where