How to measure the magnetic bearing of the object


How to measure the magnetic bearing of the object

Upendra Kumar

For the determining exact locations of the object in the map on the earth, surface uses a term by bearing. It is useful for surveying in surface and underground is also used for direction purpose of an object

Bering of a line. The horizontal angle which the line makes with some reference direction or meridian.

There are three types of a meridian.

  • True meridian
  • Magnetic meridian
  • Arbitrary meridian
Upper two are generally used in surveying.

Magnetic meridian. The direction indicated by freely suspended and properly balanced magnetic needle, unaffected by local attraction force is called magnetic meridian, and the horizontal angle made with the object or line is called magnetic bearing. It is expressed in degrees minutes and second.

True meridian. The true meridian or geographical meridian passing through a point on the earth surface is the line in which the plane passing through the given point, and the north and south poles intersects the surface of the earth.

Magnetic Declination. The horizontal angle which the magnetic meridian makes with the true or geographical meridian is known as the magnetic declination or the declination the needle.

        Magnetic Meridian= True meridian + - magnetic declination

Magnetic bearing measured with Compass, theodolite, and miner dail, the miner dial is used for the underground survey.

  • Observation of magnetic bearing with used of compass--- Compass is the instrument for use in measuring bearing from magnetic meridian. It is used in the surveying of wooded country, rough traverse, filling in details,  preliminary survey. It is less accurate than a theodolite.
  •      The compass consists of a magnetic needle, a graduated circle and line of sight.  
      Generally, prismatic compass in surveying, the advantage of the prismatic compass is it has 
       a reflecting prism for reading the reading.
  •      Observing Bearings--- (i) Centering: The centre of the compass placed over the station (A)by dropping a small piece of stone so that it falls on the top of the peg marking the station.
  • (ii) Levelling: The compass should be levelled by eye,  It should be clamped when levelled.
  • Turn up the prism and sighting vane in ventricle position. Raise or lower the prism until the graduation on the ring are clearly visible when looked through the prism.
  • Range the station( B) by the hair looked through the slit above the prism.
  • When the needle comes to rest, read the reading through the prism.

To observe a magnetic bearing of a line by theodolite.

1. Set up the instrument over A and level it accurately.
2. Set the verifier A to the zero of the circle. Unclamp the lower plate and release the magnetic needle. Rotate the instrument about its outer axis until the magnetic needle takes it's outer axis until the magnetic needle takes its normal position.
3. Loosen the upper plate. Turn the telescope and bisect the station B exactly by using the upper clamp and its tangent screw.
4. Read both verniers. The mean of the two readings gives the magnetic bearing of the line AB.

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