COMPASS TRAVERSING

COMPASS TRAVERSING

TYPES OF COMPASS

  There are two types of compass:

1). Prismatic compass and

2). Surveyor’s compass.

PRISMATIC COMPASS. – In this compass, the readings are taken with the help of a prism. The following are essential parts of this compass.

(a). Compass Box. The compass box is a circular metallic box (the metal should be non-magnetic) of diameter 8 to 10 cm. A pivot with a sharp point is provided at the centre of the box.

(b). Magnetic Needle and Graduated Ring. The magnetic needle is made of a broad, magnetised iron bar. The bar is pointed at both end. The magnetic needle is attached to a graduated aluminium ring.

  The ring is graduated from 0⁰ to 360⁰ clockwise, and the graduations begin from the south end of the needle. Thus 0⁰ is marked at the south end, 90⁰ at the west, 180⁰ at the north and 270⁰ at the east. The degrees are again subdivided into half degree. The figures are written upside down. The arrangement of the needle and ring contains a agate cap pivoted at central pivot point. A rider of brass or silver coil is provided with the needle to counterbalance its dip.

(c). Sight Vane and Prism. The sight vane and the reflecting prism are fixed diametrically opposite to the box. The sight vane is hinged with the metal box and consist of a horsehair at the centre. 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 eye.

(d). Dark Glasses. Two dark glasses are provided with the prism. The red glass is meant for sighting illuminous objects at night and blue glass for reducing the stain on the observer’s eye in bright daylight.

(e). Adjustable Mirror. A is provided with the sight vane. The mirror can be lowered or raised and can also be inclined. If any object is too low or too high with respect to the line of sight, the mirror can be adjusted to observe it through reflection.

(f). Brake Pin. A brake pin is provided just at the base of sight vane. If presses gently, it stops the oscillations of the ring.

(g). Lifting Pin. A lifting pin is provided just at the below the sight vane. When the sight vane is folded, it presses lifting pin. The lifting pin then lifts the magnetic needle out of the pivot to prevent damage to the pivot head.

(h). Glass Cover. A glass cover is provided on the top of the box to protect the aluminium ring from (Fig. 1).

Fig. 1

THE SURVEYOR'S COMPASS. —The surveyor's compass is an instrument for determining the direction of a line with reference to the direction of a magnetic needle. The needle is balanced at its centre on a pivot so that it swings freely in a horizontal plane. The pivot is at the centre of a horizontal circle which is graduated to degrees and half-degrees and numbered from two opposite zero points each way to 90°. The zero points are marked with the letters N and S, and the 90° points are marked E and W. The circle is covered with a glass plate to protect the needle and the graduations, the part enclosed being known as the compass-box. A screw is provided for raising the needle from the pivot by means of a lever. The needle should always be raised when the compass is lifted or carried, to prevent dulling the pivot-point; a dull pivot-point is a fruitful source of error. Both the circle and the pivot are secured to a brass frame, on which are two vertical sights so placed that the plane through them also passes through the two zero points of the circle. This frame rests on a tripod

and is fastened to it by means of a ball-and-socket joint. On the frame are two spirit levels at right angles to each other, which afford a means of levelling the instrument. This ball-and-socket joint is connected with the frame by means of a spindle which allows the compass-head to be revolved in a horizontal plane, and to be clamped in any position.

True Meridian: The line passing through the geographical north pole, geographical south pole and any point surface of the earth, is known as the ‘true meridian’ or ‘geographical meridian’ (Fig. 2). The true meridian at a station is constant. The true meridian passing through different points of the earth’s surface are not parallel but converge towards the pole. But for surveys in small area, the true meridian passing through different points are assumed parallel.    

Magnetic Meridian: The magnetic needle possesses the property of pointing in a fixed direction, namely, the Magnetic Meridian (Fig. 2).

 The horizontal angle between the direction of this meridian and of any other

line may be determined by means of the graduated circle, and this angle is called the Magnetic Bearing of the line (Fig. 2), or simply its Bearing. By means of two such bearings the angle between two lines may be obtained. Bearings are reckoned from 0° to 90°, the 0° being either at the N or the S point and the 90° either at the E or the W point. The quadrant in which a bearing

falls is designated by the letters N.E., S.E., S.W., or N.W. For example, if a line makes an angle of 20° with the meridian and is in the southeast quadrant its bearing is written S 20° E.

Sometimes the bearing is reckoned in a similar manner from the geographical meridian, when it is called the true bearing (Fig. 2). In general, this will not be the same as the magnetic bearing. True bearings are often called azimuths and are commonly reckoned from the south point right-handed (clockwise) to 360°; i.e., a line running due West has an azimuth of 90°, a line due North an azimuth of 180°. Sometimes, however, the azimuth is reckoned from the north as in the case of the azimuth of the Pole-star.

Arbitrary Meridian: Sometimes for the survey of a small area, a convenient direction is assumed as a meridian, known as the ‘arbitrary meridian’. Sometimes the starting line of a survey is taken as the ‘arbitrary meridian’. The angle between arbitrary meridian and a line is known as the ‘arbitrary bearing’ of the line.

