Prismatic compass(Surveying)

Compass is an instrument which is used to measure the direction of a survey line with respect to magnetic north-south.

Prismatic compass and Surveyor’s compass is the important compass used to measure the direction of a survey line.

Prismatic compass is an instrument used to measure bearing with respect to the magnetic meridian.

The different components of the prismatic compass are the Magnetic needle, graduated ring, object and eye vane, prism.

The graduated ring is directly attached to the needle. The ring does not rotate along the line of sight. Graduations are of whole circle bearing (WCB) type.

 The graduations on the aluminium ring increase clockwise from 0° to 360˚ with the zero of the graduations coinciding with the south end of the needle.

The graduations having 0˚ at the south, 90˚ at west, 180˚ at north and 270˚ at west.

The graduations are engraved inverted on the aluminium ring.

The main advantage with prismatic compass is that sighting and recording the reading can be done simultaneously.

The magnetic needle of the prismatic compass is of broad type and the needle doesn’t act as an index.

The object vane consists of metal vane with a vertical hair whereas eye vane consists of a small metal vane with slit.

Prismatic compass is of portable type which can be used as a hand instrument without a tripod.

The compass is provided with a glass covering to protect aluminium ring and needle.

The triangular prism is fitted below the eye slit, having both horizontal and vertical faces convex so that the reading can be taken easily with more magnified.

The object vane can be folded on the glass lid when the prismatic compass is not in use. The prism can also be folded over the edge of the box.

PERNMENT ADJUSTMENT IN PRISMATIC COMPASS

• Adjustment of vanes
• Adjustment of needles
• Adjustment of a pivot
• Adjustment of level

Permanent adjustments are those adjustments which are done only when the fundamentals relations within the parts are disturbed.

Therefore they are not required to be repeated at every set up of the instrument.

TEMPORARY ADJUSTMENT IN   PRISMATIC COMPASS

• Centring
• Levelling
• Focusing the prism

These are the adjustments that are carried out at every set up of the instrument before taking the bearings.

Centring is the process of keeping the instrument exactly over the station. Plumb bob is used to judge the centring or in its absence, a small pebble or stone is used to judge by dropping it from the centre of the bottom of the instrument.

Levelling the prismatic compass should be placed in such a way that the graduated disc is swinging freely and appears to be level from the top edge of the case.

Focusing the prism, the prism is slided up or down for focusing till the reading on the ring appears to be sharp, correct and magnified.