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Fore Bearing and Back Bearing | Surveying

Bearings measured in the direction of progress of the survey are known as fore bearing and bearings measured opposite to the direction of the survey are known as back bearing.


The bearing of a line is the direction with respect to a given meridian; Meridian is a fixed reference line. While setting out a survey line, the bearing readings are necessary. ​

Simply,

  • Fore Bearing - Bearing measured in the direction of progress of the survey

  • Back Bearing - Bearing measured opposite to the direction of survey

Fore bearing and Back bearing of Survey Line


Hope you are pretty familiar with bearing, types of bearing and some of the related terms.


In the figure below, OA is the survey line to be set out.

  • Fore Bearing (FB) of OA = θ1

  • Back Bearing (BB) of OA = θ2

Fore bearing and back bearing

Relation Between Fore Bearing and Back Bearing


We will have two cases here:

  • Case 1(Fore Bearing is less than 180 degrees)

  • Case 2(Fore Bearing is more than 180 degrees)

Case 1(Fore Bearing is less than 180 degrees)


Let OA be the survey line.


According to the definition of fore and back bearing, in the figure:

θ1 is the fore bearing of OA

ϕ1 is the back bearing of OA

Fore Bearing is less than 180 degrees

Using fundamental geometry, we can write-

ϕ1 = θ1 + 180 degrees

So, Back bearing = Fore bearing + 180 degrees


Case 2(Fore Bearing is more than 180 degrees)


Let OB be the survey line.


According to the definition of fore and back bearing, in the figure:

θ1 is the fore bearing of OB

ϕ1 is the back bearing of OB

Fore Bearing is more than 180 degrees

Using fundamental geometry, we can write-

θ1 = ϕ1 + 180 degrees

So, Fore bearing = Back bearing + 180 degrees


This is all about the concept of fore and back bearing. The next thing we are going to see, how we can convert an angle to bearing and vice versa.


Finding Internal Angles from Bearing


Let us take AB and BC are two survey lines, and we are asked to find out the internal angle ABC.

Finding internal angles from bearing

θ1 is the fore bearing of line AB and θ2 is the fore bearing of line BC.

Using Geometry,


The internal angle ABC = Back