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:
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”.)
