Introduction to surveying, ranging and chaining

“It is the art of determining the
relative positions of different object on the surface of
the earth by measuring the horizontal distance
between them and by preparing a map to any suitable
scale.”

• The object of surveying is to prepare a map or plan to show
the relative positions of the objects on the surface of the
earth. The map or plan is drawn to some suitable scale.
• It also shows boundaries of districts, states, and countries
too.
• It also includes details of different engineering features such
as buildings, roads, railways, dams, canals etc

The surveying may be used for following purposes:
1. To prepare a topographical map which shows hills, valleys, rivers, forests,
villages, towns, etc of country.
2. To prepare a cadastral map showing the boundaries of fields, plots, houses,
and other properties.
3. To prepare an engineering map which shows the position of engineering
works such as buildings, roads, railways, dams, canals, etc.
4. To prepare military map showing roads and railways, communication with
different parts of country.
5. To prepare a contour map to know the topography of the area to find out the
best possible site for roads, railways, bridges, reservoirs, canals, etc
6. To prepare archeological map including places where ancient relics exist.
7. To prepare a geological map showing areas including underground resources.
8. For setting out work and transferring details from the map on the ground.

Classification of Surveying
A. Primary Classification
Plane Surveying
Geodetic Surveying
B. Secondary Classification
1. Based on Instruments
2. Based on Methods
3. Based on Object
4. Based on nature of field

We know that the shape of the earth is spheroidal.
Thus the surface is obviously curved. Surveying
is primarily divided into two types considering
the curvature of the earth’s surface.
1. Plane Surveying
2. Geodetic Surveying

1. Plain Surveying
• The plain surveying is that type of surveying in which
earth surface is considered as a plane and the curvature
of the earth is ignored.
• In such surveying a line joining any two stations is
considered to be straight.
• The triangle formed by any three points is considered
as a plane triangle, and the angles of the triangle
are considered as plain angles.
• Surveying is carried out for a small area of less than
250 km2 .
• It is carried out by local or state agencies like R & B
department, Irrigation department, Railway
department.

2. Geodetic Surveying
• The geodetic Surveying is that type of surveying
in which the curvature of the earth is taken into
account.
• It is generally extended over larger areas.
• The line joining any two stations is considered as
curved line.
• The triangle formed by any three points is
considered to be spherical and the angles of the
triangle are considered to be spherical angles.
• Geodetic surveying is conducted by the survey of
India Department and is carried out for a larger
area exceeding 250 km2

Plain Surveying Vs Geodetic Surveying
No. Plain Surveying Geodetic Surveying
1 The earth surface is considered as
plain Surface.
The earth surface is considered
as Curved Surface.
2. The Curvature of the earth is
ignored
The curvature of earth is taken into
account.
3 Line joining any two stations is
considered to be straight
The line joining any two stations is
considered as spherical/curved
line.
4. The triangle formed by any three
points is considered as plain.
The Triangle formed by any three
points is considered as spherical.
5. The angles of triangle are
considered as plain angles.
The angles of the triangle are
considered as spherical angles.
6. Carried out for a small area <
250 km2
Carried out for a small area > 250 km2

Survey can be classified on different bases:
1. Based on Instrument:
2. Based on Methods:
3. Based on Objects:
4. Based on the Nature of Field

Survey can be classified into various categories depending on
methods used and nature of the field.
1. Based on Instrument:
i. Chain Survey: This is the simplest type of surveying in which
only linear measurements are made with a chain or a tape.
Angular measurements are not taken.
ii. Compass Survey: In Compass Survey, the angles are
measured with the help of a magnetic compass.
iii. Plane Table Survey: It is a graphical method of surveying in
which field works and plotting both are done simultaneously.

iv. Theodolite survey: In theodolite survey the horizontal angles
are measured with the theodolite more precisely than compass
and the linear measurements are made with a chain or tape.
v. Tacheometric Survey: A special type of theodolite known as
tachometer is used to determine horizontal and vertical
distances indirectly.
vi. Photographic Survey: Photogrammetry is the science of taking
measurements with the help of photographs taken by aerial
camera from the air craft.
vii. Electronic Distance Measurement (EDM) Survey: In this type
of survey all measurements (length, angles, co-ordinates) are
made with the help of EDM instrument ( i.e.. Total Station).

2. Based on Methods:
i. Triangulation Survey: Triangulation is basic
method of surveying, when the area to be surveyed is
large, triangulation is adopted. The entire area is
divided into network of triangles.
ii.Traverse Survey: A Traversing is circuit of survey
lines. It may be open or closed. When the linear
measurements are done with a chain and a tape and the
directions or horizontal angles are measured with a
compass or a theodolite respectively the survey is called
traversing.

