Surveyor?s compass is not most convenient and portable form of magnetic compass. Prismatic compass can be used as a hand instrument or can be fitted on a tripod also.

Magnetic meridian through a point in direction shown by a freely floating and balanced magnetic needle free from all or attractive forces. The direction of magnetic meridian can be established with help of a magnetic compass.

The survey stations should be so selected that a good system of lines is obtained fulfilling following conditions such as lines must run through level ground as possible, as far as possible, main survey lines should not pass through obstacles.

Here angle ABC is 90°. From, ? BCD, CD = BC sec 60° = 100 × 2 = 200m.

The Diurnal variation or daily variation is systematic departure of declination from its mean value during a period of 24 hours. It generally varies with phase of sunspot period.

Magnetic declination at a place is horizontal angle between true meridian and magnetic meridian shown by needle at time of observation.

Distance perpendicular to chain = l(1 ? cos?)/s = 10 (1 ? cos 6°)/5. By simplifying we get 0.0109 cm.

In prismatic compass, graduations are in W.C.B system, having 0° to south end, 90° at west, 180° at north and 270° at east. The graduations are engraved inverted.

The irregular variations are due to what are known as magnetic storms, earthquakes and solar influences. They may occur at any time and cannot be predicted.

Edge bar type of needle is in surveyor?s compass. The needle is of broad needle type and needle doesn?t act as an index in case of a prismatic compass.