Page 1
Curves
Introduction
Curves are generally used on highways and railways where
it is necessary to change the direction of motion. This chap
ter focuses on the elements of curves and methods for set
ting out of curves on ground.
Curve may be circular, parabolic or spiral and is always
tangential to the two straight directions.
Circular curves are of three types:
1. Simple curves
2. Compound curves
3. Reverse curves
Simple Curves
Simple curves consists of a single arc of a circle. It is tan
gential to both the straight lines.
1. Back tangent (AT
1
): The tangent before curve is
called back tangent or fi rst tangent.
2. Forward tangent (BT
2
): The tangent following the
curve is called forward tangent or second tangent.
3. Point of intersection, vertex (V): It is the point of
intersection of two tangents AT
1
and BT
2
.
?
2
?
2
?
A
R
O
B
D
C
V '
(PI) V
(PT) T 2
(PC) T 1
R
4. Point of curve (T
1
): It is the beginning of the curve
where the alignment changes from a tangent to a
curve.
5. Point of tangency (T
2
): It is the end of the curve
where the alignment changes from a curve to a
tangent.
6. Defl ection angle or Deviation angle (?): The
diff erence between the slopes of the two tangents.
Curves and
Triangulation Survey
Part III_Unit 12_Chapter 05.indd 1 5/31/2017 4:56:03 PM
Page 2
Curves
Introduction
Curves are generally used on highways and railways where
it is necessary to change the direction of motion. This chap
ter focuses on the elements of curves and methods for set
ting out of curves on ground.
Curve may be circular, parabolic or spiral and is always
tangential to the two straight directions.
Circular curves are of three types:
1. Simple curves
2. Compound curves
3. Reverse curves
Simple Curves
Simple curves consists of a single arc of a circle. It is tan
gential to both the straight lines.
1. Back tangent (AT
1
): The tangent before curve is
called back tangent or fi rst tangent.
2. Forward tangent (BT
2
): The tangent following the
curve is called forward tangent or second tangent.
3. Point of intersection, vertex (V): It is the point of
intersection of two tangents AT
1
and BT
2
.
?
2
?
2
?
A
R
O
B
D
C
V '
(PI) V
(PT) T 2
(PC) T 1
R
4. Point of curve (T
1
): It is the beginning of the curve
where the alignment changes from a tangent to a
curve.
5. Point of tangency (T
2
): It is the end of the curve
where the alignment changes from a curve to a
tangent.
6. Defl ection angle or Deviation angle (?): The
diff erence between the slopes of the two tangents.
Curves and
Triangulation Survey
Part III_Unit 12_Chapter 05.indd 1 5/31/2017 4:56:03 PM
7. Tangent distance: It is the distance from PC to PI
(also the distance from PI to PT)
T
1
V = T
2
V = R tan
?
2
·
R is the radius of the curve.
8. External distance or apex distance (E): The
distance from vertex V to the centre of the curve C.
E = VC = R sec
?
2
1 
?
?
?
?
?
?
9. Length of curve (L): Distance from PC to PT,
L = T
1
CT
2
= R ? ? in radians.
10. Length of long chord: It is the chord joining PC to
PT.
TT R
12
2
2
= sin
?
11. Midordinate/versed sine: The distance from mid
point of the long chord to midpoint of the curve.
CD R =
?
?
?
?
?
?
1
2
cos
?
12. Normal chord: A chord between two successive
regular stations on a curve.
13. Subchord: Any chord shorter than the normal chord.
14. Righthand curve: If the curve deflects to the right
of the direction of the progress of survey, it is called
the righthand curve.
15. Lefthand curve: If the curve deflects to the left of
the direction of the progress of survey, it is called the
lefthand curve.
Chainages
Chainage of T
1
= Chainage of V – Tangent length
Chainage of T
2
= Chainage of T
1
+ Length of Curve
Degree of a Curve (D)
• The central angle subtended by a chord of a fixed length.
• In India, the fixed length of chord is taken as 20 m to cal
culate the degree of curve.
360 degrees = 2pR
D degrees = 20 m
? Therefore, R =
1146
D
metres
Methods of Setting out a Curve
Linear Methods
In this method, only a chain or tape is used. Linear methods
are used when:
• A high degree of accuracy is not required.
• The curve is short.
1. By ordinates or offsets from the long chord:
Midordinate,
OR R
L
R
0
2
2
= 
?
?
?
?
?
?
O
x
= RX RO
22 0
  ()
=
XK X
R
() 
2
(approx)
O
x
= Ordinate at distance of ‘X’ from central ordinate.
To set out the curve, the long chord is divided into an
even number of equal parts. Offsets calculated at each
point are then set out at each of these points.
