Site Leveling

8/2/2019 Site Leveling

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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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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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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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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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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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Digital Level


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Digital Level


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Wooden Tripod Aluminum Tripod


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Levelling Staff


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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

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