UNIT II - Syllabus

COMPASS AND PLANE TABLE SURVEYING

Compass – Basic principles - Types - Bearing -

Systems and conversions- Sources of errors -

Local attraction - Magnetic declination-Dip-

Traversing - Plotting - Adjustment of closing

error – applications - Plane table and its

accessories - Merits and demerits - Radiation -

Intersection - Resection – Traversing- sources of

errors – applications.

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2

3

MEASUREMENT OF

DIRECTIONS – COMPASS

SURVEYING

OBJECTIVES

1. Define and explain the term magnetic

bearing

2. Explain the construction of a prismatic

compass and its use.

3. Explain the difference between prismatic

and surveyor’s compasses.

4. Explain the method of traversing with a

compass and chain.

4

OBJECTIVES

5. Convert whole circle bearing to reduced bearings and vice versa and find internal angles from bearings.

6. Explain the methods used to plot and make adjustments to a traverse.

7. Explain the terms local attraction, magnetic declination an dip and precautions needed to eliminate errors.

8. Explain the adjustments, upkeep and maintenance of compass.

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6

AGENDA

Earth’s Magnetism

Bearings

Magnetic Compass

Conversion of Bearings

Angles and bearings

Local Attraction

Magnetic Declination

Plotting

Precision

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EARTH’S MAGNETISM

EARTH AS A MAGNET

• Earth acts like a huge magnet

• This is due to the iron core

• Earth’s magnet is so powerful

that it affects every magnetic

substance on the surface

• A freely suspended magnetic

needle thus takes up a position

along the earth’s magnetic

lines of force

• This principle is used in

compass surveying

Earth’s magnetism

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MAGNETIC POLES

Magnetic poles are different from

geographic poles

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MAGNETIC NEEDLES

Magnetic needles are of many types

i) Edge bar needle

ii) Broad form needle

The needles have a central point by which it

is supported on a pivot. The pivot is

provided with a hard agate tip to reduce

wear and tear.

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BASIC PRINCIPLE

1. A freely suspended magnetic needle lies along the magnetic lines of force.

2. The magnetic lines of force are horizontal at the equator.

3. The magnetic lines of force are inclined away from equator as they converge to the poles.

4. The inclination of the lines of force is the dip angle.

5. A magnetic needle giving the direction of magnetic lines of force is used in compass survey

Magnetic compass

12 BACK

Definitions

True meridian:

Line or plane passing through geographical

north pole ,geographical south pole and any

point on the surface of the earth is known as

the True meridian or geographical meridian.

The angle between the True meridian and a

line is known as True bearing of the line. It is

also called as Azimuth.

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Def: (contd..)

Magnetic meridian:

When the magnetic needle is

suspended freely and balanced

properly, unaffected by magnetic

substances, it indicates a direction.

This direction is known as magnetic

meridian. The angle between the

magnetic meridian and a line is known

as magnetic bearing or simple bearing

of the line.

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Contd...

Arbitrary meridian:

Convenient direction is assumed as a

meridian.

Grid meridian:

Sometimes for preparing a map some

state agencies assume several lines

parallel to the true meridian for a

particular zone these lines are termed

as grid meridian.

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• Designation of magnetic bearing

– Whole circle bearing (WCB)

– Quadrantal bearing (QB)

• WCB: The magnetic bearing of a line measured

clockwise from the North Pole towards the line is

known as WCB. Varies 0-360°

• Quadrantal Bearing: The magnetic bearing of a line measured clockwise or anticlockwise from NP or SP (whichever is nearer to the line) towards the east or west is known as QB. This system consists of 4-quadrants NE, SE, NW, SW. The values lie between 0-90°

– QB of OA = N a E

• Reduced Bearing: When the whole circle bearing of a line is converted to quadrantal bearing it is termed as reduced bearing.

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Contd..

• Magnetic declination:

The horizontal angle between the magnetic

meridian and true meridian is known as

magnetic declination.

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MAGNETIC BEARING

Bearing is the angle to a

line from any

reference direction

• When the reference

direction is the

magnetic north-south,

the angle is magnetic

bearing

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Reference direction

Line

B

A

θ

B

A

N

S

θ

MAGNETIC BEARING

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MAGNETIC BEARINGS

Whole circle bearing: Space is divided into

four quadrants by the North-South and

East-West directions.

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1st quadrant 2nd quadrant

3rd quadrant 4th quadrant

N

S

E W

MAGNETIC BEARINGS

Whole Circle bearing is measured from the

North direction, clockwise.

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North

θ θ

Line in 4th quadrant Line in 1st quadrant

North

WHOLE CIRCLE BEARINGS

Whole circle bearing can have a value from

0 to 360 degrees

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North

θ

North

θ

Line in 2nd quadrant Line in 3rd quadrant

REDUCED BEARINGS

Reduced bearing is the acute angle

measured from North or South directions.

