CONTROL SURVEYING
LECTURE –27(RESECTION & INTERSECTION)
BY: Er. Rabindra Shah
rabindrasah556@gmail.com
Department of Geomatics Engineering
JAY GOVIND SAH POLYTECHNIC INSTITUTE,
Golbazar-4, Siraha
Intersection
Intersection is the method of locating a point by sighting it from two
or more known control points.
This method is suitable for the case where there is difficult to occupy
the point to be located.
There are different approaches to compute the coordinates of the
intersection point
Method 1: Bearing Method
Method 2: Angle Method
Method 1: Bearing Method
Let point P be located by sighting from
known points A and B
The bearings of lines AP and BP are
computed first by observing PAB and
PBA
Computing the bearing of line AB with
known coordinates of points A and B
Then coordinate of P is calculated as:
Note that P is always assumed to be right of
Exercise
Find the coordinates of a point P, which was observed from two
known points A and B whose coordinate are as follows:
Station Easting Northing
A 1000m 1000m
B 500m 1200m
And Bearings of lines AP and BP are 650 0
respectively.
Method 2: Angle Method
This method involves the coordinates
of point P using the observed angles
and
Coordinate of P is computed as:
A P
A
Exercise
P
B
Find the coordinates of P, which was observed from known points A
and B whose coordinates are as follow:
Station Easting(m) Northing(m)
A 345780.67 3050394.05
B 351629.10 3044899.07
And angle BAP ( ) and ABP( ) are 53.359660g and 35.150278g
respectively.
Suitability:
This method is very commonly used for plotting details.
It is preferred when the distance between the stations is too large, or
the stations are inaccessible, or the ground is undulating. The most
suitable example is of broken boundaries which can be very
conveniently plotted by this method.
Resection
Resection is the method of determining the position of the point
occupied by the instrument by taking measurement of horizontal
angles between at least three known control points
Considered as a weaker solution than other methods. So, it is
preferred to take horizontal angles measurement at more than three
control points so that more checks and adjustment can be applied.
Very quick method to fix the position
Reduce fieldwork as it needs only single instrument setup
Teinstra Method
Let ABC be the known control points
in clockwise manner
and are the angle between the
directions PB and PC, PC and PA,
and PB and PA respectively in
clockwise direction.
An anticlockwise convention
throughout is also permissible but
random is not.
Teinstra Method
Where,
Compute the coordinates of the station
Compute the coordinates of the
station G using Teinstra Method as
if it were resected from the stations
B(356788.67, 3044328.27)
C(356442.71, 3048778.96) and
D(350044.25, 3050752.70)
g
g
g
g
g
m=248.77
83
n=211.993
l=264.3244
643.4708
Suitability:
This method is employed when during surveying the surveyor feels
that some important details can be plotted easily by choosing any
station other than the triangulation stations. The position of such a
station is fixed on the drawing sheet by resection.
Danger Circle
The triangle formed by joining the
three control points from which
resection is to be fixed is called great
triangle and the circumscribing this
great triangle is called great circle
If the desired point of resection lies on
or near the circumference of the great
circle then the circle is known as
danger circle
Danger Circle
The circle is danger in the sense that
the strength of fixing goes on
decreasing as the point lies nearer the
circumference
It is recommended that the resection
point should not lie on or periphery of
circumference by (1/3)rd of the radius
of the great or danger circle
Precaution to be taken for resection
The strength of fix depends upon the relative position of the known points
and resection point. In order to get strong fix following precaution should
be taken:
When the occupied station is inside the great triangle, the station should
be selected near the centroid of the great triangle. Strength of fix is
maximum at centroid of great triangle
Occupied should never be selected near the circumference of the great
circle. It reduces the strength of fix. It should not be near the periphery of
circumference by (1/3)rd of the radius of the great or danger circle
Occupied station should never be selected on the circumference of the
great circle, because at the circumference the position of P is
indeterminate
Observation and computation should be carried out very carefully to avoid
possible errors and mistakes
Advantages of Resection
It needs less field work because only a single setup can fix the
position of the occupied station
Measurement of distance is not needed that also further reduces the
volume of field work and lance the cost of project
Position of a point at any desired place can be established without
visiting the known control points, so it is quickest method for
establishing a control
No need of figural adjustment, only adjustment for horizontal
misclosure is needed
Disadvantages of resection
As only a single station is occupied it provides a weaker solution than
intersection
Visibility of a least three known control is needed which might not be
possible from the desired location
Difficult to judge whether the occupied station lies on the
circumference of the great circle or not
The computation is cumbersome as it needs lengthy formula
Advantages of Intersection
It provides a stronger solution than resection
We can locate inaccessible points
Visibility from two known stations are enough to find unknown point
Shorter formula than resection
Disadvantages of Intersection
Needs setup in at least 2 known stations
It is slower method than resection
Costlier than resection
Field Operation
Planning
Recconnaissance
Monumentation
Signalling
Observation
Recording
Computation and adjustment
Thank You
