What is Meridian and Bearing ? | Civil Easy Learning

What is Meridian and Bearing ?

Meridian:
Some reference direction based on which direction of line is measured

True meridian ( Constant)

Magnetic meridian ( Changing)

Arbitrary meridian  

Bearing: 

Horizontal angle between the meridian and one of the extremities of line

True bearing

Magnetic bearing

Arbitrary bearing

True meridian

Line passing through geographic north and south pole and observer’s position

Position is fixed

Established by astronomical observations

Used for large extent and accurate survey (land boundary)

Magnetic meridian

Line passing through the direction shown by freely suspended magnetic needle

Affected by many things i.e. magnetic substances

Position varies with time (why? not found yet)

Assumed (Arbitrary) meridian

Line passing through the direction towards some permanent point of reference

Used for survey of limited extent

Disadvantage

Meridian can’t be re-established if points lost.

True bearing:
The angle made by a survey line with reference to true
meridian is called true bearing. It is always remains constant.
Magnetic bearing:
The angle made by a survey line with reference to
magnetic meridian is called magnetic bearing. It changes from place to
place and time.
Arbitrary bearing:
 The angle made by a survey line with reference
to arbitrary meridian is called arbitrary bearing.

Representation of Bearing

Bearings are expressed in the following two systems.
1) Quadrantal (Reduced)  bearings system.
2) Whole circle bearings system.

 Reduced bearing:

In this system bearings of lines are measured with reference to north or south  which ever is closer to given survey line 
Here the measurement of angles can be done both in clockwise or anticlockwise direction 
When the whole circle bearing is converted to quadrantal bearing then it is called as reduced bearing 
Thus reduced bearing is similar to quadrantal bearing 
Here  the values lies between 0 to 90 degree's.
The system of measuring this bearing is known as Reduced Bearing System (RB System).

Whole circle bearing (Azimuth):

The bearing of a line measured with reference to magnetic meridian  in clockwise directions  here the angles lies between 0 to 360 degree's.
The quadrantal starts from north and progress in clockwise direction 
First quadrant is lies between  0 to 90 degree's.
Second quadrant is lies between  90 to 180 degree's.
Third quadrant is lies between  180 to 270 degree's.
Fourth quadrant is lies between  270 to 360 degree's.

The system of measuring this bearing is known as Whole Circle  Bearing System (WCB System)

MEASUREMENTS OF BEARINGS:

Conversion of Whole Circle bearings into Quadrantal Bearings

Conversion of Quadrantal bearings into  Whole Circle Bearings

Fore bearing and back bearing:

FORE BEARING (F.B)

The bearing of a line measured in forward direction of a survey line is called fore bearing  (F.B).

F.B = BACK BEARING +-1800

BACK  BEARING (B.B)

The bearing of a line measured in direction  opposite of a survey line is called back bearing (B.B)

 1. Convert the following whole circle bearings of lines to quadrental bearings.

a) OA 32

 b) OB 109

 c) OC 211

 d) OD 303

Solution:

Refer to fig: 

a) W.C.B of OA = 32 degrees
Quadrantal bearing = N 32 E
b) W.C.B of OB = 109
Quadrantal bearing = 180 – W.C.B
= 180-109 = S 710E
c) W.C.B of OC = 211
Quadrantal bearing = W.C.B – 180
 = 211– 180= S 31 W
d) W.C.B of OD = 303
Quadrantal bearing = 3600– W.C.B
= 360– 303 = N 570 W


2) Convert following reduced bearings to the whole circle bearings:
(i)N 52o 30’E (ii)S 30o 15’E (iii)S 85o 45’W (iv)N 15o 10’W

Solution:

(i) R.B. = N 52o30’E & which is in the NE quadrant,
Therefore W.C.B =same as R.B = 52o30’
(ii)S 30o 15’E which is in the SE quadrant,
Therefore W.C.B =180o-30o15’ = 149o45’
(iii)S 85o45’W which is in the SW quadrant,
Therefore W.C.B =180o+85o45’ = 265o45’
(iv)N 15o10’W which is in the NW quadrant,
Therefore W.C.B =360o - 15o10’ = 344o50’