Local Attraction:

The bearings measured by prismatic compass are magnetic bearings measured with reference to magnetic north of the earth. Apart from the fact that magnetic meridian changes from place to place on earth and with time of observation, external magnetic influence existing locally at a place can influence the readings seriously. The local presence of magnetic rocks, iron ore deposits, steel structures, railway lines, iron electric poles etc. can seriously deflect the magnetic needle of compass from its normal positions. Such disturbances in accuracy of measurements are termed local attraction. The actual measurements of bearings can also be disturbed if the surveyors, carelessly keeps bunch of iron keys, iron knives or buttons, steel framed spectacles. Even the chains and arrows, used in surveying, near the compass can also affect bearings. Proximity of such objects should be avoided as far as possible during compass surveying.

The existence of local attraction can be detected by recording the fore bearing and back bearing of the survey line from both end stations. Any difference between back bearing and fore bearing other than 180o will indicate the presence of local attraction if no instrumental and observational error is involved in the measurement process.