2. By successive bisection of arcs or chords:
• Join tangent points T
1
, T
2
and bisect them at D.
Erect a perpendicular DC whose length is equal to
versed sine of the curve.
CD RR R
L
=
?
?
?
?
?
?
= 
?
?
?
?
?
?
1
22
2
2
cos
?
• Join T
1
C and T
2
C and bisect them at D
1
and D
2
.
Erect perpendiculars D
1
C
1
and D
2
C
2
which are
equal to R 1
4

?
?
?
?
?
?
cos
?
• By successive bisection of these chords, more
points may be obtained and by joining all C, C
1
,
C
2
, … points, curve is obtained.
T 2
D
C 2
D
2
C 1
C
T
1
D 1
3. By offsets from the tangents: If the deflection angle
and the radius of curvature are both small, the curves
can be set out by offsets from the tangent. The offsets
from tangent are of two types.
Part III_Unit 12_Chapter 05.indd 2 5/31/2017 4:56:04 PM
Page 3
Curves
Introduction
Curves are generally used on highways and railways where
it is necessary to change the direction of motion. This chap
ter focuses on the elements of curves and methods for set
ting out of curves on ground.
Curve may be circular, parabolic or spiral and is always
tangential to the two straight directions.
Circular curves are of three types:
1. Simple curves
2. Compound curves
3. Reverse curves
Simple Curves
Simple curves consists of a single arc of a circle. It is tan
gential to both the straight lines.
1. Back tangent (AT
1
): The tangent before curve is
called back tangent or fi rst tangent.
2. Forward tangent (BT
2
): The tangent following the
curve is called forward tangent or second tangent.
3. Point of intersection, vertex (V): It is the point of
intersection of two tangents AT
1
and BT
2
.
?
2
?
2
?
A
R
O
B
D
C
V '
(PI) V
(PT) T 2
(PC) T 1
R
4. Point of curve (T
1
): It is the beginning of the curve
where the alignment changes from a tangent to a
curve.
5. Point of tangency (T
2
): It is the end of the curve
where the alignment changes from a curve to a
tangent.
6. Defl ection angle or Deviation angle (?): The
diff erence between the slopes of the two tangents.
Curves and
Triangulation Survey
Part III_Unit 12_Chapter 05.indd 1 5/31/2017 4:56:03 PM
7. Tangent distance: It is the distance from PC to PI
(also the distance from PI to PT)
T
1
V = T
2
V = R tan
?
2
·
R is the radius of the curve.
8. External distance or apex distance (E): The
distance from vertex V to the centre of the curve C.
E = VC = R sec
?
2
1 
?
?
?
?
?
?
9. Length of curve (L): Distance from PC to PT,
L = T
1
CT
2
= R ? ? in radians.
10. Length of long chord: It is the chord joining PC to
PT.
TT R
12
2
2
= sin
?
11. Midordinate/versed sine: The distance from mid
point of the long chord to midpoint of the curve.
CD R =
?
?
?
?
?
?
1
2
cos
?
12. Normal chord: A chord between two successive
regular stations on a curve.
13. Subchord: Any chord shorter than the normal chord.
14. Righthand curve: If the curve deflects to the right
of the direction of the progress of survey, it is called
the righthand curve.
15. Lefthand curve: If the curve deflects to the left of
the direction of the progress of survey, it is called the
lefthand curve.
Chainages
Chainage of T
1
= Chainage of V – Tangent length
Chainage of T
2
= Chainage of T
1
+ Length of Curve
Degree of a Curve (D)
• The central angle subtended by a chord of a fixed length.
• In India, the fixed length of chord is taken as 20 m to cal
culate the degree of curve.
360 degrees = 2pR
D degrees = 20 m
? Therefore, R =
1146
D
metres
Methods of Setting out a Curve
Linear Methods
In this method, only a chain or tape is used. Linear methods
are used when:
• A high degree of accuracy is not required.
• The curve is short.
1. By ordinates or offsets from the long chord:
Midordinate,
OR R
L
R
0
2
2
= 
?
?
?
?
?
?
O
x
= RX RO
22 0
  ()
=
XK X
R
() 
2
(approx)
O
x
= Ordinate at distance of ‘X’ from central ordinate.
To set out the curve, the long chord is divided into an
even number of equal parts. Offsets calculated at each
point are then set out at each of these points.
2. By successive bisection of arcs or chords:
• Join tangent points T
1
, T
2
and bisect them at D.
Erect a perpendicular DC whose length is equal to
versed sine of the curve.
CD RR R
L
=
?
?
?
?
?
?
= 
?
?
?
?
?
?
1
22
2
2
cos
?
