levelling surveying basics

1. TYPES OF LEVELS

2. It deals with angular and linear measurement in
a vertical plane.
NEED:
For execution of Engineering Projects it is very
necessary to determine elevations of different
points along the alignment of proposed project.
Levelling :
The art of determining
relative altitudes of
points on the surface of
the earth or beneath
the surface of earth is
called levelling.

3. APPLICATIONS:
i) Taking rail levels .
ii)Initial ground levels for earthwork
calculations.
iii)Levels for measurement of earthwork
etc.

4. COMPONENTS OF LEVEL

5. BASIC COMPONENTS
OF LEVEL:
Telescope– to providealineofsight
Level Tube – to make line of sight
horizontal
Levelling head – to bring the bubble of tube level at the
centreofitsrun.
Tripod– to supporttheabovethreeparts ofthelevel.
TELESC O PE :
Telescope is an optical instrument used
for magnifyingand viewing the images ofdistantobjects.
It consists oftwo lenses.
The lens fitted near the eye is called the eye piece and the
other fittedattheend near to the objectis calledthe objective
lens.

6. The objective provides areal inverted image
infront ofthe eye piece at adistance lesser
than its focal distance.
Two essential conditions are involved. :
i) The real image ofthe obj
ect,must
be formed.
Ii) the plane ofimage must coincide with that
ofcross hairs.
Focusing of Telescope :
The operation of
obtaining aclear image ofthe obj
ect in the
plane ofcross hairs is known as focusing.

7. 2.
LevelTube : Also known as BubbleTube consists of aglass tube
placed in abrass tube which is sealed with plaster of paris.
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.
Outer surface of the bubble tube is graduated in both the
directions from the centre.
The line tangential to the circular are at its highest point i.e. the
middle of tube is called the axis of bubble tube. When the bubble
is central the axis of bubble becomes Horizontal.
The level tube is attached on the top of telescope by means of
capstan headed nuts.

8. 3)Levelling head : Levelling head generally
consists oftwo parallel plates with 3 foot screws.Upper plate is
known as Tribrach and lower plate is trivet which can be
screwed on to the tripod.Levelling head has to perform 3
distant functions :
i) to support the telescope
ii) to attach the level to the tripod
iii) to provide ameans for level (foot screws)

9. IMPORTANT TERMS:
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)LINE OF COLLIMATION- - the line which passes through
the Intersection of the cross hairs of the eye piece and optical
centre of the objective and its continuation is called as line of
collimation. This is also known as line of sight.
c)REDUCED LEVEL (RL) – Height or depth of a point above or
below the assumed datum is called Reduced level.

10. d) BENCH MARK – (BM) – B.M.is a fixed reference
point of known elevation. It may be of the following
types.
i) GTS Bench mark (Geodetic Triangulation Survey) :
These Bench marks are established by national agency
like Survey of India. They are established with highest
precision. Their position and elevation above MSL is
given in a special catalogue known as GTS Maps ( 100
km.interval).

11. ii) Permanent Bench Mark :
 They are fixed points of reference establishwith reference to GTS
Bench mark (10 km.interval).
iii) Arbitrary Bench mark :These are reference
points whose elevations are arbitrarily assumed. In
most of Engineering projects,the difference in
elevation is more important than their reduced levels
with reference to MSL as given in a special catalogue
known as GTS Maps ( 100 Km. interval).

12. e)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. MSL adopted by Survey of India is
now Bombay which was Karachi earlier.
f)Level Surface : The surface which is parallel to the
mean sphereoidal surface of the earth is known as level
surface.
g)Line of Collimation : It is the 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.

13. h) Height of Instrument (HI) : The elevation of the line of
sight with respect to assumed datum is known as HI.
i) 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.
j) Fore Sight : (F
.S.) – It is the last staff reading taken from
a setting of the level. It is also termed as minus sight.
Fore sight is the sight taken on a levelling staff held at a
point of unknown elevation to ascertain the amount by
which the point is above or below the line of sight. This
is also called minus sight as the foresight reading is always
subtracted from height of Instrument.

14. 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) :
The foresight taken on a levelling staff held at a
point between two turning points, to
determine the elevation of that point, is known
as intermediate sight.

