Compass Surveying – Prismatic Compass:

· Whenever a number of base lines are to be run for obtaining the details as in traversing, just linear measurements made by chain surveying will not be sufficient.

· The angles included between the adjacent lines should also be measured

· Compass is one of the instruments used to measure the angles. Prismatic Compass:

Description:

· A magnetic needle is balanced over a pivot in a circular box of 85 mm to 110 mm in diameter.

· A graduated aluminium ring is attached to the magnetic needle.

· An agate cap keeps the aluminium ring stable.

· The box is covered by a glass lid.

· Object vane and eye vane are provided at diametrically opposite ends.

· Eye vane caries a reflecting prism which can be raised or lowered as desired.

· A vertical horse hair or fine wire is provided at the middle of the object vane

· The graduations in the aluminium ring are made in the clockwise direction starting with 0 o at South and 180o at North with inverted markings.

· A triangular prism fitted below the eye slit enables magnification of readings to suit observer’s eye.

· Based on this prism arrangement, the compass is named prismatic compass.

· Compass is fixed over a tripod with ball and socket arrangement.

· A braked pin is provided below the object vane to damp the oscillations of the magnetic needle while taking readings.

Working Principle:

· The magnetic field aligns itself with the magnetic meridian (N-S direction)

· The line of sight is actually the line joining the object vane and eye vane

· The angle between the N-S direction and the line of sight is observed in the compass

· This angle is actually the angle between N-S direction and the line on the ground

· This angle made by the line with the N-S direction is called the bearing of the line.

· Compass is used to measure the bearing of the different lines from which the angles included between the adjacent lines are computed.

How to take reading using compass:

· The compass is centered over the station by dropping a small piece of stone from the centre of the bottom of the compass.

· A plumb bob is used for centering.

· The compass is levelled by adjusting the ball and socket till the top of the box is horizontal.

· The graduated ring should move freely after having levelled the instrument.

· Suppose the bearing of a line PQ is to be observed.

· The compass is centered over P.

· It is levelled.

· The prism and the object vane are kept in vertical position.

· The compass is turned slowly till the ranging rod already erected at Q is bisected.

· In this position, the ranging rod, the object and the eye vane all lie in the same line.

· The focusing prism is raised or lowered till the readings were clear and sharp.

· The reading in the ring cut by the object hair line is taken after damping the oscillations of the ring by pressing the brake pin.

Definitions:

Magnetic Bearing:

· It is the angle between the magnetic meridian and the line.

· The angle is always measured in the clockwise direction

· It is the direction shown by a freely suspended magnetic needle

· The magnetic meridian is also called bearing.

True Bearing:

· True bearing of a line is the angle between the true meridian and the line.

· The angle is always measured in the anticlockwise direction.

· The true meridian is the line joining the geographical north and south bearings.

Whole Circle Bearing:

· The bearing of lines measured from the North is called Whole Circle Bearing.

· The angle is reckoned in the clockwise direction from 0o coinciding with the north.

Quadrant Bearing:

· The whole circle is divided into four quadrants.

· The bearing is expressed with N or S as prefix and E or W as suffix.

· Quadrant Bearing is also known as Reduced Bearing.

Fore Bearing and Back bearing:

· Every line has two bearing namely fore bearing and back back bearing

· Fore bearing is the bearing taken in the direction of surveying and Back bearing is the bearing taken in the reverse direction.

· The difference between the fore bearing and the back bearing should be 180o .

· It means that one or both stations of the line are subjected to local attraction.

· Thus, local attraction is the influence caused on the measured bearings of lines due to the presence of materials like railway track, current carrying wires or cables, etc.,