Local Attraction
A compass needle is affected by the presence of masses of iron and steel
such as lamp posts electric cables, steel girders etc., they deflect the needle and
the effect of this disturbance is called local attraction. Due to local attraction, the
difference between the fore bearing and back bearing of a survey line will not be
equal to 180.
The observed bearings of lines affected by local attraction are
corrected by starting from the unaffected line and the correct bearings of the
successive lines are calculated.
Method of correction for traverse:
First method:
Sum of the interior angle should be equal to (2n-4) x 90. if not
than distribute the total error equally to all interior angles of the traverse. Then
starting from unaffected line the bearings of all the lines are corrected using
corrected interior angles.
Second method:
Unaffected line is first detected. Then, commencing from the
unaffected line, the bearing of other affected lines are corrected by finding the
amount of correction at each station
Example:
The following are the bearings of the lines of the closed traverse ABCDA
taken with a compass in a place where local attraction was suspected.
Correct the bearings of the lines for local attraction
Solution:
It is observed that the difference between the fore and back bearings of the
line AB is exactly 180
hence stations A and B are free from local attraction and
the bearings observed A and B are correct. The difference of FB and BB of
other lines is not 180.
Local attraction is present at those stations. The observed
F.B. of BC is also correct since B is unaffected by local attraction.
Observed F.B. of BC = 115 15’
Add = 180 0’
Correct BB of BC = 295 15’
Less Observed BB of BC = 294 15’
Error due to local attraction at C = 1 00’
Since the error is negative all bearing observed at C must be corrected by
adding 1 00’
Observed F.B. of CD = 180 45’
Add correction = 1 00’
Correct FB of CD = 181 45’
Deduct 180 = 180 00’
Correct BB of CD = 1 45’.
Observed BB of CD = 3 45’.
Error due to local attraction at D = 2 00’.
Hence all bearings observed at D must be corrected by -2 00’ for local attraction.
Observed F.B. of DA = 283 45’
Add correction at D = - 2 00’
Correct FB of DA = 281 45’
Less = 180 00’
Correct BB of DA = 101 45’
This is the same as the observed BB of DA which shows that there is no
local attraction at A.
The corrected bearings of the lines will be as follows:
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