CHAIN SURVEYING
Is the branch of
Surveying in which the distance is measured with a Chain and tape, this
operation is called chaining.
The principle of chain
surveying is TRIANGULATION this
means that the area to be surveyed is divided into a number of small triangles
which should be Well conditioned.
Preferably,
all the sides of a triangle should be nearly equal having each angle nearly 60°
to ensure minimum distortion due to errors in measurement of sides and
plotting.
Generally, such an ideal
condition is practically not possible always due to configuration of the
terrain and, therefore, attempt should be made to have well-conditioned
triangles in which no angle is smaller than 30° and no angle is
greater than 120°.
The arrangement of
triangles to be adopted in the field, depends on the shape, topography, and the
natural or artificial obstacles met with.
Chain survey recommended
when
1.
The ground surface is more or less level.
2.
Area is to be surveyed.
3.
A small-scale map is to be prepared.
4.
The formation of well-conditioned triangle
is easy.
Chain surveying is
unsuitable when
1.
First the area is crowded with many details.
2.
The area consists of two many undulation.
3.
Area is very large.
4.
Formation of well-conditioned triangle
become difficult due to obstacles.
SURVEY STATION
MAIN SURVEY
STATION
Station taken along the
boundary of an area as controlling point are known as main survey station. the line joining the main
station are called main survey line.
Main Survey Station- A,
B, C,D
Tie Station- E, F
Main Survey Line- AB, BC,
CD, DA
Tie Survey Line- DE, FB
Base Line- AC
Check Line- BH, BG
TIE STATION OR SUBSIDIARY STATIONS
Station which are on main
survey line or any other survey line are known as subsidiary station.
BASE LINE
The line on which the
framework of a survey is built is known as baseline the long survey line which
is run through the middle of the area to be surveyed.
CHECK LINE
Line is run to check
accuracy of the traverse consisting of a framework of triangles. It joins the
apex point of a triangle to some fixed point on its base is known as check
line. it is taken to check the accuracy of triangle sometime this line help to
locate interior details.
OFFSETS
The lateral
measurement taken from an object to the chain line is called offset.
Offsets are taken to
locate object with reference to the chain line.
1.
Perpendicular offsets
When the lateral
measurement is taken perpendicular to the chain line they are known as
perpendicular offset.
2.
oblique offsets
oblique you upset are
taken when the object is at long distance from the chain line it is not
possible to set up a right angle due to some difficulties.
CHAINAGE
Chainage is the
horizontal distance as measured along a combination of curves and straight
lines (curvilinear) between two points.
Chainage It is the
distance of a well-defined point from the starting point. In chain surveying it
is normally referred to as the distance of the foot of the offset from the
starting point on the chain line.
The operation of measuring the distance is termed as chaining/taping.
The Point of Beginning
of the line is usually denoted as 00+00 (The Starting Point) with stationing at
least every 100 feet along the line denoted as 1+00 (for 100 feet)…2+00 (for
200 feet)…3+00 (for 300 feet), etc. where the first number represents the
number of 100 foot stations and the digits after the (+) sign represent any
remaining portion less than 100 feet.
For example: 31+57.95
would represent 3,157.95 feet.
TYPE OF CHAIN
1.
Metric chains
Metric chains are the most commonly used chain in
India. These types of chains come in many lengths such as 5, 10, 20 and 30
meters.
Most commonly used is 20m and 30m chain.
Brass ring is placed at every meter.
The total length of the chain is marked on the brass
handle at the ends.
20 meter chain 100 link @ 0.2m, Tallies after 10
links(2m). [0.2*10=2m]
30 meter chain 150 link @ 0.2m, Tallies after 25
links(5m). [0.2*25=5m]
2.
Revenue
Chain
The standard size of this type of chain is 33ft. The
number of links are 16, each link being 2(1/16) ft. This chain is
commonly used in cadastral survey.
33ft = 10.06 m link @ (33/16 =62.87).
3.
Gunter’s
chain or surveyor’s chain
Gunter chain comes in standard 66ft. This chain
consists of 100links, each link being 0.66ft or 7.92inches.
The length 66ft is selected because it is convenient in land measurements.
1.
1
mile = 8 Furlongs = 80 Gunter chains
2.
1
Acre = 10 square
Gunter’s chains
3.
10
Gunter chains = 1
Furlong
4.
Engineer’s
chain
This chain comes in 100ft length. It consists
of 100 links each link being 1ft long.
