linear measurement in field of civil engineering

1. METHODS
1. Direct methods
• in the this method, the distance is actually measured
during field work using a chain or tape.
2. Optical methods
• in this method, principal of optics are used.
3. E.D.M. methods
• Electromagnetic Distance Measuring (E.D.M.)
instruments have been developed quite recently

2. INSTRUMENTS USED IN
CHAINING
 Chains
 Tapes
 Arrows
 Ranging rods
 Offset rods
 Pegs
 Plumb-bob

3.  Chains:

4. VARIOUS TYPES OF CHAINS
USED IN SURVEYING ARE:
1) METRIC CHAIN
2) GUNTER’S CHAIN or
SURVEYOR’S CHAIN
3) ENGINEER’S CHAIN
4) REVENUE CHAIN
5) STEEL BAND or BAND
CHAIN

5. MERTIC CHAIN:
 Galvanised mild steel wire of 4 mm diameter
known as link .
 The end of the link are bent into a loop and
connected together by mean of three oval or
circular rings which provide flexibility to the chain
 Brass handles with swivel joint so that the chain
can be turned round without twisting .
 The length of the chain is considered from
outside surface of one handle to the outside
surface of the other handles at the end .
 In metric chain length of one link 20.00 cm .

6. GUNTER’S CHAIN OR SURVEYOR’S CHAIN:
 A 66 feet long chain consist of 100 links , each of 0.66 ft.
Is known as Gunter’s chain.
 10 square chain are equal to 1 acre,
 10 chain = 1 furlong
 8 furlong (80 chains) =1 mile
 suitable for taking length in miles and areas in acre.
ENGINEERING’S CHAIN:
•A 100 Feet long chain consisting of 100
link each of 1 foot
•Brass tags are fastened at every 10
links.
•Used to measure length in feet and area
in square yards.

7. REVENUE CHAIN AND BAND CHAIN:
 33 ft . long chain consisting of 16 links.
 Used for distance measurements in feet &inches for
smaller area.
Band chain:
 Steel band are preferred than chain because they are
more accurate, but the disadvantage is that get broken
easily and difficult to repair in the field.
 20 and 30 m long , 12 to 16 mm wide and 0.3 to 0.6mm
thick.
 Numbered at every metre and divided at every 20 cm.

8.  Tally Tags :

9.  Tapes :

10. TAPES ARE USED FOR MORE ACCURATE
MEASUREMENT :
 1) CLOTH or LINEN TAPE
 2) FIBER TAPE
 3) METALLIC TAPE
 4) STEEL TAPE
 5) INVAR TAPE

11. [1] CLOTH OR LINEN TAPE
 LINEN tapes are closely woven linen and varnished
to resist moisture.
 10m,20m,and 30m long in length and 12 to 15mm
wide .
 Used for offset measurements.
 These tapes are light and flexible.
[2] FIBRE GLASS TAPES:
 Similar to linen and coated but these are made of glass
fibre.
 The tapes are quite flexible, strong and non-
conductive.
 Do not stretch or shrink in temperature or moisture.

12. [4] STEEL TAPES:
made of steel ribbon of width varying
from 6 to 16mm .
length are 10m,20m,30m,and 50m.
[5]INVAR TAPE:
made of an alloy of nickel 36% and steel
64% having very low co-efficient of thermal
expansion .
These are 6mm wide and generally
available in lengths of 30m,50m,100m.

13. [3]METALLIC TAPE:
A linen tape reinforced with brass
or copper wires to prevent
stretching or twisting of fibres is
called a metallic tape.
These wires are visible to naked
eyes.
Used for taking offset in chain
surveying.

14.  Arrows :

15. ARROWS
Arrows are made of steel wire of
diameter 4mm.
One end is bent into a ring of diameter
50mm and the other end is pointed.
Overall length is 400 mm.
Used for counting the number of chains
while measuring a chains line
Inserted into the ground after every
chain length measured on the ground.

16.  Ranging rods :

17. RANGING RODS
 Used for ranging some intermediate points on
the survey line .
 2 to 3 m in length and are painted with
alternate bands of black and white colours with
length of each equalising 20cm.
 The location of any survey station can be
known from long distances only by means of
ranging rods.
 If the distance is too long, a rod of length 4.0
to 6.0 m is called ranging pole.

18.  Offset rods :

19. OFFSET RODS
Similar to the ranging rod with the
exception that instead of the flag.
A hook is provided at the top for
pushing and pulling the chain or
the tape.
Used for measuring small offset

20.  Pegs :

21. PEGS
Timber or steel and used to
mark the position of the station
or terminal point of a survey line
.
Wooden pegs are 15 cm long
and are driven in to ground with
the help of a hammer.

22.  Plumb bob :

23. PLUMB – BOB:
 Used to transfer points on the ground.
 Also used for fixing the instrument exactly
over the station point marked on the
ground by checking the centre of the
instrument whether coincides with the
centre of the peg or station or not, by
suspending the plumb-bob exactly at the
centre of the instrument.
 Plumb-bob is thus used as centring aid in
theodolite and plane table etc.

