Compass surveyingUNIT-II

Compass Surveying

In Compass survey chain or tape is used for linear measurements and compass is used for fixing direction.

In compass freely suspended magnetic needle directs to north- south and the bearing of line is obtained by line of sight.

Principles of Compass SurveyingWhen large area are involved, compass surveying is used. Traversing is that type of survey in which a number of

connecting survey lines form the frame work and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and a tape respectively.

When the lines form a circuit which ends at the starting point, it is known as a closed traverse.

If the circuit ends elsewhere, it is said to be an open traverse.

Types of Compass

The following are the instruments used fordirect measurement of directions: • Prismatic Compass• Surveyors compass

Instruments used for compass surveying:

The various instruments used in the compass survey are :

Prismatic compassTapeRanging rodsTripodArrows

Azimuth:

• Azimuth is defined as a horizontal angle measured clockwise from a north base line or meridian.

Bearing

• Bearing of a line is its direction relative to a given meridian.

• A meridian is any direction such as: i) True Meridianii) Magnetic Meridianiii) Arbitrary Meridian

• True Meridian:True meridian is defined as the plane that passes

through true north poles and true south poles at the place of observation.

• True Bearing:True bearing of a line is the horizontal angle which

it makes with the true meridian.• Magnetic Meridian:

Magnetic meridian through a point is the direction shown by a freely floating and balanced magnetic needle of a magnetic compass.

• Magnetic Bearing:The magnetic bearing of a line is the horizontal

angle which it makes with the magnetic meridian. The direction of magnetic meridian can be established with the help of a prismatic compass.

• Arbitrary Meridian:Arbitrary meridian is any convenient direction

towards a permanent and prominent mark or signal, such as a church spire or top of the chimney.

• Arbitrary Bearing:Arbitrary bearing of a line is the horizontal angle

which it makes with any arbitrary meridian. A theodolite is used to measure it.

Designation of Bearing:

• The common systems of bearings are:a) The whole circle bearing system(W.C.B.) or Azimuthal system.b) The Quadrantal bearing(Q.B.) system.

a) The Whole circle Bearing:

• In this system, the bearing of a line is measured with magnetic north in clockwise direction.

• The value of bearing varies from 0⁰ to 360⁰.

• The W.C.B. of• AB is θ1, • AC is θ2, • AD is θ3 and • AF is θ4.

b) The Quadrantal Bearing system:• In this system, the bearing of a line is measured

from north or south, whichever is nearer.• Here both North and South are used as

reference meridians and the directions can be either clockwise or anti-clockwise depending upon the position of the line.

• The Q.B. of a line varies from 0⁰ to 90⁰.• The bearing of this system is also known as

Reduced Bearings(R.B.)

• The Q.B. of a • line AB is α and is written

as N α E,• The bearing of AC is β

and is written as S β E,• AD and AF being S θ W

and N φ W.

Conversion of W.C.B. into R.B.LINE W.C.B. between Rule for R.B. Quadrant

AB 0⁰ and 90⁰ R.B. = W.C.B. NE

AC 90⁰ and 180⁰ R.B. = 180⁰ - W.C.B. SE

AD 180⁰ and 270⁰ R.B. = W.C.B. - 180⁰ SW

AF 270⁰ and 360⁰ R.B. = 360⁰ - W.C.B. NW

LINE R.B. Rule for R.B. W.C.B. between

AB N α E W.C.B.= R.B. 0⁰ and 90⁰

AC S β E W.C.B. = 180⁰ - R.B. 90⁰ and 180⁰

AD S θ W W.C.B. =180⁰ + R.B. 180⁰ and 270⁰

AF N φ W W.C.B.= 360⁰ - R.B. 270⁰ and 360⁰

Conversion of R.B. into W.C.B.

Adjustments of Prismatic Compass:

The following are the adjustments usually necessary in the prismatic compass:• Centering• Leveling• Focusing the prism.

CENTERING:The center of the compass is placed vertically over the station

point by dropping a small piece of stone below the center of the compass, it falls on the top of the peg marking that station.

LEVELLING:By means of ball and socket arrangement the Compass is then

leveled the graduated ring swings quite freely. It may be tested by rolling a round pencil on the compass box.

FOCUSSING THE PRISM :The prism attachment is slided up or down focusing till the

readings are seen to be sharp and clear.

Errors in Compass observations:The errors may be classified as

• Instrumental errors

• Personal errors

• Errors due to natural causes

Instrumental errors• They are those which rise due to the faulty

adjustments of the instruments. They may be due to the following reasons:– The needle not being perfectly straight.– Pivot being bent– Sluggish needle– Blunt pivot point– Improper balancing weight– Plane of sight not being vertical– Line of sight not passing through the center of

graduated ring

Personal errors

They may be due to the following reasons:Inaccurate leveling of the compass box.Inaccurate centering.Inaccurate bisection of signals.Carelessness in reading and recording.

Natural Errors:

• They may be due to following reasons:–Variation in declination– Local attraction due to proximity of local attraction

forces. –Magnetic changes in the atmosphere due to clouds

and storms.– Irregular variations due to magnetic storms etc.