• Join T
1
C and T
2
C and bisect them at D
1
and D
2
.
Erect perpendiculars D
1
C
1
and D
2
C
2
which are
equal to R 1
4

?
?
?
?
?
?
cos
?
• By successive bisection of these chords, more
points may be obtained and by joining all C, C
1
,
C
2
, … points, curve is obtained.
T 2
D
C 2
D
2
C 1
C
T
1
D 1
3. By offsets from the tangents: If the deflection angle
and the radius of curvature are both small, the curves
can be set out by offsets from the tangent. The offsets
from tangent are of two types.
Part III_Unit 12_Chapter 05.indd 2 5/31/2017 4:56:04 PM
• Radial offsets
• Perpendicular offsets
E
O
R
Ox
T 1
D
D
x
A
D
T 1 D
V
E
R
O
R
Radial offset:
OR xR
x
=+ 
22
(Exact)
· ˜
x
R
2
2
(approx)
Perpendicular offset:
OR Rx
x
= 
22
(Exact)
· ˜
x
R
2
2
(approx)
4. By offsets produced from the chords (deflection
distances)
• This method is very much useful for long curves
and is generally used when a theodolite is not
available.
C
D
C
1
C
3
C
1
O
1
B
1
T
1
A
1
C
2
O
2
D
R
R
C
3
C
2
C
1
O
3
O
D
A
• Assuming C
1
, C
2
, …, C
n
subchord lengths and
calculating O
1
, O
2
, …, O
n
.
First offset,
O
C
R
1
1
2
2
=
Second offset,
O
C
R
2
2
2
2
=
O
CC C
R
n
nn n
=
+

()
1
2
O
1
, O
2
,… O
n
= 1st, 2nd, …, nth offset.
• Great disadvantage in this method is that the error
in fixing a point is carried forward.
Angular Methods
In this method, an instrument such as a theodolite is used
with or without a chain (or tape).
1. Rankine’s method of deflection (tangential) angles:
A deflection angle ( d) to any point on the curve is the
angle at PC between the back tangent and the chord
from the PC to that point.
• d (in minutes) =
1718 9 . C
R
C is the length of the chord.
R is the radius of the curve.
• One theodolite (to measure angles), one chain or
tape (to measure distances) are used in this method.
• The deflection angle for any chord is equal to the
deflection angle for the previous chord plus the
tangential angle for that chord.
?
1
= d
1
; ?
2
= ?
1
+ d
2
; ?
n
= ?
n–1
+ d
n
2. Two theodolite method:
• In this method, two theodolites are used one at PC
and the other at P .T.
• This method is used when the ground is unsuitable
for chaining.
• Only angular measurements are used (no chain or
tape).
• It is based on the principle that the angle between
the tangent and the chord is equal to the angle
which that chord subtends in the opposite segment.
3. Tacheometric method:
• Chaining is completely eliminated and the method
is less accurate than Rankine’ s.
• Setting of curve is done with stadia theodolite.
(a) Compound curves: A curve with two or more
simple curves turn in the same direction and
join at common tangent points.
(b) Reverse curves: Two curves turn in the oppo
site directions.
[The characteristics, length and sight distance
requirements of transition and vertical curves
are discussed in transportation]
Triangulation Survey
Introduction
The triangulation is the system which consists of a num
ber of interconnected triangles in which length of one line
and the angles of triangles are measured very precisely.
Part III_Unit 12_Chapter 05.indd 3 5/31/2017 4:56:05 PM
Page 4
Curves
Introduction
Curves are generally used on highways and railways where
it is necessary to change the direction of motion. This chap
ter focuses on the elements of curves and methods for set
ting out of curves on ground.
Curve may be circular, parabolic or spiral and is always
tangential to the two straight directions.
Circular curves are of three types:
1. Simple curves
2. Compound curves
3. Reverse curves
Simple Curves
Simple curves consists of a single arc of a circle. It is tan
gential to both the straight lines.
1. Back tangent (AT
1
): The tangent before curve is
called back tangent or fi rst tangent.
2. Forward tangent (BT
2
): The tangent following the
curve is called forward tangent or second tangent.
3. Point of intersection, vertex (V): It is the point of
intersection of two tangents AT
1
and BT
2
.
?
2
?
2
?
A
R
O
B
D
C
V '
(PI) V
(PT) T 2
(PC) T 1
R
4. Point of curve (T
1
): It is the beginning of the curve
where the alignment changes from a tangent to a
curve.
5. Point of tangency (T
2
): It is the end of the curve
where the alignment changes from a curve to a
tangent.
6. Defl ection angle or Deviation angle (?): The
diff erence between the slopes of the two tangents.