At every 10 links a brass ring or tags are
provided for indication of 10 links.
Readings are taken in feet and decimal.
Testing and Adjustment of Chain
As the chain is a metal made, it may undergo many
changes due to temperature effect or human error and etc. So, for all lengths
of chain a tolerance is given,
10m chain = + or – 3mm
20m chain = + or – 5mm
30m chain = + or – 8mm
Errors in chain Surveying
1.
Personal
Errors
Wrong
reading, wrong recording, reading from wrong end of chain etc., are personal
errors. These errors are serious errors and cannot be detected
easily. Care should be taken to avoid such errors.
2.
Compensating
Errors
The
compensating errors are those which are liable to occur in both direction
i.e (positive and negative) and finally hence tend to compensate are known as
compensating error.
Hence, they
are likely to get compensated when large number of readings are taken.
They are directly proportional to √L
In chaining, these may be caused by the following: -
·
Incorrect
holding of the chain:
- The follower may not bring his handle of the chain to
the arrow, but may hold it to one or other side of the arrow.
·
Fractional
parts of the chain or
tape may not be correct if the total length of the chain is adjusted by
insertion or removal of a few connection rings from one portion of the chain,
or tape is not calibrated uniformly throughout its length.
·
Graduations
in tape may not be exactly same throughout.
·
During
stepping operation crude method of plumbing (such as dropping of stone
from the end of chain) is adopted.
·
When
chain angles are set out with a chain which is not uniformly adjusted or
with a combination of chain and tape.
3.
Cumulative
Errors
The
cumulative errors are those which occur in the same direction and tend
to add up or accumulate i.e. either to make the apparent measurement always too
long or too short.
In each
reading the error may be small, but when large number of measurements are made,
they may be considerable, since the error is always on one side.
Examples of such errors are:
· Bad ranging
·
Bad
straightening
·
Temperature
variation
·
Variation
in applied pull
·
Non-horizontal
it
They are directly proportional to L.
4.
Positive
errors
(making the
measured lengths more than the actual) are caused by the following:-
The length of the chain or tape is shorter than the
standard, because of bending of links, removal of too many links in adjusting
the length, ‘knots’ in the connecting links, cloggings of rings with clay,
temperature lower than that at which the tape was calibrated, shrinkage of tape
when becoming wet.
·
The
slope correction is not applied to the length measured along the sloping
ground.
·
The
sag correction is not applied when the tape or the chain is suspended in the
air.
·
Measurements
are made along the faulty alignment.
·
The
tape is in suspension or in high winds.
5.
Negative
errors
(making
the measured lengths less than the actual) may be caused because the length of the tape or chain
may be greater than the standard because of the
·
wear
or flattening of the connecting rings,
·
opening
of ring joints,
·
temperature
higher than the one at which it was calibrated.
·
Applied
more pull than the standard pull.
TAPES
Tapes are used in surveying to take linear
measurements. They are available in different lengths and can be made of
different materials.
There are 5 types of tapes available in surveying for
linear measurements and they are as follows
1.
Linen
Tape.
2.
Metallic
Tape.
3.
Steel
Tape.
4.
Synthetic
Tape.
5.
Invar
Tape.
Linen tape, also known as cloth tape is a varnished strip made
of closely woven linen and it is varnished to resist moisture.
15mm wide. Available in 10m,15m.
Used for taking offset for ordinary work.
Metallic Tape, when linen tape is reinforced with brass and copper
wire to make it durable, then it is called metallic tape.
available in different lengths of 10m, 15m, 20m,
30m, and 50m.
These are used for survey works such as topographical
survey works.
Steel tape,
is made of steel or stainless steel.
It consists of a steel strip of 6mm to 16mm wide.
Synthetic tapes are made of glass fibers coated with PVC.
These are light in weight and flexible.
Synthetic tapes may stretch when subjected to tension.
Hence, these are not suitable for accurate surveying works.
Invar tapes,
36% of nickel and 64% of steel.
6mm wide strip and is available in different lengths of 30m, 50m,
100m.
The coefficient of thermal expansion of invar alloy is
very low. It is not affected by changes in temperature. Hence, these tapes are
used for high precision works
Accuracy Sequence of tapes
Invar Tape > Steel Tape > Metallic Tape >
Linen Tape SSC
OTHER EQUIPMENT
Ranging rod is a surveying instrument used for making a LINE
STRAIGHT. And also marking the position of stations, and for sightings of those
stations.