3. Based on Objects:
i. Geological Survey: In this both surface and subsurface
surveying are conducted to locate different minerals and
rocks. In addition, geological features of the terrain such as
folds and faults are located.
ii. Mine Survey: Mine Survey includes include both surface and
underground surveys. It is conducted for the exploration of
mineral deposits and to guide tunneling and other operations
associated with mining.
iii. Archeological Survey: It is conducted to locate relics of
antiquity, civilization, kingdoms, forts, temples, etc.
iv. Military Survey: It has a very important and critical
applications in the military. Aerial surveys are conducted for
this purpose. It is conducted to locate strategic positions for
the purpose of army operations.

4. Based on Nature of Field:
i. Land Survey: Land Survey is done on land to prepare plan
and maps of a given area. Topographical, city and cadastral
surveys are some of the examples of land surveying.
ii. Marine Survey: This survey is conducted on or near the
body of water such as lake, river, coastal area. This Survey
consists of locating shore lines of water bodies.
iii.Astronomical Survey: This survey is conducted for the
determining of latitudes, longitudes, azimuths, local time, etc.
for various places on earth by observing heavenly bodies ( sun
or the stars).

• According to the first principle,
the whole survey area is
first enclosed by main
stations (i.e.. Control stations)
and main survey lines.
• The area is then divided
into a number of divisions
by forming well conditioned
triangles.
1. Always work from whole to the part:

1. Always work from whole to the part:
• The main survey lines are measured very accurately
with precise survey instruments.
• The remaining sides of the triangle are measured.
• The purpose of this method of working is to control
accumulation of errors.
• During measurement, if there is any error, then it will
not affect the whole work, but if the reverse
process is followed then the minor error in
measurement will be magnified.

2. To locate a new station by at least two
measurements ( Linear or angular) from fixed reference
points.
• According to the second principle the points are
located by linear or angular measurement or by both
in surveying. If two control points are established
first, then a new station can be located by linear
measurement.
• Let A & B are control points, a new point C can be
established.

• Following are the methods of locating point C from such reference points A & B.
• The distance AB can be measured accurately and the relative positions of the point
can be then plotted on the sheet to some scale.
(a) Taking linear measurement from A and B for C.
(b) Taking linear measurement of perpendicular from D to C.
(c) Taking one linear measurement from B and one angular measurement as ∕_ ABC
(d) Taking two angular measurement at A & B as angles /_CAB and / ABC.
(e) Taking one angle at B as /_ ABC and one linear measurement from A as AC.

1. Linear
2. Angular
UNITS
• 1 FOOT = 12 INCH
• 1 Y
ARD = 3 FEET
• 1 CHAIN = 66 FEET
• 1 FURLOG = 10 CHAIN
• 1 MILE = 5280 FEETAND 1.6KM

A. Direct methods
B. Optical methods
C. E.D.M methods

[A] Direct Method of Measuring Distance:
1. Pacing:
Where approximate result is required, distance may be determined
by pacing. This method is used for reconnaissance survey, for
preparation of military plans. Also used for approximate checking
distance. The method consists of walking over a line and counting the
number of paces (80cm) the required distance may be obtained by
multiplying the number of paces by the average length of pace.
The length of pace varies with the:
• Individual, age, height and physical condition
• The nature of the ground (uphill and down hill)
• The slope of the country and
• The speed of pacing

2. Passometer:
It is a pocket instrument. It automatically records the number of
paces. It should be carried vertically, in waistcoat pocket or
suspended from a button. The mechanism being operated by motion
and strain of the body.

3. Pedometer:
It is similar to passometer. But it registers the distance walked by the
persons carrying it. The distance is read by means of an indicator. It
is fitted with a stud or knob, which when pressed release indicator to
zero, it may be carried in the same way as the passometer.

4. Odometer:
It measures the distance approximately. It can be attached to the
wheel of any vehicle, such as carriage, cart bicycle, etc. It registers
the number of revolution of the wheel. Knowing the circumference of
the wheel, the distance traversed may be obtained by multiplying
the number of revolutions. By the circumference of the wheel

5. Speedometer:
The Speedometer of an automobile may be used to measure
distances approximately. It gives better results than pacing, provided
the route is smooth.

6. Perambulator:
It can measure distance rapidly. It consist a single wheel provided
with forks and a handle. It is wheeled along the line, the length of
which is desired. The distance traversed is automatically registered
on the dial. The reading approximates on rough ground.

