How to use prismatic compass in surveying?

• A prismatic compass is a simple form of the magnetic compass.
• It is portable, can be held in hand or in a tripod stand during survey.
• Sighting and reading can be done simultaneously without changing the position of the eye.
• The readings measured in clockwise direction, i.e., from the south (0 degrees) to West (90 degrees) and North (180 degrees) and East (270 degrees).

Instruments Required for Prismatic Compass Survey

• Prismatic compass with tripod stand.
• Measuring Tape
• Arrows
• Ranging Rods
• Plumbing Fork or Plumb Bob
• Field Book

Temporary Adjustments for Prismatic Compass

1. Centering
2. Levelling
3. Focusing
4. Centering: The instrument is kept exactly over the station point. It can be done either by a plumb-bob adjusting the tripod stands.
5. Levelling: The adjustment of the instrument in such a way that the graduated disc rotates freely and when it is viewed from the side of the top edge it must appear level.
6. Focusing: The adjustment by sliding up and down of the prism till the readings are observed sharp and clear.

Working of the Prismatic Compass

• Prismatic compass is usually mounted on a light tripod stand.
• When the needle of the compass is suspended freely towards the north.
• All the angles measured with prismatic compass with respect to north (magnetic meridian).
• Measure the bearing of a survey line with reference to magnetic meridian in clockwise direction in horizontal plan.