Curves and
Triangulation Survey
Part III_Unit 12_Chapter 05.indd 1 5/31/2017 4:56:03 PM
7. Tangent distance: It is the distance from PC to PI
(also the distance from PI to PT)
T
1
V = T
2
V = R tan
?
2
·
R is the radius of the curve.
8. External distance or apex distance (E): The
distance from vertex V to the centre of the curve C.
E = VC = R sec
?
2
1 
?
?
?
?
?
?
9. Length of curve (L): Distance from PC to PT,
L = T
1
CT
2
= R ? ? in radians.
10. Length of long chord: It is the chord joining PC to
PT.
TT R
12
2
2
= sin
?
11. Midordinate/versed sine: The distance from mid
point of the long chord to midpoint of the curve.
CD R =
?
?
?
?
?
?
1
2
cos
?
12. Normal chord: A chord between two successive
regular stations on a curve.
13. Subchord: Any chord shorter than the normal chord.
14. Righthand curve: If the curve deflects to the right
of the direction of the progress of survey, it is called
the righthand curve.
15. Lefthand curve: If the curve deflects to the left of
the direction of the progress of survey, it is called the
lefthand curve.
Chainages
Chainage of T
1
= Chainage of V – Tangent length
Chainage of T
2
= Chainage of T
1
+ Length of Curve
Degree of a Curve (D)
• The central angle subtended by a chord of a fixed length.
• In India, the fixed length of chord is taken as 20 m to cal
culate the degree of curve.
360 degrees = 2pR
D degrees = 20 m
? Therefore, R =
1146
D
metres
Methods of Setting out a Curve
Linear Methods
In this method, only a chain or tape is used. Linear methods
are used when:
• A high degree of accuracy is not required.
• The curve is short.
1. By ordinates or offsets from the long chord:
Midordinate,
OR R
L
R
0
2
2
= 
?
?
?
?
?
?
O
x
= RX RO
22 0
  ()
=
XK X
R
() 
2
(approx)
O
x
= Ordinate at distance of ‘X’ from central ordinate.
To set out the curve, the long chord is divided into an
even number of equal parts. Offsets calculated at each
point are then set out at each of these points.
2. By successive bisection of arcs or chords:
• Join tangent points T
1
, T
2
and bisect them at D.
Erect a perpendicular DC whose length is equal to
versed sine of the curve.
CD RR R
L
=
?
?
?
?
?
?
= 
?
?
?
?
?
?
1
22
2
2
cos
?
• Join T
1
C and T
2
C and bisect them at D
1
and D
2
.
Erect perpendiculars D
1
C
1
and D
2
C
2
which are
equal to R 1
4

?
?
?
?
?
?
cos
?
• By successive bisection of these chords, more
points may be obtained and by joining all C, C
1
,
C
2
, … points, curve is obtained.
T 2
D
C 2
D
2
C 1
C
T
1
D 1
3. By offsets from the tangents: If the deflection angle
and the radius of curvature are both small, the curves
can be set out by offsets from the tangent. The offsets
from tangent are of two types.
Part III_Unit 12_Chapter 05.indd 2 5/31/2017 4:56:04 PM
• Radial offsets
• Perpendicular offsets
E
O
R
Ox
T 1
D
D
x
A
D
T 1 D
V
E
R
O
R
Radial offset:
OR xR
x
=+ 
22
(Exact)
· ˜
x
R
2
2
(approx)
Perpendicular offset:
OR Rx
x
= 
22
(Exact)
· ˜
x
R
2
2
(approx)
4. By offsets produced from the chords (deflection
distances)
• This method is very much useful for long curves
and is generally used when a theodolite is not
available.
C
D
C
1
C
3
C
1
O
1
B
1
T
1
A
1
C
2
O
2
D
R
R
C
3
C
2
C
1
O
3
O
D
A
• Assuming C
1
, C
2
, …, C
n
subchord lengths and
calculating O
1
, O
2
, …, O
n
.
First offset,
O
C
R
1
1
2
2
=
Second offset,
O
C
R
2
2
2
2
=
O
CC C
R
n
nn n
=
+

()
1
2
O
1
, O
2
,… O
n
= 1st, 2nd, …, nth offset.
• Great disadvantage in this method is that the error
in fixing a point is carried forward.
Angular Methods
In this method, an instrument such as a theodolite is used
with or without a chain (or tape).
1. Rankine’s method of deflection (tangential) angles:
A deflection angle ( d) to any point on the curve is the
angle at PC between the back tangent and the chord
from the PC to that point.
• d (in minutes) =
1718 9 . C
R
C is the length of the chord.
R is the radius of the curve.
