DEFINITIONS:-
1. BEARING OF THE LINE:-
The horizontal angle made by the survey line with some fixed reference
direction is bearing of line.
2. The fixed reference is known as Meridian
3. The meridians are
4. True meridian
5. Magnetic meridian
6. Arbitrary meridian
7. True Meridian:
It is the line passing through the point
and joining the North and South
poles of the earth.
8. True Bearing: It is he horizontal angle made by
the survey line with the true meridian
9. Magnetic Meridian: The direction shown by a freely
suspended well balanced Magnetic Needle at any point is termed as Magnetic
Meridian
10.
Magnetic Bearing: Horizontal angle made by the survey line with the magnetic meridian.
11.
Arbitrary Meridian: Any arbitrarily assumed direction chosen to measure the bearings is
termed as Arbitrary Meridian
12.
Arbitrary Bearing: The horizontal angle made by the survey line with the arbitrary
meridian
13.
Bearings
are designated by two systems
Whole Circle Bearing ( WCB )
Quadrantal Bearing or reduced bearings( QB or RB )
WHOLE CIRCLE BEARING (W.C.B.) :
The bearing of a line measured from
the North in clockwise direction.
The value of WCB varies from 00 to 3600
QUADRANTAL
BEARING OR REDUCED BEARING (Q.B. OR
R.B.)
The bearing of a Line observed either
from the North or South
Measured in Clockwise (or) Anticlockwise direction
Towards East or West
§ The value of QB ranges between 00
to 900
14. Declination: The horizontal angle subtended
between the true meridian and the magnetic meridian.
·
If
the Magnetic Meridian is towards East of True Meridian, the declination is East
·
If
the Magnetic Meridian is towards West of True Meridian, the declination is West
The relation between the true
bearing and magnetic bearing of a line can be written as
1.
True
Bearing = Magnetic Bearing ± Declination.
2.
For
East declination, +ve Sign is to be used where as –ve sign is used for West
declination.
3.
Magnetic
Bearing = True Bearing ± Declination
4.
For
East declination, –ve sign is to be used where as -ve sign is used for West
declination.
LOCAL ATTRACTION
1.
A
magnetic needle indicates the north direction when freely suspended or pivoted.
2.
Some
times the needle comes near some magnetic substances such as iron ore, steel
structures, electric cables conveying current etc.
3.
Then
it is found to be deflected from its true direction and does not show the
actual magnetic north.
4.
This
disturbing influence of magnetic substances is known as Local Attraction.
5.
If
all the lines of a traverse are found to be affected by local attraction
a.
The
line with minimum error is identified
b.
The
fore bearing and back bearing of this line are adjusted by distributing the
error equally
c.
Then
starting from this adjusted line, the fore and back bearings of other lines are
corrected
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