7. Judging distance:
This is very rough method of determining distance. It is used
reconnaissance survey.
8. Time Measurement:
This distance is roughly determined by time intervals of travel.
Knowing the average time per km for a person at walk or a horse,
the distance traversed may be easily obtained.

9. Chaining:
Measuring the distance with chain or rope or tape is the most
accurate and common method, called as chaining. For work of
ordinary precision a chain is used. Where great accuracy is
required, a steel tape is used.

[B] Optical Method of Measuring Distance:
• In the optical methods, principles of optics are used.
• The distance is not actually measured in field but it is computed
indirectly.
• The instrument used for making observations is called tacheometer.

[B] Optical Method of Measuring Distance:

[C] EDM Method of Measuring Distance:
• Electronic Distance Measuring (E.D.M) instruments have been developed
quite recently.
• These are practically replacing the measurement of distances using
chains or tapes.
• There is a large variety of such instruments and depending upon the
precision required the instruments should be used.

The following instruments are required for measurements
with chain and tape:
1. Chains
2. Tape
3. Ranging rods
4. Offset rods
5. Arrows
6. Pegs
7. Plumb bob and
8. Line ranger

1. Chain:
Various types of chains used in surveying are:
I. Metric Chain
II. Gunter’s Chain or Surveyor’s Chain
III. Engineer’s Chain
IV. Revenue Chain
V. Steel Band or band chain

I. Metric Chain:
• Normally, a chain is prepared with 100 or 150 pieces of
galvanized mild steel wire of 4 mm diameter known as link.
• The ends of the links are bent into loop and connected together by
means of three oval rings which provide the flexibility to the chain
and make it less liable to kinking.
• Both ends of the chain have brass handle with swivel joint so that
the chain can be turned round without twisting.

I. Metric Chain:
• In a metric chain at every one meter interval of chain, a small brass ring is
provided.
• Brass tallies are also provided at every 5.0 m length of chain. Each tally has
different shape which indicates 5 , 10, 15m from any one side of the chain,
metric chains are available in 20 m and 30 m length.
• A 20 m chain has 100 links each of 20 cm and 30 m chain has 150 links. Length
of chain is embossed on the brass handles of the chain.

II. Gunter’s Chain or Surveyor’s Chain:
• A 66 feet long chain consists of 100 links each of 0.66 ft it is
known as Gunter‟s Chain
• Here, 10 sq chain are equal to 1 acre,
• 10 chains= 1 furlong and 8 furlongs = 1 mile
• This chain is suitable for taking length in miles and areas in acres.

II. Gunter’s Chain or Surveyor’s Chain:

III. Engineer’s Chain:
• A 100 ft chain of 100 links each of 1 foot is known as Engineer's
chain.
• Brass tags are fastened at every 10 links.
• This chain is used to measure length in feet and area in square
yards.

IV. Revenue Chain:
• Revenue chain is 33 ft long chain consisting of 16 links.
• This chain is used for distance measurements in feet & inches for
small areas.

V. Steel Band or Band Chain:
• Steel bands are preferred than chains because they are more
accurate, but the disadvantages is that they get broken easily
and are difficult to repair in the field.
• They are 20 and 30 m long, 12 to 16 mm wide and 0.3 to 0.6
mm thick.
• They are numbered at every metre and divided by brass studs at
every 20 cm

2. Tapes
Tapes are used for more accurate measurement. The tapes
are classified based on the materials of which they are
made of such as:
I. Cloth or linen tape
II. Fibre Tape
III. Metallic Tape
IV. Steel tape
V. Invar Tape

I. Cloth or LinenTape :
• Linen tapes are closely woven linen and varnished to resist
moisture.
• They are generally 10 m, 20 m, 25 m and 30 m long in length
and 12 to 15 mm wide.
• They are generally used for offset measurements. These tapes are
light and flexible.

II. Fibre Glass Tape :
• These tapes are similar to linen and plastic coated tapes but these
are made of glass fibre.
• The tapes are quite flexible, strong and non- conductive.
• These can be used in the vicinity of electrical equipment.
• These tapes do not stretch or shrink due to changes in temperature
or moisture.
• These tapes are available in length of 20 m, 30 m and 50m
length.

III. Metallic Tape:
• A linen tape reinforced with brass or copper wires to prevent
stretching or twisting of fibres is called a metallic tape.
• As the wires are interwoven and tape is varnished these wires are
visible to naked eyes.
• This is supplied in a lather case with a winding device. Each metre
length is divided into ten parts (decimetres) and each part is
further sub-divided into ten parts.
• It is commonly used for taking offset in chain surveying.

