Methods of Correcting Local Attraction
a. Method of Elimination by Included Angles:
- Draw a tentative figure from the observed data.
- Identify the line where the fore bearing and back bearing differ exactly by 180° (unaffected by local attraction).
- Compute the included angle at each station.
- Start from the unaffected line and correct the bearings of the successive sides. Move in an anticlockwise direction for interior angles and clockwise for exterior angles.
b. Method of Elimination by Applying Corrections to Bearings:
- Draw the tentative figure.
- Identify the unaffected line.
- Calculate the magnitude and direction of the local attraction error at each affected station.
- Correct the bearings starting from the unaffected line.
c. When No Line has a 180° Difference:
If no line has a 180° difference in fore and back bearings:
- Calculate the interior angles and check their sum against (n−2)×180°, where n is the number of sides.
- Distribute the error equally among all angles.
- Identify the line whose fore and back bearings differ least from 180° in the Whole Circle Bearing System or 0° in the Quadrantal Bearing System.
- Correct the bearing of that line by distributing half the error to each bearing.
- Calculate the correct bearings of other lines using the previous methods.