15. TYPES OF LEVELS:
i) Dumpy level
ii)Tilting level
iii)Automatic level
iv)DigitalAuto
level

16. DUMPY LEVEL
This are the basic levels used in construction
work.
The telescope is attached to a single bubble
and the assembly is adjusted either by means
of ascrewed ball-joint or by footscrews
which are adjusted first in one direction then
at 90 degrees.

17. DUMPY LEVEL

19. ADVANTAGES:
● It is simple compact and stable.
● Fewer adjustments to be made.
● Simple construction with fewer movable
parts.
●Longer life of the adjustments.
DISADVANTAGE:
● The telescope is rigidly fixed to its
support therefore cannot be rotated
about its longitudinal axis

20. 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.
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.

23. ADVANTAGES :
● Helpful in quick levelling.
● Ball and socket arrangement permits the
head to be tilted and quickly locked
nearly level.

24. AUTO LEVEL

26. The Automatic level : Also termed as self
aligning level. It has an compensator which
consists of an arrangement of three prisms.The
two outer ones are attached to the barrel of
the telescope.The middle prism is suspended by
fine wiring and reacts to gravity .The instrument
is first levelled appproximately by the circular
bubble ,the compensator then deviate the line
of sight by the amount that the telescope is out
of sight.

27. ADVANTAGES:
i) Much simpler to use
ii)High precision – Mean elevation error on staff
graduated to 5mm division varies between +0.5
to 0.8 mm per km of forward and backward
levelling.
iii)High speed :The speed of Dumpy level
is about 25% lower than tilting level.
iv)Freedom from errors – Accuracy is
increased by an errect telescope image.
v) Range of application – level can be used on
medium and large sized projects and setting
bench marks.

28. DIGITAL LEVEL:
It uses digital-electronic image processor
using a charged-coupled device(CCD) for
determining heights and distances.
Its accuracy in rod reading is 0.5mm and
the maximum range is 100m.

30. ADVANTAGES :
● Data can be transferred to computer
directly.
● Error in writing of data and calculations
can be minimised.
DISADVANTAGE:
● It needs clear and better illuminated area
for levelling hence not useful in cloudy
area.

31. • Digital leveling systems for surveying rely on an electronic
laser to scan a level staff with bar code markings.
• This type of level reduces the chance of human error when
interpreting graduation marks, and a digital display ensures
accurate readings.
• When job sites need fast and accurate height determinations,
precision digital leveling equipment serves as valuable
timesaving technology.
• An upside to digital levels is the potential for data integration.
A surveyor can store readings in the internal memory and
refer to them later, or transfer data to computers using
external memory ports.
• Contractors should be aware that digital levels require a
battery, which is often rechargeable, to operate.
• Digital levels use electronic image processing to evaluate the
special bar-coded staff reading, This bar-coded pattern is
converted into elevation and distance values using a digital
image matching procedure within the instrument.

32. ADVANTAGES OF DIGITAL LEVEL
Correction of collimation error: Can be reliably determined and
saved using the four
integrated Check and Adjust procedures or it can be entered
manually
Minimize human error: Fatigue-free observation as visual staff
reading by the observer is
not required.
User friendly menus with easy to read, digital display of results.
Earth curvature correction: The measurements made are
automatically free of the
influence of the earth’s curvature.
Measurement of consistent precision and reliability due to
automation.
Automatic data storage eliminates booking and its associated
errors.
We can operate in low light conditions.
Fast, economic surveys resulting in saving in time (up to 50% less
effort has been
claimed by manufacturers).
Data on the storage medium of the level can be downloaded to a
computer enabling quick
data reduction for various purposes.

33. COMPONENTS OF DIGITAL LEVEL
Main components of digital level consist of two parts: Hardware (Digital level and
levelling
staff) and Software.
1. Both digital level and associated staff are manufactured so that they can be used for both
conventional and digital operations.
2. Typically digital level has the same optical and mechanical components as a normal
automatic level.
3. However, for the purpose of electronic staff reading a beam splitter is incorporated which
transfers the bar code image to a detector diode array.
4. The light, reflected from the white elements only of the bar code, is divided into infrared
and visible light components by the beam splitter.
5. The visible light passes on to the observer, the infrared to diode array.
6. The acquired bar code image is converted into an analogous video signal, which is then
compared with a stored reference code within the instrument.