Made with the well-seasoned wood such as teak, pine or
deodar and GI pipes.
Diameter- 25mm
Length- 2 meter
Alternate BLACK & WHITE Strip of length 0.2
meter.
Lower end is pointed and provided with iron shore.
Offset Rods
These are similar to ranging rods except at the top
where a stout open ring recessed hook is provided, It is also provided with
two short narrow vertical slots at right angles to each other, passing through
the center of the section, at about eye level.
It is mainly used to align the offset line and
measuring the short offsets. With the help of hook provided at the top of the
rod, the chain can be pulled or pushed through the hedges or other
obstructions, if required. Offsets may also be made in the field with the help
of cross-staff or optical square.
Arrows
When the length of the line to be measured is more
than a chain length, there is need to mark the end of the chain length. Arrows
are used for this purpose.
Arrows are made up of 4 mm diameter steel wire
with one end sharpened and another end bent into a loop.
Diameter of loop is- 50mm, Total length is 400mm.
Pegs
Wooden pegs are used in measuring a length of a line
to mark the end points of the line.
The pegs are made of hard wood of 25 mm × 25 mm
section, 150 mm long with one end sharp.
When driven in ground to mark station points, they
project about 40 mm.
Clinometer is an instrument used for measuring angles of slope
(or tilt).
Cross-Staff
It is essentially an instrument used for setting
out right angles. In its simplest form it is known as Open Cross-Staff
(a). It consists of two pairs of vertical slits providing two lines of sight
mutually at right angles.
Another modified form of the cross-staff is known as French
Cross-Staff (b). This consists of an octagonal brass tube with slits on all
eight sides. This has a distinct advantage over the open cross-staff as with it
even lines at 45° can be set out from the chain line.
The latest modified cross-staff is the Adjustable
Cross-Staff (c). It consists of two cylinders of equal diameter placed one
above the other. The upper cylinder can be rotated over the lower one graduated
in degrees and its subdivisions. The upper cylinder carries the Vernier and the
slits to provide a line of sight. Thus, it may be used to take offsets and to
set out any desired angle from the chain line.
Optical Square
Optical Square It is more accurate than the cross
staff.
It is small and compact hand instrument and works on
the principle of reflection. Generally, it is a
5 cm in diameter and 1.25 cm deep.
used for locating objects situated at larger
distances. a metal cover to protect it
from dust, moisture etc.
It consists of horizontal mirror (H) and index
mirror (l) placed at an angle of 45 degree to each other.
The mirror H is half silvered and the upper
half is plain while the mirror I is fully silvered. There are three
openings a, b and c on the sides.
Let AB is the chain line and it is required to locate
an object O during the process of surveying. The optical square is held in such
a manner that a ray of light from object O passes through slot c, strikes the
mirror, gets reflected and strikes the silvered portion of the mirror H. After
being reflected from H, the ray passes through the pin hole and becomes visible
to the eye. The observer looking through the hole a can directly see the
ranging rod at B through the un-silvered portion of the mirror H and he image
of the ranging rod placed at O. Thus, when both the ranging rods coincide, the
line OD becomes perpendicular to the chain line. If they do not coincide, the
optical square has to move back and forth to get the correct position of D.
Ranging
The term ranging is used to establish a set of
intermediate points on a straight line whose two ends have already been fixed
on the ground, whereas
Chaining means the measuring the length of a straight line with chain or tape.
Chaining a long line necessarily involves ranging.
There are two methods of ranging-
When intermediate ranging rod are fixed on a straight
line by direct observation from end station, the process is known as direct
ranging.
When end station is not visible due to there being
high ground between them intermediate ranging rod fixed on the line in an
indirect way this method is known as indirect ranging or (reciprocal
ranging)
Taping on a Sloping or Uneven Ground
If the slope of the ground is more than 3° or 1 in 20,
there is considerable difference between the slope distance and the horizontal
distance.
One of the following procedures may be adopted to
determine the horizontal equivalent of the measured slope distance in such cases:
1. Direct method 2. Indirect method.
Direct method: In the direct method, also known as stepping
method, the horizontal equivalent of the slope distance is directly
measured, as shown in Fig. It is more convenient to measure downhill than
to measure uphill and, therefore, uphill measurement should be avoided
as far as possible. In the case of downhill measurements, the horizontal
distance is measured in steps as shown in Fig. 3.8a using the procedure
explained for taping on the flat ground. For taping uphill as shown in Fig.
