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Fig. 2.11 Declination set off on a compass circle
The magnetic field is affected by metallic objects and direct-current electricity,
both of which cause a local attraction. As an example, when set up besides an old-
time street car with overhead power lines, the compass needle would swing toward
the car as it approached, then follow it until it was out of an effective range.
If the source of an artificial disturbance is fixed, all bearings from a given
station will be in error by the same amount. Angles calculated from bearings taken
at station will be correct, however. Local attraction is present if the forward and
back bearings of a line differ by more than the normal observation errors. Consider
the following compass bearings read on a series of lines:

0
AB…………………………N24 15’W
0
BA…………………………S24 10’E
0
CD…………………………N60 00’E
0
DC………………………….S61 15’W
Forward bearing AB and back bearing BA agree reasonably well, indicating
that little or no local attraction exists at A or B. However, the bearings at D differ
0
from corresponding bearings taken at C by roughly 1 15’ to the west of north.
Local attraction therefore exists at point D and deflects the compass needle
0
approximately 1 15’ to the west of north.
It is evident that to detect local attractions, successive stations on a compass
traverse have to be occupied and forward and back bearings read, even though the
directions of all lines could be determined by setting up an instrument only on
alternate stations.

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