IV. Steel Tape
• The steel tape is made of steel ribbon of width varying from 6 to
16 mm.
• The commonly available length are 10 m, 15 m, 20 m, 30 m and
50 m. It is graduated in metres, decimetres, and centimetres.
• Steel tapes are used for accurate measurement of distances.

V. Invar tape
• Invar tape are made of alloy of nickel 36 % and steel 64 %
having very low co-efficient of thermal expansion.
• These are 6 mm wide and generally available in length of 30 m,
50m, 100m.
• It is not affected by change of temperature therefore, it is used
when high degree of precision is required.

3. Ranging Rods:
• Ranging rods are used for ranging some
intermediate points on the survey line.
• Ranging rods are generally 2 to 3 m in
length and are painted with alternate bands
of black or white or red and white colour
with length of each equalizing 20 cm.
• 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 used and is called ranging
pole.

4. Offset Rods:
• The offset rod is similar to 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.
• It is also used for measuring small offsets

5. Arrows:
• Arrows are made of tempered steel wire
of diameter 4 mm.
• one end of the arrow is bent into ring of
diameter 50 mm and the other end is
pointed.
• Its overall length is 400 mm.
• Arrows are used for counting the number
of chains while measuring a chain line.
• An arrow is inserted into the ground after
every chain length measured on the
ground.

6. Pegs:
• Pegs are made of timber or steel and they are used to mark the
position of the station or terminal points of a survey line.
• Generally, pegs are 15 cm long and are driven into the ground
with the help of a hammer.

7. Plumb Bob:
• Plumb-bob is used to transfer points on
the ground.
• It is also used for fixing the instruments
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 not, by
suspending the plumb-bob exactly at the
centre of the instrument under it.
• Plumb bob is thus used as centring aid in
theodolites and plane table.

8. Line Ranger:
• It is an optical instrument used for locating
a point on a line and hence useful for
ranging.
• It consists of two isosceles prisms placed one
over the other and fixed in an instrument
with handle.
• The diagonals of the prisms are silvered so
as to reflect the rays.
• Its advantage is it needs only one person
to range.
• The instrument should be occasionally
tested by marking three points in a line
and standing on middle point observing
the coincidence of the ranging rods.

8. Line Ranger:
• If the images of the two ranging rods do not appear in the same line,
one of the prism is adjusted by operating the screw provided for it.

8. Line Ranger:
• To locate point C on line AB (ref. Fig.) the surveyor holds the instrument in hand
and stands near the approximate position of C.
• If he is not exactly on line AB, the ranging rods at A and B appear separated as
shown in Fig. (b).
• The surveyor moves to and from at right angles to the line AB till the images of
ranging rods at A and B appear in a single line as shown in Fig. (c).
• It happens only when the optical square is exactly on line AB.
• Thus the desired point C is located on the line AB.

9. Cross Staff:
• Three different types of cross staffs used for setting perpendicular offsets.
• All cross staffs are having two perpendicular lines of sights.
• The cross staffs are mounted on stand.
• First line of sight is set along the chain line and without disturbing setting right
angle line of sight is checked to locate the object.

9. Cross Staff:
• With open cross staff (Fig(a)) it is possible to set perpendicular
only, while with French cross staff (Fig(b)), even 45º angle can be
set.
• Adjustable cross staff can be used to set any angle also, since there
are graduations and upper drum can be rotated over lower drum.

10.Cross Staff:
• With open cross staff (Fig(a)) it is possible to set perpendicular
only, while with French cross staff (Fig(b)), even 45º angle can be
set.
• Adjustable cross staff can be used to set any angle also, since there
are graduations and upper drum can be rotated over lower drum.

• Chain surveying is the type of surveying in which only
linear measurements are taken in the field.
• This type of surveying is done for surveying of small
extent to describe the boundaries of plots of land and to
locate the existing feature on them.
• It is the method of surveying in which the area is divided
into network of triangles and the sides of the various
triangles are measured directly in the field with a chain
or a tape and no angular measurements are taken.

• Triangulation is the principle of chain surveying.
• The principle of chain surveying is to divide the area
into a number of triangles of suitable sides. As a
triangle is the only simple plane geometrical figure
which can be plotted from the length of the three sides
even if the angles are not known. A network of triangles
is preferred to chain surveying.
• If the area to be surveyed is triangle in shape and if the
lengths and sequence of its three sides are recorded,
the plan of the area can be easily drawn.