• One theodolite (to measure angles), one chain or
tape (to measure distances) are used in this method.
• The deflection angle for any chord is equal to the
deflection angle for the previous chord plus the
tangential angle for that chord.
?
1
= d
1
; ?
2
= ?
1
+ d
2
; ?
n
= ?
n–1
+ d
n
2. Two theodolite method:
• In this method, two theodolites are used one at PC
and the other at P .T.
• This method is used when the ground is unsuitable
for chaining.
• Only angular measurements are used (no chain or
tape).
• It is based on the principle that the angle between
the tangent and the chord is equal to the angle
which that chord subtends in the opposite segment.
3. Tacheometric method:
• Chaining is completely eliminated and the method
is less accurate than Rankine’ s.
• Setting of curve is done with stadia theodolite.
(a) Compound curves: A curve with two or more
simple curves turn in the same direction and
join at common tangent points.
(b) Reverse curves: Two curves turn in the oppo
site directions.
[The characteristics, length and sight distance
requirements of transition and vertical curves
are discussed in transportation]
Triangulation Survey
Introduction
The triangulation is the system which consists of a num
ber of interconnected triangles in which length of one line
and the angles of triangles are measured very precisely.
Part III_Unit 12_Chapter 05.indd 3 5/31/2017 4:56:05 PM
This chapter aims at the establishment of geodetic survey
using triangulation.
Geodetic Surveying
• To determine precisely the relative or absolute position
on the earths’ surface.
• The stations at which the astronomical observations
for azimuth and longitude are also made are known as
Laplace stations.
Objects of Geodetic Triangulation
1. To provide the most accurate system of horizontal
control points.
2. To assist in the determination of the size and shape of
the earth.
Classification of Triangulation System
(Based on Accuracy)
First Order or Primary Triangulation
This is of the highest order and is employed either to deter
mine the earths figure or to furnish the most precise control
points.
General specifications:
1. Average triangle closure < 1 second
2. Maximum triangle closure > / 3 seconds
3. Length of base line—5.5 km
4. Probable error in astronomic azimuth—0.5 seconds.
Second Order or Secondary Triangulation
The stations are fixed at close intervals so that the sizes of the
triangles formed are smaller than the primary triangulation.
General specifications:
1. Average triangle closure—3 seconds
2. Maximum triangle closure—8 seconds
3. Length of base line—1.5–5 km
4. Probable error in astronomic azimuth—2.0 seconds
ThirdOrder or Tertiary Triangulation
General specifications:
1. Average triangle closure—6 seconds
2. Maximum triangle closure—12 seconds
3. Length of base line—0.5–3 km
4. Probable error in astronomic azimuth—5 seconds
Triangulation Figures or Systems
1. Single chain of triangles: This figure is used where
a narrow strip of terrain is to be covered. Though
it is rapid and economical, it is not so accurate for
primary work since the number of conditions to be
fulfilled in the figure adjustment is relatively small.
2. Double chain of triangles: It is used to cover larger
area.
3. Centred figures: These are used to cover area and
give very satisfactory results in flat country. Centred
figures may be quadrilaterals, pentagons or hexagons
with central stations.
4. Quadrilaterals: Quadrilateral with four corner
stations and observed diagonal forms the best figures.
They are best suited for hilly country and most
accurate.
Criteria for Selection of the Figure
1. The figure should be such that the computations can
be done through two independent routes.
2. The figure should be such that at least one, and
preferably both routes should be wellconditioned.
3. All the lines in a figure should be of comparable
length. Very long lines should be avoided.
4. The figure should be such that least work may secure
maximum progress.
5. Complex figures should not involve more than about
twelve conditions.
• In very extensive survey, the primary triangula
tion laid in two series of chains usually in N–S
and E–W respectively is filled by secondary and
tertiary triangulation figures. This is known as the
grid iron system and is adopted for France, Spain,
Austria and India.
• In another system called central system, which
extends outward in all directions from base line
and covered by a network of primary triangulation
is adopted for United Kingdom.
Wellconditioned Triangle
• The shape of the triangle should be such that any error in
the measurement of angle shall have a minimum effect
upon the lengths of the calculated side. Such a triangle is
called wellconditioned triangle with base angles equal
to 56°14'. Form practical considerations, an equilateral
triangle is the most suitable. However triangle with an
angle < 30° and >120° should be avoided.
• The accuracy attained in each figure depends on
¦ The magnitude of the angles in each individual triangle
and
¦ The arrangement of triangles.
Strength of Figure
• The strength of figure is to be considered in triangulation
as the computations can be maintained within a desired
degree of precision.
