CONTROL SURVEYING
TECHNIQUES
Three traditional surveying techniques
•Triangulation
•Trilateration
•Traverse
are in general use for determining the exact horizontal
positions of points on the earth's surface.
In recent years, modern technological developments have
added several new methods utilizing artificial earth satellites.
Triangulation
The most common type of geodetic survey is known as
triangulation.
It differs from the plane survey in that more accurate
instruments are used, instrumental errors are either removed or
predetermined so that they can be compensated for in the
computations.
The positions established by triangulation are mathematically
related to each other.
Triangulation
Triangulation consists of the measurement of the angles of a
series of joined or overlapping triangles. The principle of
triangulation is based on simple trigonometric procedures.
Vertices being the control points are called as triangulation
stations
If the distance along one side (base line) of a triangle and the
angles at each end of the side are accurately measured, the other
two sides and the remaining angle can be computed.
Triangulation
The measured side of the base triangle is called a base line.
Measurements are made as carefully and accurately as possible
with specially calibrated tapes.
The EDM’s, operating on electro-optical and electronic
principles respectively, have replaced the older methods of base
measurement in the recent surveys. The work can be completed
more rapidly and accurately than with tape.
A simple triangulation Net
Triangulation
Triangulation is extended over large areas by connecting and
extending series of triangles and forming a network or
triangulation system.
The network is adjusted in a manner which reduces the effect
of observational errors to a minimum.
A denser distribution of geodetic control is achieved in a
system by subdividing or filling in with other surveys.
Usefulness of Triangulation
Triangulation surveys are carried out,
To establish accurate control for plane and geodetic surveys
covering large areas,
To establish accurate control for photogrammetric surveys for
large area
To assist in the determination of the size and shape of the earth,
To determine accurate locations for setting out of civil
engineering works and topographic survey.
Trilateration
Only distances are measured in trilateration and each
side is measured repeatedly to insure precision.
The entire network is then adjusted to minimize the
effects of the observations errors.
The angles of the triangles are computed so the
geodetic positions are obtained as in triangulation.
Trilateration
A combined triangulation and trilateration system represents
the strongest network for creating horizontal control.
Classification of triangulation
system
Classification of triangulation system is based on;
• Length of baseline
• Length of side of triangles
• Maximum triangle closure
Classification of triangulation
system (1)
1) First Order or Primary Triangulation
-is of higher order
-involve the whole country
SPECIFICATIONS
-length of baseline 5 to 15km
-length of side of triangles 30 to 150km
-maximum triangle closure not more than 3sec.
Classification of triangulation
system (2)
2) Second Order or Secondary Triangulation
-number of points fixed within the Framework of
Primary triangulation
-triangle formed are small than primary triangulation
SPECIFICATIONS
-length of baseline 1.5 to 5km
-length of side of triangles 8 to 65km
-maximum triangle closure not more than 8sec.
Classification of triangulation
system (3)
3) Third Order or Tertiary Triangulation
-number of points fixed within the Framework of
secondary triangulation and form immediate control for
detailed engineering and other works
SPECIFICATIONS
-length of baseline 0.5 to 3km
-length of side of triangles 1.5 to 10km
-maximum triangle closure not more than 12sec.
Triangulation figure/system
a) Chain of triangle
• AB is a baseline
• All the angles of a triangle is observed
Chain of triangle (contn…..)
Theother length of the triangle sides in the
chain may be computed
Usedwhere a narrow strip of terrain is to be
covered
System not so accurate for primary work/higher
precision
Triangulation figure/system
b) Chain of polygon (centered figure)
• used to cover big area and give very satistifactory
results in a flat country
• Centered figure may be quadrilaterals, pentagon or
hexagons with central station
Chain of polygon (contn…..)
Systemprovides the desired checks on
computation
Progress of work is slow due to more settings
of instruments
Triangulation figure/system
c) Chain of Quadrilaterals
• Has four corner stations and observed diagonal – form best
figure
Chain of Quadrilaterals (contn…..)
Best suited for hilly country
System is the most accurate,
why?
computed lengths of the sides can be carried
through by different combinations of sides and angles.
Triangulation net accuracy
The accuracy of a triangulation net depends on not only the
methods and precision used in making observation but also the
shapes of figure in the net
The strength of figure
The system to measure the accuracy of shapes.
The strength of figure is a factor considered in establishing a
triangulation system to maintain the computations within a
desired degree of precision.
It plays an important role in deciding the layout of a
triangulation system
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 sides
The strength of figure-expression
24 2
L d R
3
where L2 = the square of the probable error that would occur in
the sixth place of the logarithm of any side,
d = the probable error of an observed direction in seconds of
arc,
R = a term which represents the shape of a figure
The strength of figure
D C
R
D
A A B B
2 2
D = the number of directions observed excluding the known
side of the figure,
D = 2(n – 1)
δA , δB , δC = the difference in the sixth place of logarithm of
the sine of the distance angles A, B, C, etc., respectively,
C = (n′ − S′ +1)+(n − 2S + 3)
The strength of figure
n′ = the total number of sides including the known side of the
figure,
n = the total number of sides observed in both directions
including the known side,
S′ = the number of stations occupied, and
S = the total number of stations.
NOTE: The smaller the number in “R” obtained, the more
strength of the figure
The strength of figure (contn…)
• From practical considerations, an equilateral triangle is the
most suitable
•In general, triangles having an angle small than 30 degree or
greater than 120 degree should be avoided, since the error
propagation increases in the side computations