Grid Meridian: Sometime, for preparing a map assume several lines are parallel to those on adjoining sheets, although they are numbered differently. The vertical lines are not true north and south lines unless on one particular line which corresponds with a meridian that is roughly the central meridian of the country. The whole network forms what is called a grid and the northward direction of the vertical lines is called grid north. Grid bearings are bearings referred to the direction of grid north as given by the vertical lines. At any point the difference between the directions of true north and grid north is a small angle called the convergence, and this varies according to the position of the point east or west of the central meridian, being greater in magnitude the farther away the point is from this meridian.

Designation of magnetic bearing: Magnetic bearings are designated by two systems:

a). Whole-circle bearing (WCB), and

b). Quadrantal bearing (QB)

Whole-circle bearing (WCB): The bearing of an object is the angle between some fixed direction and the direction of the object. Thus, in (Fig. 3), if is the position of the observer

and OP the fixed direction from which bearings are reckoned, the

bearing of the point A is the (Fig. 3) angle POA.

  Usually bearings are reckoned clockwise from through 90⁰, 180⁰ and 270⁰ to 360⁰ from the fixed direction, so that in fig. 3 the angles marked α, β and θ are the bearings from to A, B and C respectively. Bearings reckoned in this way are called whole-circle bearings.

Quadrantal bearing (QB): The magnetic bearing of a line measured clockwise or counter clockwise from the North pole or South pole (whichever is nearer the line) towards the East or West is known as the ‘quadrantal bearing’ (Fig. 4).  

Reduced bearing (RB): In computing, and in work with the magnetic compass, it is often convenient to use what are called reduced bearings. A reduced bearing is the angle between the main vertical line marking the direction to which bearings are referred and the given line, measured 0⁰ from to 90⁰ the shortest way, east or west and north or south of the point, to that line. Thus, in fig. 5, in which the circle is divided into four quadrants

numbered I, II, III and IV, the reduced bearings of the lines OA,

OB, OC and OD are indicated by the Greek letters α, β, y and δ, and

are all reckoned the shortest way, east or west, from the line SN. If

the bearings are given on the whole-circle system, it can easily be seen

from the figure that we have the following rules for obtaining reduced

bearings:

    If the whole-circle bearing lies in the first quadrant, i.e. between and 90⁰, the reduced bearing is the same as the whole-circle bearing.

   If the whole-circle bearing lies in the second quadrant, i.e. between 90⁰ and 180⁰, the reduced bearing is 180⁰ minus whole-circle bearing.

   If the whole-circle bearing lies in the third quadrant, i.e. between 180⁰ and 270⁰, the reduced bearing is whole-circle bearing minus 180⁰.

  If the whole-circle bearing lies in the fourth quadrant, i.e. between 270⁰ and 360⁰, the reduced bearing is 360⁰ minus whole-circle bearing.

It should be noted that a reduced bearing never exceeds 90⁰ in value, and, when bearings are derived and expressed in the first place as whole-circle bearings, and reduced bearings are used only as a convenience in computing, a reduced bearing need take no account of the quadrant in which the line lies. If, however, it is desired to specify the quadrant in which a reduced bearing lies, this is done by putting the letter N or S before the figures giving the actual bearing, according as to whether the latter is

measured from the direction of north or south, and then inserting

after the figures the letters E or W to show whether the bearing lies east or west of the north and south line. Thus, the bearings a, j8, y and S in fig. 5 would be written as NαE, SβE, SyW and NδW respectively. Magnetic compasses are often graduated on the quadrantal system, with the letters, N, E, S and W marked on the card or rim, and accordingly magnetic bearings are commonly booked and expressed in terms of reduced bearings, with the proper distinguishing letters before and after them to specify the quadrant.

The rules given above are so simple that it is hardly worth while attempting to memorize them, as any given case can easily be worked out from first principles. As practice is gained, the computation becomes almost automatic without conscious effort. The importance of reduced bearings in computing lies in the fact that most mathematical tables only tabulate the values of the trigonometrical functions and their logarithms in terms of angles lying between and 90⁰. Accordingly, when whole-circle bearings are used, it is usually necessary to convert them into reduced bearings before entering the tables.

Fore bearing and Back bearing: In fig. 6, AB, is the reference direction from which bearings are reckoned. Then the bearing of the line AB is the angle marked a. At B draw BE' parallel to AB. At B bearings are reckoned clockwise from BE', and the bearing of the line BA is the angle marked α',

which, it will easily be seen, is 180⁰+α. If the direction AB is taken

as the forward direction of the line and the bearing in that direction

as the forward or fore bearing, the bearing in the back or reverse direction BA differs from the forward bearing by 180⁰, and is known as the back or reverse bearing of the line as viewed from station A. It will thus

be seen that the back bearing of AB at station A is the forward bearing

of BA at station B. By drawing diagrams for each case, the student can verify the following rules: 

 If fore bearing is in quadrants I or II, back bearing = fore bearing + 180.

If fore bearing is in quadrants III or IV, back bearing = fore bearing - 180.

These rules also follow from the rules for working out bearings from angles, because, since BA is the direction of AB turned clockwise through 180⁰, the bearing of BA can be obtained by adding the angle of reversal (180⁰) to the bearing of AB.

EXAMPLE

Fore Bearing  
     67⁰        131⁰     216⁰     348⁰        12⁰   +180⁰     +180⁰    -180⁰     -180⁰    +180⁰

   247⁰        311⁰      36⁰       151⁰      192⁰
Back Bearing……..     

(Next Post on “THE EARTH'S MAGNETISM. —Dip of the Needle”.)