Survey Stations:
Survey stations are the points at the beginning and at the end of a
chain line they may also occur at any convenient position on the
chain line.
Such station may be:
• Main Stations
• Subsidiary Stations
• Tie Stations

Survey Stations:
• Main Stations: Main Station Stations along the boundary of an
area as controlling points are known as ‘Main Stations’. The main
stations are denoted by Δ.
• Subsidiary Stations: Stations which are on the main survey lines
or any other survey lines are known as ‘Subsidiary Stations’
these stations are taken to run subsidiary lines for dividing the
area into triangles, for checking the accuracy of triangles and
for locating interior details.
• Tie Stations: These stations are also subsidiary stations taken on
the main survey lines. Lines joining the stations are known as ‘Tie
lines’. Tie lines are taken to locate interior details.

Survey Lines:
• Main Survey Lines: The line joining the main stations are
called main survey lines or chain lines. The main survey lines
should cover the whole area to be surveyed.
• Base Lines: The line on which the framework of the survey is
built is known as ‘Base line’. It is the most important line of the
survey. Generally the longest of the main survey lines is
considered as the base line. This lines should be taken through
fairly level ground, and should be measured very carefully and
accurately.

Survey Lines:
• Check Lines: The line joining the apex point of a triangle to
some fixed points on its base is known as ‘Check line’. It is taken
to check the accuracy of the triangle. Sometimes this line helps to
locate interior details.
• Tie Line: A line joining tie stations is termed as a tie line. It is
run to take the interior details which are far away from the main
lines and also to avoids long offsets. It can also serve as check
line.

The following points should be considered while selecting survey
stations:
• It should be visible from at least two or more stations.
• As far as possible main lines should run on level ground.
• All triangles should be well conditioned (No angle less than 30º).
• Main network should have as few lines as possible.
• Each main triangle should have at least one check line.
• Obstacles to ranging and chaining should be avoided.
• Sides of the larger triangles should pass as close to boundary
lines as possible.
• Trespassing and frequent crossing of the roads should be
avoided.

The following operations are involved in chain surveying.
1. Chaining
2. Ranging
3. Offsetting
These three operations are done simultaneously during
chain Surveying.

1. Chaining:
Chaining on Level Ground
• The method of taking measurement with the help of
chain or tape is termed as chaining.
Chaining involves following operations
a) Fixing the stations
b) Unfolding the chain
c) Ranging
d) Measuring the distance (Survey Line)
e) Folding the Chain

1. Chaining:
a) Fixing of Station
Stations are first of all marked with pegs and ranging rods to
make them visible.
b) Unfolding of a Chain
• To open a chain, the strap is unfastened and the two brass
handles are held in the left hand and the bunch is thrown
forward with the right hand.
• Then one chainmen moves forward by holding the other
handle until the chain is completely extended.

1. Chaining:
c) Ranging:
The process of establishing intermediate points on a straight
line between two end points is known as ranging. Ranging must
be done before a survey line is chained.

1. Chaining:
d) Measuring the distance (Survey Line):
• Two persons are required in this operation, i.e. Leader and
Follower.
• The chainman at the forward end of the chain who drags the
chain forward, is known as the leader
• The chainmen at the rear end of the chain, who holds the
zero end of the chain at the station, is known as the follower.
• To chain the line, the leader moves forward by dragging the
chain line and taking with him ranging rod and ten arrows.
• The follower stands at the starting station by holding the
other end of the chain.

1. Chaining:
• When the chain is fully extended, the leader holds the
ranging rod vertically at arms length.
• The follower directs the leader to move his rod to the left or
right until the ranging rod is exactly in the line.
• Then the follower holds the zero end of the chain by touching
the station peg.
• The leader stretches the chain by moving it up and down with
both hands, and finally place it on the line.
• He then inserts an arrow on the ground at the end of the
chain and mark it with cross.
• Again the leader moves forward by dragging the chain with
nine arrows and the ranging rod.

1. Chaining:
• At the end of the chain, he fixes another arrow as before, As
the leader moves further, the follower picks the arrow which
were inserted by the leader.
• During chaining the surveyor or an assistant should conduct
the ranging operation. In this way, chaining is continued, when
all the arrows are inserted the leader has non left with him,
the follower hands over to the leader.
• To measure the fractional length, the leader should drag the
chain beyond the station and the follower should hold the
zero end of the chain at last arrow, then odd links should be
counted.

1. Chaining:

1. Chaining:
e) Folding of Chain:
• To fold the chain, a chainmen should move forward by pulling the chain
at the middle.
• Then the two halves of the chain will come side by side. After this,
commencing from the central position of the chain, two pairs of links are
taken at a time with the right hand and placed on the left hand
alternately in both directions.
• Finally the two brass handles will appear at top. The bunch should be
then fastened by the strap.

