LECTURE NOTES -1
3.1 Purpose and Principle of Compass Surveying
Compass Surveying
The location of a point concerning two predefined points can
be expressed in other terms other than distance alone. In such cases, the direction
of the point is used for locating a point by any of the following methods:
1.
By measuring its distance from one of the given points
and its direction from the other point.
2.
By measuring its distance and direction from any
of the two known points.
3.
By measuring its direction from each of the two
known points.
The
direction of the survey lines is measured with the help of an instrument known
as Compass. The purpose of compass surveying is to determine the
relative position of a point using direction.
Principle
of Compass Surveying
The
principle of compass surveying is Traversing. Traversing consists of several
connected lines whose lengths are measured by chain or tape and angle is
measured using Compass.
There
are two types of traversing. Open traversing and closed traversing. If the
starting and endpoint of the framework coincide then it is said to be Closed
Traversing. If the starting and ending points of the framework do not coincide
it is said to be Open Traversing.
3.2
Prismatic Compass
The prismatic compass
reads the line of an object in the whole circle bearing system directly. The
parts of a prismatic compass are as follows.
1. non-metallic box
2.
Graduated ring
3.
Pivot
4.
Magnetic needle
5.
Glass cover
6.
Eye vane
7.
Eye hole
8.
Sunglasses
9.
Prism
10.
Prism cap
11.
Object vane
12.
Horsehair
13.
Adjustable mirror
14.
Lifting pin
15.
Lifting lever
1. Metal
box: The cylindrical
box with a size of 8 to 12 cm covers the compass and then the whole casting or
body of the compass.
2. Graduated ring: It is an aluminium ring labelled 0o
to 360o to calculate all potential line bearings and connected with
a magnetic needle.
3. Magnetic needle: The magnetic needle points the magnetic
north.
4. Pivot: The graduated ring and the magnetic needle rests on
the pivot.
5. Glass cover: It covers the top of the metal box so that the
needle and the graduated ring can be observed from the top.
6. Eye
vane: All observations are noted from the eye vane.
7. Eye hole: readings are noted from prism through
the eye hole.
8. Prism:
Prism is used to read graduation on the ring
and to take an exact reading by compass. It is placed exactly opposite to
object vane. The prism hole is protected by a prism cap to protect it
from dust and moisture.
9. Object vane: The object vane carries a vertical hair of fine
silk or horse hair attached to a suitable frame.
10. Sliding mirror: The object vane is provided with a hinged
mirror that can be raised upwards or lowered downwards and can slide thus
changing its position if required. This is used to sight objects that are too
high or too low.
11. Sunglasses: Sunglasses are provided on the prism to sight
luminous objects.
12. Braking pin: it is provided at the base of the object vane to
dampen the oscillations of the needle to facilitate the reading.
3.3 Procedure
of taking observations with Prismatic Compass
1.
The prismatic compass is mounted on the tripod
such that it is exactly over the ground station.
2.
A small stone is dropped from the bottom centre
of the compass. If it falls on the peg driven on the ground, the centre point
is correct.
3.
The surveyor has to judge whether the compass is
in Level or not by sighting only. If not, it is levelled by ball and socket arrangement.
4.
Turn up the prism and the sighting vane
vertically. Raise or lower the prism until the gradations on the ring are
clearly visible.
5.
Then turn the compass box until the ranging rod
at the station is bisected by the hair when sighted through the slit above the
prism.
6.
When the needle comes to rest look through the
prism and note the reading at which the hairline produced appears to cut image
of the graduated ring. Thus, the required bearing of the line is found.
