SURVEYING

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SURVEYING
Department of Civil Engineering
Lecturer
Dr. Farag Bastawy Farag
Lecture of Surveying and Geodesy Surveying
Department Shoubra Faculty of Engineering

• drfarag7_at_yahoo.com
• dr.farag.7_at_hotmail.com

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7. LEVELLING
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What is levelling?

• A measurement process whereby the difference in
  height between two or more points can be
  determined

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Reading an E-type levelling staff
The value is ?
Read value at the horizontal cross hair
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Terms used in Leveling
a) DATUM or Datum plane is an arbitrarily
assumed level surface or line with reference to
which level of other line or surface are
calculated.
b) REDUCED LEVEL (RL) is the vertical distance
between a survey point and the adopted level
datum. c) BENCH MARK (BM) B.M. is a fixed
reference point of known elevation. It may be of
the following types.

1. Permanent Bench Mark They are fixed points of
   reference establish with reference to mean sea
   level
2. Arbitrary Bench mark These are reference points
   whose elevations are arbitrarily assumed.

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Mean Sea Level (M.S.L.) M.S.L. is obtained by
making hourly observations of the tides at any
place over a period of 19 years. e) Level
Surface The surface which is a surface, all
points on which are normal to the pull of
gravity.f) Line of Collimation The line of
collimation is the imaginary line joining the
intersection of the cross hair and the optical
center of the objective and its extensions, it is
also called line of sight or collimation.
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Height of Instrument (HI) The elevation of
the line of sight with respect to assumed datum
is known as HI.h) Back sight (B.S.) - The
first sight taken on a levelling staff held at a
point of known elevation. B.S. enables the
surveyor to obtain HI sight i.e. Height of
Instrument or line of sight.i) Fore Sight
(F.S.) It is the last staff reading taken from
a setting of the level.. This is also called
minus sight as the foresight reading is always
subtracted from height of Instrument.
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k) Change Point (CP) The point on which both
the foresight and back sight are taken during the
operation of levelling is called change point. L)
Intermediate Sight (IS) Any levelling sight
which is neither BS nor a FS, is known as
intermediate sight. It may be noted that for one
setting of a level, there will be only one back
sight and one foresight but there can be any
number of intermediate sights.
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Type of Leveling Equipmentsi) Dumpy levelii)
Tilting leveliii) Automatic leveliv) Digital
Auto level
Dumpy level It is simple compact and stable.
The telescope is rigidly fixed to its support
therefore cannot be rotated about its
longitudinal axis. A long bubble tube is
attached to the top of telescope. Dumpy
literally means short and thick.
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Tilting level
It consists of a telescope attached with a level
tube which can be tilted within few degrees in
vertical plane by a tilting screw.
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The main peculiarity of this level is that the
vertical axis need not be truly vertical, since
the line of collimation is not perpendicular to
it. The line of collimation, is, however, made
horizontal for each pointing of telescope by
means of tilting screw. It is mainly designed
for precise levelling work.
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The Automatic level
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The Automatic level Also termed as self
aligning level. The fundamental difference
between automatic and the classic spirit level is
that in the former the line of sight is no longer
levelled manually using a tubular spirit level,
but is levelled automatically within a certain
tilt range. This is achieved by compensator in
the telescope.
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1. Telescope to provide a line of sight2.
Level Tube to make line of sight
horizontal3. Levelling head to bring the
bubble of tube level at the centre of its run.4.
Tripod to support the above three parts of the
level.1. Telescope Telescope is an optical
instrument used for magnifying and viewing the
images of distant objects. It consists of two
lenses. The lens fitted near the eye is called
the eye piece and the other fitted at the end
near to the object is called the objective lens.
Basic components of level
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The objective provides a real inverted image in
front of the eye piece at a distance lesser than
its focal distance.Two essential conditions are
involved. i) The real image of the object,
must be formed.ii) the plane of image must
coincide with that of cross hairs.

• Focusing of Telescope The operation of
  obtaining a clear image of the object in the
  plane of cross hairs is known as focusing.