2. Ranging:
• When a survey line is longer than a chain length, it
is necessary to align intermediate points on chain
line so that the measurements are along the line.
• The process of locating intermediate points on
survey line is known as ranging.
• There are two methods of ranging viz.,
i. Direct Ranging and
ii. Indirect/Reciprocal Ranging.

2. Ranging:
i. Direct Ranging
• If the first and last points are intervisible this method is possible.
• Stations A and B in which an intermediate point C is to be
located.
• Point C is selected at a distance slightly less than a chain length.

2. Ranging:
i. Direct Ranging
• At points A and B ranging rods are fixed.

2. Ranging:
i. Direct Ranging
• The assistant holds another ranging rod near C.
• Surveyor positions himself approximately 1 m behind station A and looking
along line AB directs the assistant to move at right angles to the line AB till he
aligns the ranging rod along AB.

2. Ranging:
i. Direct Ranging
• Then surveyor instructs the assistant to mark that point and stretch the chain
along AC.

2. Ranging:
i. Indirect/Reciprocal Ranging
• Due to intervening ground, if the ranging rod at B is not visible from station A,
reciprocal ranging may be resorted.
• Figure shows this scheme of ranging.

2. Ranging:
i. Indirect/Reciprocal Ranging
• It needs two assistants one at point M and another at point N, where from those points
both station A and station B are visible. It needs one surveyor at A and another at B.
• To start with M and N are approximately selected, say M1 and N1. Then surveyor
near end A ranges person near M to position M2 such that AM2N1 are in a line.
• Then surveyor at B directs person at N, to move to N2 such that BN2M2 are in a line.
• The process is repeated till AMNB are in a line.

• During continuous use, the length of a chain gets altered. Its
length is shortened chiefly due to the bending of links. Its length
is elongated either due to stretching of the links and joints and
opening out of the small rings.
• For accurate work it is necessary to test the chain time to time.
The chain can be thus tested by a steel tape or by a standard
chain. Sometimes, it is convenient to have a permanent test
gauge established where the chain is tested.
• When the length of a chain is measured at a pull of 8 kg at 20
degree C the length of the chain should measure 20 m ± 5 mm
and 30 m ± 8 mm for 20 m and 30 m long chain shall be
accurate to within 2 mm.

Following measures are taken to adjust the length of a chain.
1. If chain is found to be too long
It can be adjusted by;
• Closing up the joints of the rings if found to be opened out
• Reshaping damaged rings
• Removing one or more small rings
• Adjusting the links at the end.
2. If the chain is found to be too short
• Straightening the bent links
• Opening the joints of the rings
• Replacing one or more small circular rings by bigger ones.
• Inserting new rings where necessary.
• Adjusting the links at the end.

3. Offsetting:
• Lateral measurements to chain lines for locating
ground features are known as offsets.
• For this purpose perpendicular or oblique offsets may
be taken.
• If the object to be located (say road) is curved more
number of offsets should be taken.
• For measuring offsets tapes are commonly used.

3. Offsetting:
I. Perpendicular Offsets
• The offsets which are taken
perpendicular to the chain line are
termed as perpendicular offsets.
• These offsets are taken by holding
zero end of the tape at the object
and swinging the tape on the chain
line.
• The shortest distance measured
from object to the chain line is
usually the perpendicular offset.

3. Offsetting:
II. Oblique Offsets
• Oblique distance is always
greater than perpendicular
distance.
• All the offsets which are not
taken at the right angle to chain
line are known as oblique
offsets.

3. Offsetting:
For setting perpendicular offsets any one of the following
methods are used:
(i) Swinging:
(ii) Using cross staffs:
(iii) Using optical or prism square.

3. Offsetting:
(i) Swinging
• Chain is stretched along the survey line. An assistant holds
the end of tape on the object.

3. Offsetting:
(i) Swinging
• Surveyor swings the
tape on chain line and
selects the point on
chain where offset
distance is the least and
notes chain reading as
well as offset reading
in a field book on a
neat sketch of the
object.

3. Offsetting:
(i) Swinging
• Surveyor swings the tape on chain line and selects the point on chain where
offset distance is the least and notes chain reading as well as offset reading in
a field book on a neat sketch of the object.

3. Offsetting:
(ii) Using Cross Staff:
• Three different types of cross staffs used for setting
perpendicular offsets. All cross staffs are having two
perpendicular lines of sights.
• The cross staffs are mounted on stand.
• First line of sight is set along the chain line and without
disturbing setting right angle line of sight is checked to locate
the object.
.