3.8b, the rear end of the tape is held at A' above A and the other end at B on
the line AE, and the measurement proceeds upwards in similar manner as above.
Indirect Method- When the slope is more steep than steeping method is
not suitable. In such case horizontal distance is measured by following method.
1.
By
Measuring the slope with Clinometer.
2.
By
Applying Hypotenuse allowance
3.
By knowing the difference of level between two points.
1.In this method the angle PQR can be measured by a clinometer
or on the vertical circle of the transit.
Then. PQ= PQ cos
Hypotenuse allowance is one of the indirect methods of ranging on uneven
or sloping ground. In this method, a correction is applied at every chain
length and at every point where the slope changes. This facilitates in locating
or surveying the intermediate points.
Let α = the angle of slope of the ground.
AD = AB = 1 Chain = 100 links.
Then AC = (100 sec) α links and
BC = AC – AB
BC = (100 Sec α) – (100) links.
BC = 100 (Sec α- 1) links.
The amount 100 (sec α – 1) is known as hypotenuse
allowance.
3.Knowing the difference of level
Suppose, A, B, C and D are different points on sloping
ground. The difference of level between these points is determined by a
levelling instrument. Let the respective differences be h1,
h2 and h3. Then the sloping distances AB, BC and CD
are measured. Let the distances L1, L2 and L3 respectively
(Fig).
Obstacles in Chaining of a Line | Land Survey |
Surveying
The three main obstacles in chaining of a line are of
the following types:
1. Chaining Free, Vision Obstructed
2. Chaining Obstructed, Vision Free
3. Chaining and Vision Both Obstructed.
1. Chaining Free, Vision Obstructed:
In this type of obstacles, the ends of the lines are
not intervisible e.g. rising ground, hill or jungle intervening.
(i) Both ends may be visible from any intermediate
point lying on the line such as in the case of a hill. The obstacle of this
kind may easily be crossed over by reciprocal ranging and length measured by stepping
method of chaining.
(ii) Both ends may not be visible from any intermediate
point such as in the case of a jungle. The obstacle of this kind may be crossed
over by “Random line method”.
2. Chaining Obstructed, Vision Free
The typical obstacle of this type is a sheet of water,
the width of which in the direction of measurement exceeds the length of the
chain or tape. The problem consists in finding the distance between convenient
points on the chain line on either side of obstacle.
(a) When the obstacle can be chained around, e.g. a pond,
a thorny hedge etc.
(b) When the obstacle cannot be chained around e.g. a
river.
3. Chaining and Vision Both Obstructed
A building is a typical example of this class of
obstacles. The problem in this case consists both in prolonging the line beyond
the obstacle and finding the distance across it.
SCALE OF MAP
It is not always representing the actual distance of
an object on drawing.
Scale is the ratio of distance of two point on a sheet or map to the same distance
of two point in the
ground.
Small Scale-
1 cm = 100 m
Medium Scale-
1 cm = 50 m
Large Scale- 1 cm = 10 m
Smallest scale is that whose denominator is larger
value.
The scales are classified into four categories-
1.
Plain
Scale
2.
Diagonal
Scale
3.
Scale
of chords
4.
Vernier
Scale
1.
Plain
Scale
Plain Scale is one on which it is possible to measure
two dimensions only. For example, measurements such as units and lengths, meters
and decimeters etc.
2.
Diagonal
Scale
On diagonal scale, it is possible to measure three
dimensions such as meters, decimeters and centimeters, units, tens and
hundreds; yards, feet and inches etc.
A short length is divided into number of parts using
the principle of similar triangle in which sides are proportional.
1-1 represent
1/10 PQ
2-2 represent 2/10 PQ
9-9 represent 9/10 PQ
3.
Scale
of Chords
Scale of chords is used to measure an angle and is
marked on either on rectangular protractor or an ordinary box wood scale.
4.
Vernier
Scale
A device used for measuring the fractional part of
one of the smallest divisions of a graduated scale.
It usually consists of a small auxiliary scale
which slides alongside of the main scale.
Least count of the Vernier = the difference between smallest division on the
main scale(S) and smallest division on the Vernier scale(v).
Least Count = S-v
Direct Vernier
It is directly calibrated in same direction of
main scale.