• The geodetic survey has developed a very rapid and con
venient method of evaluating the strength of triangulation
figure and is based on an expression for the square of the
Part III_Unit 12_Chapter 05.indd 4 5/31/2017 4:56:05 PM
Page 5
Curves
Introduction
Curves are generally used on highways and railways where
it is necessary to change the direction of motion. This chap
ter focuses on the elements of curves and methods for set
ting out of curves on ground.
Curve may be circular, parabolic or spiral and is always
tangential to the two straight directions.
Circular curves are of three types:
1. Simple curves
2. Compound curves
3. Reverse curves
Simple Curves
Simple curves consists of a single arc of a circle. It is tan
gential to both the straight lines.
1. Back tangent (AT
1
): The tangent before curve is
called back tangent or fi rst tangent.
2. Forward tangent (BT
2
): The tangent following the
curve is called forward tangent or second tangent.
3. Point of intersection, vertex (V): It is the point of
intersection of two tangents AT
1
and BT
2
.
?
2
?
2
?
A
R
O
B
D
C
V '
(PI) V
(PT) T 2
(PC) T 1
R
4. Point of curve (T
1
): It is the beginning of the curve
where the alignment changes from a tangent to a
curve.
5. Point of tangency (T
2
): It is the end of the curve
where the alignment changes from a curve to a
tangent.
6. Defl ection angle or Deviation angle (?): The
diff erence between the slopes of the two tangents.
Curves and
Triangulation Survey
Part III_Unit 12_Chapter 05.indd 1 5/31/2017 4:56:03 PM
7. Tangent distance: It is the distance from PC to PI
(also the distance from PI to PT)
T
1
V = T
2
V = R tan
?
2
·
R is the radius of the curve.
8. External distance or apex distance (E): The
distance from vertex V to the centre of the curve C.
E = VC = R sec
?
2
1 
?
?
?
?
?
?
9. Length of curve (L): Distance from PC to PT,
L = T
1
CT
2
= R ? ? in radians.
10. Length of long chord: It is the chord joining PC to
PT.
TT R
12
2
2
= sin
?
11. Midordinate/versed sine: The distance from mid
point of the long chord to midpoint of the curve.
CD R =
?
?
?
?
?
?
1
2
cos
?
12. Normal chord: A chord between two successive
regular stations on a curve.
13. Subchord: Any chord shorter than the normal chord.
14. Righthand curve: If the curve deflects to the right
of the direction of the progress of survey, it is called
the righthand curve.
15. Lefthand curve: If the curve deflects to the left of
the direction of the progress of survey, it is called the
lefthand curve.
Chainages
Chainage of T
1
= Chainage of V – Tangent length
Chainage of T
2
= Chainage of T
1
+ Length of Curve
Degree of a Curve (D)
• The central angle subtended by a chord of a fixed length.
• In India, the fixed length of chord is taken as 20 m to cal
culate the degree of curve.
360 degrees = 2pR
D degrees = 20 m
? Therefore, R =
1146
D
metres
Methods of Setting out a Curve
Linear Methods
In this method, only a chain or tape is used. Linear methods
are used when:
• A high degree of accuracy is not required.
• The curve is short.
1. By ordinates or offsets from the long chord:
Midordinate,
OR R
L
R
0
2
2
= 
?
?
?
?
?
?
O
x
= RX RO
22 0
  ()
=
XK X
R
() 
2
(approx)
O
x
= Ordinate at distance of ‘X’ from central ordinate.
To set out the curve, the long chord is divided into an
even number of equal parts. Offsets calculated at each
point are then set out at each of these points.
2. By successive bisection of arcs or chords:
• Join tangent points T
1
, T
2
and bisect them at D.
Erect a perpendicular DC whose length is equal to
versed sine of the curve.
CD RR R
L
=
?
?
?
?
?
?
= 
?
?
?
?
?
?
1
22
2
2
cos
?
• Join T
1
C and T
2
C and bisect them at D
1
and D
2
.
Erect perpendiculars D
1
C
1
and D
2
C
2
which are
equal to R 1
4

?
?
?
?
?
?
cos
?
• By successive bisection of these chords, more
points may be obtained and by joining all C, C
1
,
C
2
, … points, curve is obtained.
T 2
D
C 2
D
2
C 1
C
T
1
D 1
3. By offsets from the tangents: If the deflection angle
and the radius of curvature are both small, the curves
can be set out by offsets from the tangent. The offsets
from tangent are of two types.
