Upendra Kumar

                                                      WHAT IS COMPASS

Compass is an instrument that is used for measuring the magnetic bearing of a line from the magnetic north. It is also used for navigation purposes. 

The component of the compass:-(i.) Magnetic Needle (ii.) A graduated Circle (iii.) A line of sight.
The compass does not measure the angle between the lines.

                                                           TYPES OF COMPASS

There are two types of compass
1. Prismatic Compas
2.Surveying Compass

1. Prismatic Compass:- The prismatic compass consists of
I. Compass Box:-      A circular box about 85 to 110 mm in diameter. In the center of the circular box has a hard steel pivot for blanching the magnetic needle. 


ii. Magnetic Needle and Graduated Ring:-The magnetic needle made of a broad magnetized iron bar and attached with a graduated aluminum ring. The ring is graduated 0º to 360º in the clockwise direction. The 0º is beginning south of the needle and west is marked 90º and north is marked 180º and east is marked 270º. Each degree is subdivided into half a degree and written in upside down. The arrangement of the needle and ring contains an agate cap pivoted on the central pivoted point. A ridder of brass or silver coil is provided with a needle to counterbalance of the dip.
iii. Sight vane and Prism:-Sight vane and Prism are fixed opposite diametrically to the box. The sight vane hinged in the metal box and horsehair fixed at the center. The prism consists of a sighting slit at the top and two small circular holes, one at the bottom of the prism and other at the side of the observer's eyes.
iv.Dark Glasses:- There are two dark glasses provided with the prism.The red glass for sighting luminous objects at night and blue glasses for reducing the strain on the observer's eyes in bright daylight.
v. Adjustable Mirror: -A mirror is provided with the sight vane. The mirror can be lowered or raised and can be inclined. If any object is too low or high too with respect to the line of sight, The mirror can be adjusted to observe through reflection.
vi. Brake Pin:- A brake pin is provided at the base of the sight vane. If pressed gently. It stopped oscillation of the ring.
vii. Lifting pin:- A lifting is provided just below the sight vane. When the sight vane is folded, it presses the lifting pin and then lifts the magnetic needle out of the pivot point to prevent damage to the pivot head.
viii. Head Cover:- A glass cover is provided on top of the box to protect the aluminum ring from the ring.
                                              USE OF PRISMATIC COMPASS
The prismatic compass is used for survey in wooded country rough traverse, filling in details, preliminary survey for a road military purposes such as sketching and night marching, etc. It is unreliable in places abounding in magnetic rock or iron ore. It is less accurate than a theodolite.

2. Surveyor compass:- The following point makes the difference between the surveyor and prismatic compass.
i. In the surveyor compass, no prism on it, The reading is read by the naked eye.
ii. It consists of an eye vane with a fine sight slit.
iii. The graduate aluminum ring attached with a circular box.
iv. The magnetic needle moves freely over the pivot. The needle shows the reading on the graduated ring.
v. The ring graduated from 0º to 90º into four quadrants. The 0º is marked in north and south direction and 90º is marked in the east and west direction.
vi. No mirror is attached to the object vane.
A compass survey is the process of surveying in which the length of the line measured with a chain or tape and direction of the lines measured with a compass.

                                    METHOD OF USING PRISMATIC COMPASS

Centering:-The compass may be held in the hand but for better results, It is usually mounted on a light striped which carries a vertical spindle in a ball and socket joint to which the box is screwed. By means of this arrangement, the instrument can be quickly leveled and also rotated in a horizontal plane and clamped in any position.
Leveling:-The compass should then be leveled by eye, by means of a ball and socket joints so that the ring may swing quite freely. It should be clamped when leveled.

To observe the bearing of a line AB
i. Center the compass over the station A and level it.
ii. Having turned up vertical the prism and the sighting vane, raise or over the prism until the graduation on the ring is clearly visible when looked through the prism.
iii. Turn the compass box until the ranging rod at the station B bisected by the hair when looked through the slit above the prism.
iv. When the needle comes to rest bring the ring to rest by pressing the knob. look through the prism and note the reading at which the hairline produced appears to cut the image of the graduated ring. Which gives the required bearing of the line AB. Reading is usually estimated to the nearest 15'.

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