SURVEY FORE BEARINGS(FB) & BACK BEARINGS(BB)

Understanding forward and back bearings

Taking a forward bearing
Taking a forward bearing.

The first compass bearing we need to take at the lake is from our unknown location to our cabin on the far shore. The bearing we end up with represents the angle between a line from Magnetic North to our location, and a line from our location to the cabin. This bearing is a “forward bearing.”

But when we go to plot this bearing on our map, we can’t plot the angle from our current location since that is what we are trying to find. Instead we plot a bearing from the cabin, as if someone there took a bearing “looking BACK towards our location.” This would be called a “back bearing” and will be 180° different from our “forward bearing.”

How to determine a back bearing

One technique is to do the math. Add or subtract 180° from you forward bearing to get your back bearing. You want the result to fall between 0° and 360°, so if the forward bearing is less than 180°, add 180° to it, and if it’s greater than 180°, subtract 180°.

Calculating a back bearing
Calculating a back bearing.

If you are bad with math in your head, the “200/20” trick might help you. When your bearing is less than 180°, add 200° and then subtract 20° (same as adding 180°). When your bearing is greater than 180°, subtract 200° and then add 20° (same as subtracting 180°).

Back bearing as viewed through a sighting compass
Forward and back bearings as seen through a sighting compass.

There are also lots of tricks that avoid doing the arithmetic entirely…

Most sighting compasses show the back bearing in a smaller font size above or below the forward bearing.

When you are taking a bearing with a base plate compass, and you want a back bearing instead of a forward bearing, box the compass needle with the south end where the north end would usually be. The resulting bearing will be 180° different.

Compass taking a back bearing
Aligning North needle with South for a back bearing.

When you are plotting your bearing, start by plotting a short segment of the forward bearing. When you extend the line, extend it in the opposite direction.

Plotting a back bearing with a protractor
Plotting a back bearing with a protractor.

Next get straight in your head, where you will be plotting the bearing from on the map, and whether you will be plotting a forward bearing or a back bearing. You will always be starting to plot your bearing from the known location and then extending your line towards the unknown location.

If you took the bearing from the known location, you will be plotting a forward bearing, starting at the known location.

If you took the bearing from the unknown location, you will be plotting a back bearing, starting at the known location.

Measurement of a fore bearing and back bearing of a traverse



Theory


Bearing of a line is defined as the angle made by the line with zero reference direction and measured in clockwise direction.


Item required


1.      Prismatic compass


2.    Ranging rods


Diagram





Procedure


Fore bearing is a line bearing taken in forward direction. Back bearing is a line bearing taken in reverse direction.


1.      First the prismatic compass is set upto at A.


2.    All the temporary adjustment were performed at station A.


3.    Then from station A ranging rod at B is bisects.


4.    The angle a, is measured it is fore bearing of line AB.


5.     Then the prismatic compass is shifted to station B.


6.    Again all the temporary adjustment were performed.


7.     From station B ranging rods at station A was.


8.    The angle between line BA and north is measured. This is angle Q2it is the back bearing of line AB.

   Finally


Fore Bearing (F.B)

  • Fore bearing is the bearing(angle) of the line in the forward direction of surveying.

  • formula used to calculate the fore bearing of the progressive line         F.B = B.B ± 180° { + sign when B.B less than 180° and – sign when B.B more than 180°}.

Back Bearing (B.B)

  • Back bearing is the bearing(angle) of the line in the opposite direction of surveying.

  • formula used to calculate the back bearing of line                              B.B = F.B ± 180°     { + sign when F.B less than 180° and – sign when F.B more than 180° }.

If the difference of F.B and B.B is 180°  then consider no error in measurements.

Example:-


Fore bearing of line AB = 260° (given above) and included angles are also given.

Back bearing of line AB ( means BA) = 260°- 180°= 80°

Fore bearing of line BC = Bearing of line BA + angle B

                                         = 80° + 62° = 142° and so on calculate bearings for other lines.

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