24. CHAIN SURVEYING
 Chain survey suitable in the
following cases
1. Area to survey is comparatively
small
2. Ground is fairly level
3. Area is open
4. Details to be filled up are simple
and less

25.  Chain survey is not
suitable in the following
condition
1. Area to survey to large
2. Ground is quite Uneven
3. Area is crowded
4. Details the shows are to many.

26. PRINCIPLES
Make triangle in chain survey.
As far possible main angles are make
an angle 60⁰
 However the arrangement of
triangle depends on shape,
topography.

27. Various technical terms used in
connection with the network of
triangle in surveying.
1. Station:
station is a point of importance at
the beginning at the end of a surveying
2. Main - station:
this are the station at the beginning
or at the end of lines formed. Main skeleton
A, B, C, D …etc

28. 3. Subsidiary station:
• These are the stations selected on
main-lines to run auxiliary/secondary
lines for the purpose of locating the
interior details.
• Denoted as a, b, c etc
• Base line
• Detail-line
• Check lines
• Tie line

29. SELECTION OF STATION
 The following point should be
considered while selecting station
point.
A station selected should be visible
from at least 2- more station.
 Main frame should have as few lines
as possible.
 All triangles should be well
conditioned
 Each triangle should have at least one
check line.

30.  Subsidiary station should be selected
such that offset to main object from
subsidiary lines are as short as possible.
 If possible should have one or two base
lines which run on level ground and
through the middle of the area.
 Avoid obstacles to ranging and chaining
 As far as possible survey lines should be
on the level ground.
Station points should be selected within
the boundary of the area to be surveyed.

31. SETTING OUT PERPENDICULAR OFFSET
a. Swinging or 3-4-5 method

32. b. Using cross staff

33. c. Using optical square or Prism square

34.  Optical square:
It is more accurate and convenient
instrument for finding foot of the perpendicular
or to set a right amgle.It is a metal box of 50mm
dia. And 120 mm deep.There three openings
such as pin hole, a small rectangular slot and a
large rectangular slot.

35. Prism square:
It works on the same principle as
optical square. In a prism square ), the
direction of a ray originating from the flag on
the right is changed by 90° after refraction
and internal reflection. To lay off the angle
by which the direction of the ray is changed,
a second flag is set up in such a way that,
when it is seen over the instrument, it is
exactly above the image of the first flag in
the instrument.

36. ERRORS IN SURVEYING
 TYPES OF ERRORS:
1. Mistakes
2. Systematic errors
3. Accidental errors

37.  Mistakes:
Mistakes are the errors due to
carelessness of the observer. They may be due
to wrong reading or recording of the
observations. These errors are very large and
can be easily detected by the following field
procedures:
a) Carefully targeting objects before taking
reading
b) Taking multiple scale readings
c) Recorded loudly announcing the readings so
that reader hears what he records.
d) Taking additional readings for checking.

38.  Systematic errors:
The errors which follow a well – defined
pattern are classified as systematic errors. They
can be determined by mathematical expressions.
They are regarded as positive, if they make result
too great and as negative if they make result too
small. Examples of such errors are use of a tape
which is shorter than the actual as per marking or
using a steel tape at a temperature different from
calibrated temperature. If tape is short, makes each
measured length longer, hence contributes positive
error. the actual length of the tape is determined
actual measured length can be calculated. This
type of errors is called cumulative errors, since
each measurement adds to the error in the same
sense.

39.  Accidental errors:
There are errors in measurements which
cannot be prevented, even with sufficient care.
These errors may be positive or negative their
magnitude may vary from reading to reading for
example taking a reading with a survey
instrument Human eye has a limitation of
distinguishing between two close readings.
Marking the end of a chain length is another
common example of accidental error.

40. • The thickness of marking and its exact
position contribute to accidental errors.
These errors ae not deterministic they are
probabilistic hence they cannot be
estimated using standard functional
relations. However, using laws of probability
they may be accounted satisfactorily.

41. SOURCES OF ERRORS
Errors may arise from the following
sources:
1. Instrumental errors
2. Natural errors
3. Human limitations
4. Carelessness

42.  Instrumental errors:
Instruments used for linear
measurements may not be having true
length due to manufacturing defects and
instruments may not show true horizontal
and vertical angles due to manufacturing
defects or out of adjustments. There are
limitations on the scales used which
contribute to instrumental errors.

43.  Natural errors:
Errors will creep in because of the
natural phenomena like variation in
temperature humidity refraction, curvature
of the earth and magnetic declination. They
are to be properly accounted to arrive at
exact values.

44.  Human limitations:
Human eye cannot distinguish
between two points closer than 0.25 mm.
when ends of a chain/tape line is marked,
the thickness of line contributes to error,
when next length is measured.

45.  Carelessness:
These errors are purely due to the
mistakes. They are quite large. They can be
avoided by following good surveying
practice by taking precautions and check
readings.

46.  Most Probable Value of
Accidental Error
Though accidental errors are
unpredictable, the following features of
these errors are observed:
a) Positive and negative errors will
occur with equal frequency
b) Small errors occur more frequently
c) Very large errors do not occur.