3. Offsetting:
(ii) Using Cross Staff:
• With open cross staff (Fig(a)) it is possible to set
perpendicular only, while with French cross staff (Fig(b)), even
45º angle can be set.
• Adjustable cross staff can be used to set any angle also, since
there are graduations and upper drum can be rotated over
lower drum.

3. Offsetting:
(ii) Using Optical Square and Prism Square:
• An optical square is also used for setting out right angles.
• It consist of a small circular metal box of diameter 5 cm and
depth 1.25 cm.
• It has a metal cover which slides round the box to cover the slits.
• The following are the internal arrangements of the optical
square.
1. A horizon glass H is fixed at the bottom of the metal box. The lower half
of the glass is unsilvered and the upper half is silvered.
2. A index glass I is also fixed at the bottom of the box which is completely
silvered.
3. The angle between the index glass and horizon glass is maintained at
450.

3. Offsetting:
(ii) Using Optical Square and Prism
Square:
4. The opening ‘e’ is a pinhole for eye E,
‘b’ is a small rectangular hole for
ranging rod B, ‘P’ is a large rectangular
hole for object P.
5. The line EB is known as horizon sight and
IP as index sight.
6. The horizon glass is placed at an angle
of 1200 with the horizon sight. The
index glass is placed at an angle of
1050 with the index sight.
7. The ray of light from P is first reflected
from I, then it is further reflected from H,
after which it ultimately reaches the eye
E

3. Offsetting:

3. Offsetting:
Principle:
According to the principle of reflecting surfaces, the angle
between the first incident ray and the last reflected ray is twice
the angle between the mirrors. In this case, the angle between
the mirrors is fixed at 450. So, the angle between the horizon
sight and index sight will be 900.
Setting up the perpendicular by optical square
a. The observer should stand on the chain line and
approximately at the position where the perpendicular is to
be set up.

3. Offsetting:
b. The optical square is held by the arm at the eye level. The
ranging rod at the forward station B is observed through the
unsilvered portion on the lower part of the horizon glass.
c. Then the observer looks through the upper silvered portion
of the horizon glass to see the image of the object P.
d. Suppose the observer finds that the ranging rod B and the
image of object P do not coincide. The he should move
forward or backward along the chain line until the ranging
rod B and the image of P exactly coincide
e. At this position the observer marks a point on the ground to
locate the foot of the perpendicular.

1. Correction for temperature
2. Correction for Sag
3. Correction for tension
4. Correction for Altitude
5. Correction for slope
6. Correction for alignment
7. Correction for standard length

1. Correction for Temperature:
The length of the tape increases while the temperature is
increased and decreases if the temperature is lowered.
Where
L = measured length of a line
t = mean temperature during measurement in 𝑜𝑐.
ts = nominal temperature of standardization.
Ct = correction due to temperature
= co-efficient of linear expansion.(6.5*10−6o F) or 11.2*10-6o C

2. Correction for Sag:
When the tape is suspended from two support in air,
is assumes the shape of centenary.
Where
Cs = Correction for sag
w = weight per unit length
l = measured length of open
p = pull applied during measurement

3. Correction for Tension:
If the pull applied to the tape during measurements is more than it is
standardized its length increase and the measured distance become less than
the actual.
(Correction for tensions is there for positive.)
Let
P = the tension of pull at field (N).
Ps = standardized tension. (N).
L = length measured.
A = Cross – sectional of steel band
E = Young's modulus of elasticity of the steel bond.
Cten = Correction for tension

3. Correction for Altitude:
If the surveying is connected to the functional mapping of the country,
the distance will need to be reduced to the common datum at that
system normally.
The correction for altitude for
surface work is negative & positive
for tunneling work or mining work
below MSL.

A chain line may be interrupted in the following
situations:
1. When chaining is free, but vision is obstructed
2. When chaining is obstructed, but vision is free
3. When chaining and vision both are obstructed

Such problems arises when a rising ground or a
jungle area interrupts the chain line. Here the end
stations are not inter-visible. There may be two
cases:
Case I: Both ends may be visible from intermediate points
on the line.
Case II: Both ends may not be visible from any intermediate
point.

Case I: The end stations may be visible from some
intermediate points on the rising ground. In this case, Reciprocal
ranging is resorted to, and the chaining is done by the stepping
method.

Case II: The end stations are not visible from intermediate
points when a forest area comes across the chain line. In this
case, the obstacle may be crossed over using a random line as
explained below.