Part III_Unit 12_Chapter 05.indd 2 5/31/2017 4:56:04 PM
• Radial offsets
• Perpendicular offsets
E
O
R
Ox
T 1
D
D
x
A
D
T 1 D
V
E
R
O
R
Radial offset:
OR xR
x
=+ 
22
(Exact)
· ˜
x
R
2
2
(approx)
Perpendicular offset:
OR Rx
x
= 
22
(Exact)
· ˜
x
R
2
2
(approx)
4. By offsets produced from the chords (deflection
distances)
• This method is very much useful for long curves
and is generally used when a theodolite is not
available.
C
D
C
1
C
3
C
1
O
1
B
1
T
1
A
1
C
2
O
2
D
R
R
C
3
C
2
C
1
O
3
O
D
A
• Assuming C
1
, C
2
, …, C
n
subchord lengths and
calculating O
1
, O
2
, …, O
n
.
First offset,
O
C
R
1
1
2
2
=
Second offset,
O
C
R
2
2
2
2
=
O
CC C
R
n
nn n
=
+

()
1
2
O
1
, O
2
,… O
n
= 1st, 2nd, …, nth offset.
• Great disadvantage in this method is that the error
in fixing a point is carried forward.
Angular Methods
In this method, an instrument such as a theodolite is used
with or without a chain (or tape).
1. Rankine’s method of deflection (tangential) angles:
A deflection angle ( d) to any point on the curve is the
angle at PC between the back tangent and the chord
from the PC to that point.
• d (in minutes) =
1718 9 . C
R
C is the length of the chord.
R is the radius of the curve.
• One theodolite (to measure angles), one chain or
tape (to measure distances) are used in this method.
• The deflection angle for any chord is equal to the
deflection angle for the previous chord plus the
tangential angle for that chord.
?
1
= d
1
; ?
2
= ?
1
+ d
2
; ?
n
= ?
n–1
+ d
n
2. Two theodolite method:
• In this method, two theodolites are used one at PC
and the other at P .T.
• This method is used when the ground is unsuitable
for chaining.
• Only angular measurements are used (no chain or
tape).
• It is based on the principle that the angle between
the tangent and the chord is equal to the angle
which that chord subtends in the opposite segment.
3. Tacheometric method:
• Chaining is completely eliminated and the method
is less accurate than Rankine’ s.
• Setting of curve is done with stadia theodolite.
(a) Compound curves: A curve with two or more
simple curves turn in the same direction and
join at common tangent points.
(b) Reverse curves: Two curves turn in the oppo
site directions.
[The characteristics, length and sight distance
requirements of transition and vertical curves
are discussed in transportation]
Triangulation Survey
Introduction
The triangulation is the system which consists of a num
ber of interconnected triangles in which length of one line
and the angles of triangles are measured very precisely.
Part III_Unit 12_Chapter 05.indd 3 5/31/2017 4:56:05 PM
This chapter aims at the establishment of geodetic survey
using triangulation.
Geodetic Surveying
• To determine precisely the relative or absolute position
on the earths’ surface.
• The stations at which the astronomical observations
for azimuth and longitude are also made are known as
Laplace stations.
Objects of Geodetic Triangulation
1. To provide the most accurate system of horizontal
control points.
2. To assist in the determination of the size and shape of
the earth.
Classification of Triangulation System
(Based on Accuracy)
First Order or Primary Triangulation
This is of the highest order and is employed either to deter
mine the earths figure or to furnish the most precise control
points.
General specifications:
1. Average triangle closure < 1 second
2. Maximum triangle closure > / 3 seconds
3. Length of base line—5.5 km
4. Probable error in astronomic azimuth—0.5 seconds.
Second Order or Secondary Triangulation
The stations are fixed at close intervals so that the sizes of the
triangles formed are smaller than the primary triangulation.
General specifications:
1. Average triangle closure—3 seconds
2. Maximum triangle closure—8 seconds
3. Length of base line—1.5–5 km
4. Probable error in astronomic azimuth—2.0 seconds
ThirdOrder or Tertiary Triangulation
General specifications:
1. Average triangle closure—6 seconds
2. Maximum triangle closure—12 seconds
3. Length of base line—0.5–3 km
4. Probable error in astronomic azimuth—5 seconds
Triangulation Figures or Systems
1. Single chain of triangles: This figure is used where
a narrow strip of terrain is to be covered. Though
it is rapid and economical, it is not so accurate for
primary work since the number of conditions to be
fulfilled in the figure adjustment is relatively small.
2. Double chain of triangles: It is used to cover larger
area.
3. Centred figures: These are used to cover area and
give very satisfactory results in flat country. Centred
figures may be quadrilaterals, pentagons or hexagons
with central stations.
4. Quadrilaterals: Quadrilateral with four corner
stations and observed diagonal forms the best figures.
They are best suited for hilly country and most
accurate.
Criteria for Selection of the Figure
1. The figure should be such that the computations can
be done through two independent routes.