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2.Level Tube Also known as Bubble Tube
consists of a glass tube placed in a brass tube
which is sealed with plaster of paris. The whole
of the interior surface or the upper half is
accurately ground so that its longitudinal
section, is an arc of a circle. Level tube is
filled with either or alcohol, the remaining
space is occupied by an air bubble. The centre
of air bubble always rest at the highest point of
the tube.
Diaphram A frame carrying cross hairs usually
made of either silk thread or platinum wire and
placed at the plane at which vertical image of
the object is formed by the objective.
Vertical hair of the diaphram enables the
surveyor to check the verticality of levelling
staff whereas horizontal hairs are used to read
the staff graduations.
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a) Simple levelling The operation of
levelling for determining the difference in
elevation, if not too great between two points
visible from single position of the level is
known as simple levelling.PROCEDURE
Following steps are involved.
Principles of levelling
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Level the instrument correctly2. Direct the
telescope towards the staff held3. Take the
reading of Central, horizontal hair of the
diaphram, where it appears to cut the staff
ensuring that the bubble is central.4. Send the
staff to next point5. Direct the telescope
towards C and focus it again6. Check up the
bubble if central, if not bring it to the Central
position by the foot screw nearest to the
telescope.7. Take the reading of Central
Horizontal cross hair.
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b) Differential levelling or fly levelling
This method is used in order to find the
difference in elevation between two points.i)
If they are too far apartii) if the difference
in elevation between them is too great.iii) If
there are obstacles intervening. In such case it
is necessary to set up the level in several
positions and to work in series of stages.
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i) Rise and fall methodii) Height of
collimation method
The difference of level of the points AB is
equal to the algebraic sum of these difference
between the sum of back sights and sum of the
fore sights i.e. S BS - S FS
Booking and reduction of the levels may be done
by following 2 methods.
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RISE AND FALL METHOD In this method, the
difference of level between two consecutive
points for each setting of the instrument is
obtained by comparing their staff readings. The
difference between their staff readings indicates
a rise if back sight is more than foresight and a
fall if it is less than foresight. The Rise and
Fall worked out for all the points given the
vertical distances of each point relative to the
proceeding one. If the RL of the Back staff
point is known, then RL of the following staff
point may be obtained by adding its rise or
substracting fall from the RL of preceding
point.
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Height of Collimation Method In this method
Height of Instrument (H.I) is calculated for each
setting of the instrument by adding the back
sight (B.S) to the elevation of B.M.Height of
instrument (H.I) R.L of B.M B.S


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Levelling
(HI)
Height of the Plane of Collimation (HPC ) or
S2
(unknown)
RL B
HI R.L (A ) S1
R.L (B) HI - S2
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(CP)
RL A is known
RL A BS
RL B
HI - FS
Now the RL B is known
So we can repeat the process
RL B BS
RL C
HI - FS
Generally HI Known RL Back Sight
Unknown RL HI - Fore Sight
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i) Temporarily Adjustments adjustments which
are made for every setting of a level.ii)
Permanent adjustments- required if some error is
there in instrument. i) Temporary Adjustments
includes a) setting up the level b) levelling
up c) elimination of parallax
Adjustment of level
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A surveying optical telescope
Focusing
1. Rotate eyepiece to give a sharp, clear image
of the cross hairs
2. Rotate focusing screw to give a sharp, clear
image of the object being observed.
The aim of focusing is to remove (eliminate)
PARALLAX
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Bubble
When bubble is centred the instruments standing
axis is approximately vertical.
The compensators in the instrument take over and
adjust the optical Line of Collimation so that it
is horizontal (hopefully)
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Parallax
When focussing any optical instrument it is
vitally important that we eliminate Parallax.
Move the eye up and down (or from left to right)
over the eyepiece of the telescope.
If the cross hairs move relative to the object
being observed then Parallax exists and the
focussing is not satisfactory.
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Elimination of Parallax
Focus the cross hairs (using the Eyepiece) Focus
the object (using the Focussing screw)
Move eye up and down over the eyepiece
Images appear to move
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Elimination of Parallax
Parallax has been removed Therefore focussing is
good
Parallax exists and must be removed by better
focussing
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