Case II: Let AB be the line whose length is required. From A run a
line(AB1), called a random line, in any convenient direction, but as
nearly towards B as can be judged and continue it until the point B is
visible from B1. chain the line to B1 where BB1 is perpendicular to
AB1 and measure BB1. Then,

Case II: If any other length AC1 is measured along AB1 a point C
is located on the line AB by measuring the perpendicular distance
C1C=(AC1/AB1)×B1B. In this manner a sufficient number of
points can be located. The line is then cleared and the distance
measured.

When a pond plantation, tank, river, etc. The
problem is to find the distance between two
convenient points on the chain line on either side
of the obstacle. There are two cases :
Case I: In which it is possible to chain round obstacle
e.g. a pond, a bend in the river, etc.
Case II: In which it is not possible to chain round the
obstacle, e.g. a river.

(a) Select two convenient points A and B on the chain line PR
and on either side of the obstacle(fig.3.38). Erect equal
perpendicular AC and BD by the 3, 4, 5 method ,or the
optical square and measure the length CD. Then AB=CD

(b) As before select A and B. Set out a perpendicular AB of
such a length that CB clears the obstacle, and measure AB
and CB.
Then AC=√(BC²-AB²).

(c) The measurement may be effected
by constructing a right angled triangle.
Select a point A(fig.3.40) on the chain
line on one side of the obstacle and set
out AC to clear the obstacle. At C erect
a perpendicular CB with the optical
Square to clear the obstacle, and
determine the point B on the Chain line
on the other side. Measure AC and CB.
Then , AB=√AC²+CB²

(d) Select two convenient points A and B
on the chain line PR on opposite sides
of the obstacle(fig.3.41). Select a point
C so that AC and BC Clear the obstacle.
Produce the line AC to E so that CE=AC.
Similarly, continue BC to D so that CD
equals to BC. Measure DE. The triangles
CDE and CBA being equal in all
respects, AB=DE.

Case II: In which it is not possible to chain round the obstacle,
e.g. a river.
(a) Select two points A and B on the
chain line PR on opposite banks of the
river.
Set out a perpendicular AD and bisect
it at C mid point of AD .
At D erect a perpendicular DE and
mark the point E in line with C and B.
Measure DE.
Since the triangles ABC, CED are
similar,
Hence AB=DE

e.g. a river.
(b) For Large river, Select two points A
and B on the chain line PR on either side
of the river.
Mark another point E on the chain line.
At A and E erect perpendiculars AC
and ED such that D, C and B are in the
same line.
Measure AC, AE, and ED.
If a line CF is drawn parallel to AE,
meeting ED in F, the triangles ABC and
CFD are similar.

e.g. a river.
(b)
Therefore,
AB/AC=CF/FD
but
CF=AE and FD=ED-AC
or AB/AC=AE/(ED-AC)
whence,
BA=(AC×AE)/(ED-AC)

e.g. a river.
(c) As before select two points A and
B. Set off a perpendicular AD at A.
With a cross staff or an optical
square, erect a perpendicular to DB
at D, cutting the chin line at C.
Measure AD and AC.
Since the triangles ABD and ACD
are similar,
AB/AD=AD/AC.
Hence, AB=AD²/CA

e.g. a river.
(d) If a box sextant is available, the
following method may be use.
Fix two points A and B as before.
At A erect a perpendicular AC of any
convenient length so that the triangle
ABC is well conditioned.
Measure AC, and the angle ACB with
the box sextant.
The distance AB may then be calculated
from
AB = AC tan /_ACB = AC tanθ

e.g. a river.
(e) This method is used when a survey
line crosses a river obliquely.
Set out line AE at a convenient angle
with the survey line PR and range point
D in line with E and A, making AD=AE.
Using an optical square, set out
perpendiculars DB and EC to the line DE
at D and E, intersecting the survey line
PR at B and C respectively.
Measure AC. The triangles ADB and
AEC being congruent, the required
length AB is equal to AC.

3. When chaining and vision both are obstructed
In this case the problem consists in prolonging the line beyond the obstacle
and determining the distance across it. A building is a typical example of
this class of obstacle.
Choose point A and B on the chain line PR. At A and B erect
perpendiculars AE and BF of equal length. Check the diagonals BE and
AF, which should be equal and also EF, which should be equal to AB.
Prolong the line EF past the obstacle and select two points G and H on it.
At G and H set out perpendiculars GC and HD equal in length to AE. The
points C and D are obviously on the chain line PR and BC= FG.
Great care must be taken in setting
out the perpendiculars very
accurately and to see that their
lengths are exactly equal.

Introduction to surveying, ranging and chaining

Introduction to surveying, ranging and chaining

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Introduction to surveying, ranging and chaining