The Direct Vernier scale divisions are shorter than
the Main Scale divisions.
If it is required to read 1/nth part of the smallest
division on the main scale, (n-1) main divisions are taken and divided into n
equal divisions on the Vernier scale.
n = the number of
divisions on the vernier.
v = the value of the smallest division on the Vernier
scale.
d= the value of the smallest division on the Main
scale.
Retrograde Vernier
It is directly calibrated in opposite direction
of main scale.
In this type of the vernier, (n+1) divisions of the
main scale are taken and divided into n divisions on the vernier scale.
(i) Vernier divisions are longer than the main scale
divisions,
(ii) The graduations of main scale are marked in the
direction opposite to that of the vernier scale-one from right to left and the
other from left to right.
The only advantage of a retrograde vernier is that the
graduations are bigger than those of a direct vernier. But as it has to be read
in opposite direction, which is rather difficult, it is not commonly used.
Extended vernier nv=(2n-1) d
Double Vernier
With a simple vernier, readings can be taken in one
direction only, but a double vernier is required when the graduations on main
scale are marked in both discussions from the common zero, such as in Abney’s
level.
In a double vernier, two simple Vernier’s are placed
end to end forming one scale with the zero in the center. One is used for
readings in the clockwise direction and the other for the readings in the
anticlockwise direction.
In the case of a vernier attached to the vertical
circle of a transit theodolite which is divided into the quadrants, two sets of
graduations are marked on a single vernier instead of providing a double
vernier. In reading this vernier, only that set is used which increases in the
same direction as the graduations on the quadrant which is being read.
There are some other special forms of vernier such as
an extended vernier used on the astronomical sextant, and the double folded
vernier used in compasses etc.
SHRINKAGE RATIO
Shrinkage ratio / factor is < 1 or equal to 1.
Correction due to incorrect Tape Length
Corrections in Chain Surveying
1.
Correction
for Absolute Length
If the actual tape length is not equal to standard
value then the correction should be applied.
The correction for the measured length is given by the
formula,
Ca = LC / l ------------------- (1)
Where Ca
= the correction for absolute length.
L = the measured length of a
line (in m)
C = the correction to be
applied to the tape.
l = the nominal length of a
tape (in m)
2.
Correction
for Temperature
The tape length is changes due to temperature, while
taking measurements the correction Ct is applied. It is given by the formula,
Ct = a (Tm – To) L-----------(2)
Where Ct
= the correction for temperature, in m.
a = the coefficient of
thermal expansion. 11 x10-6 /C.
Tm = the mean
temperature during measurement
To = the temperature at which
the tape is standardized
L = the measure length in m.
The coefficient of Inver tape (3.6*10-7/C)
is very small so it give better results.
Tm > To Correction is positive
Tm > To Correction is negative
3.
Correction
for Pull
During measurement applied full maybe either more or
less at which tape or chain was Standardized due to elastic property of
material this strain will vary according to variation of applied pull. So, the
correction should be applied is given by the expression.
Where Cp =
the correction for pull in metres
P = the pull applied during
measurement, in newtons (N).
Po= the pull under
which the tape is standardized in newtons (N).
L = the measured length in
metres.
A = the cross-sectional area
of the tape, in sq.cm.
E = the modulus of elasticity
of steel.
The value of E for steel
may be taken as 19.3 to 20.7 x 1010 N/m2 and
that for invar 13.8 to 15.2 x 1010 N/m2.
Pm > Po Correction is positive
Pm > Po Correction is negative
4.
Correction
for Sag
The correction for sag (or sag correction) is the
difference in length between the arc and the subtending chord.
It is required only when the tape is suspended during
measurement.
Since the effect of the set on the tapes is to make the
measured length too great this correction is always subtractive. It is given by the formula,
Cs =
the sag correction for a single span, in metres.
L =
the distance between supports in metres.
W=
weight per meter length (Total mass of the tape in kilograms) W= mgl1
m = the mass of the tape, in kilograms per metre.
l1 =
the distance between supports in metres
P = the applied pull, in newtons (N).
5.
Normal
Tension
The pull or tension which when applied when the tape
is suspended to the air, equalize the correction due to pull and sag is known as
normal tension.
For one tape length, Cpull = Csag
Pm2
(Pm – Po)
= W2AE/24
The
value of P is calculated by trial and error method.
6.
Correction for Slope
This correction is always subtractive from the
measured length.
