External Influences:

Perfect instruments and their perfect use might not make the measurements error free because of the following reasons:

(a) Magnetic storms, lunar perturbations, sunspots, or minor tremors in earth may cause irregular variations in bearing measurements.

(b) Secular, annual and/or diurnal differences in declination affect the bearing accuracy due to variation in magnetic meridian.

(c) The local attraction due to presence of iron ore in ground, or steel structures, electric lines etc. in the vicinity of survey stations.