2. The figure should be such that at least one, and
preferably both routes should be wellconditioned.
3. All the lines in a figure should be of comparable
length. Very long lines should be avoided.
4. The figure should be such that least work may secure
maximum progress.
5. Complex figures should not involve more than about
twelve conditions.
• In very extensive survey, the primary triangula
tion laid in two series of chains usually in N–S
and E–W respectively is filled by secondary and
tertiary triangulation figures. This is known as the
grid iron system and is adopted for France, Spain,
Austria and India.
• In another system called central system, which
extends outward in all directions from base line
and covered by a network of primary triangulation
is adopted for United Kingdom.
Wellconditioned Triangle
• The shape of the triangle should be such that any error in
the measurement of angle shall have a minimum effect
upon the lengths of the calculated side. Such a triangle is
called wellconditioned triangle with base angles equal
to 56°14'. Form practical considerations, an equilateral
triangle is the most suitable. However triangle with an
angle < 30° and >120° should be avoided.
• The accuracy attained in each figure depends on
¦ The magnitude of the angles in each individual triangle
and
¦ The arrangement of triangles.
Strength of Figure
• The strength of figure is to be considered in triangulation
as the computations can be maintained within a desired
degree of precision.
• The geodetic survey has developed a very rapid and con
venient method of evaluating the strength of triangulation
figure and is based on an expression for the square of the
Part III_Unit 12_Chapter 05.indd 4 5/31/2017 4:56:05 PM
probable error (L
2
), that would occur in the sixth place of
the logarithm of any side,
Ld R
22
4
3
=
R =
DC
D
A AB A

++ ?
?
?
?
S dd dd
22
Where
d = Probable error of an observed Direction (in
seconds)
D = Number of directions observed (forward/back)
d
A
= Difference per second in he sixth place of loga
rithm of the sine of the distance angle A of each
triangle.
d
B
= Same as d
A
but for the distance angle B.
C = Number of angles and side conditions to be satis
fied in the net from the known line to the side in
equation.
C = (n' – s' + 1) + (n – 2s + 3)
n = Total number of lines
n' = Number of lines observed in both directions.
s = Total number of stations.
s' = Number of occupied stations.
(n' – s' + 1) = Number of angle conditions.
(n – 2s + 3) = Number of side conditions.
• The relative strength of figure can be computed in terms
of factor R.
• Lower the value of R, stronger the figure.
• Value of R computed for the strongest chain of trian
gles is called R
1
and that for the second strongest chain
R
2
. Generally, strength of a figure is almost equal to the
strength of the strongest chain. Therefore R
1
is a measure
of the strength of figure.
• For angles measured with the same precision, the strength
of figure depends upon:
¦ Number of directions observed.
¦ The number of geometrical conditions imposed by the
shape of the figures, together with the number of sta
tions occupied in the field.
¦ The size of distance angles used in computation.
Signals and Towers
Tower: A tower is a structure erected over a station for the
support of the instrument and observing party and is pro
vided when station or the signal or both are to be elevated.
• A signal is a device erected to define the exact position of
an observed station.
Nonluminous Signals
Diameter of signal in cm = 1.3D to 1.9D
Height of signal in cm = 13.3D
Where, D = distance (length of sight) for nonluminous
signals (km).
Luminous or Sun Signals
Used when length of sight distance > 30 kms.
The heliotrope and heliograph and special instruments
used as sun signals. The heliotrope consists of a plane mir
ror to reflect the sun’s rays and a line of sight to enable the
attendant to direct the reflected rays towards the observing
stations. Another form of heliotrope is ‘galton sun signal’.
Phase of Signals It is the error of bisection which arises,
when the signal is partly in light and partly in shade. The
observer needs only illuminated portion and bisects it. It
is thus apparent displacement of the signal. Thus the phase
correction is necessary.
S
(a)
(b)
90
o
– ( ) ß  a
2
1
a
(Plan)
S
1
A
90
o
ED
1
?
Signal
S
A
S
1
ß
90
o
( ) ß  a 
2
1
a
ß
2
?
F
E
B
a
90 – a
C
1. When observation is made on the bright portion,
Phase correction,
ß
a
=
r
D
cos
2
2
radians
Where
a = Angle which the direction of sun makes with
line of sight.
r = Radius of the signal.
D = Distance of sight.
2. When the observation is made on the bright line,
ß
a
=
r
D
cos
2
radians
The phase correction is applied algebraically to the
observed angle, according to the relative position of
the sun and the signal.
Total Station There are three methods of measuring dis
tance between any two given points.
Part III_Unit 12_Chapter 05.indd 5 5/31/2017 4:56:05 PM
Read More