It has a value from 0 to 90 degrees

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North

South

W E

REDUCED BEARINGS

Reduced bearings have to be designated by the direction from which it is measured, north or south and also the direction towards which it is measured.

Examples N θ E, N θ W, S θ E or S θ W.

Whole circle bearings are designated by the angle only while reduced bearings should have these directions mentioned as a part of the bearing.

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REDUCED BEARINGS

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REDUCED BEARINGS

Whole circle bearing

Reduced bearing

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N

S

E W

N

θ

S(180 – θ)E

DESIGNATION OF RB

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DESIGNATION OF RB

Reduced bearing

Whole circle bearing

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BACK

N

180+θ SθW

N

E

S

W

Fore bearing, Back bearing

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In WCB the difference between FB and BB should be exactly 180° BB=FB+/-180° Use the +ve sign when FB<180° Use the –ve sign when FB> 180°

Problem

• Convert the following WCBs to QBs

– (a) WCB of AB = 45°30’

(Ans 45°30’)

– (b) WCB of BC = 125°45’

(Ans 180- 125°45’ = 54° 15’)

• Fore bearing of the following lines are given. Find back

bearing

– AB=S 30°30’ E

– BC=N 40°30’ W

• The magnetic bearing of a line AB is 135°30’ what will be

the true bearing, if the declination is 5°15’ W. 31

Isogonic and Agonic lines:

Lines passing through points of equal declination

are known as Isogonic lines.

The lines passing through points of zero

declination is said to be Agonic lines.

Dip of the magnetic needle: If the needle is

perfectly balanced before magnetisation, it does

not remain in the balanced position after it is

magnetised. This is due to the magnetic

influence of the earth. The needle is found to be

inclined towards the pole. This inclination of

the needle with the horizontal is known as dip

of the magnetic needle.

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Local attraction:

The magnetic needle is affected by magnetic

substances such as iron ore , electric cables

carrying current it is found to be deflected from

the true direction. It does not show the actual

north. This is called local attraction.

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Traversing

• Open traverse

• Closed traverse

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METHODS OF TRAVERSING

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Chain traversing:(By Chain angle)

Compass traversing: Fore bearings and back

bearings between the traverse leg are measured

• Plane table traversing: Plane table is set at

every traverse station in clockwise and

anticlockwise direction and the circuit is finally

closed. During traversing the sides of the

traverse are plotted according to any suitable

scale.

• Theodolite traversing: Horizontal angles

between the traverse legs are measured. The

length of the traverse legs are measured by

chain/tape or by stadia method

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Checks on traverse: Open traverse

• Taking cut-off lines: measured the bearings and lengths

of cut off lines after plotting and tally with actual values.

• Taking an auxiliary point: Take P permanent point as

auxiliary point measured bearings and lengths of P from

each traverse point. If survey is accurate, while plotting

all the measured bearing of P should meet at P.

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Check – Closed traverse. • Check on closed traverse:

– Sum of the measured interior angles (2n-4) x 90°

– Sum of the measured exterior angles (2n+4) x 90 °

– The algebric sum of the deflection angles should be

equal to 360°. Right hand deflection is considered

+ve, left hand deflection –ve

• Check on linear measurement

– The lines should be measured once each on two

different days (along opposite directions). Both

measurement should tally.

– Linear measurement should also be taken by the

stadia method. The measurement by chaining and

stadia method should tally 39

PRISMATIC COMPASS

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PRISMATIC COMPASS

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PRISMATIC COMPASS

Main Parts:

1. Magnetic needle – broad form, symmetrical, with rider weight to remain horizontal

2. Graduated ring – aluminium, graduated to half minute, with zero on south end

3. Object vane – metal frame, hinged for folding, with a fine vertical thread or wire.

4. Prism and eye vane – for taking reading of the ring

5. Agate cap and pivot- on which the ring and needle move.

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PRISMATIC COMPASS

Main Parts …

6. Box and glass cover – The assembly set in a box and has glass cover on top.

7. Lifting lever- lifts the needle off the pivot to reduce wear and tear.

8. Brake Pin – to reduce oscillations of needle

9. Screw head – To screw the compass on to a tripod.

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READING THE PRISMATIC

COMPASS

The reading is taken from the prism end.

When the line of sight is in the direction of magnetic meridian, the reading, must be zero. The reading is done at the south end and hence the zero is marked there.

Prismatic compass gives whole circle bearing. The ring is marked with zero at south end and up to 360 degrees clockwise

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SURVEYOR’S COMPASS

Old form not commonly used.

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GRADUATIONS AND READING

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COMPARISON

Prismatic Compass

1. Broad needle

2. Ring moves with needle

3. Graduations 0 to 360 clockwise

4. Whole circle bearings

5. Numbering inverted

6. Eye vane and prism used to read

7. Reading taken at south end

8. Can be used hand-held

9. Sighting and reading simultaneous

Surveyor’s Compass

1. Edge-bar needle

2. Ring fixed to box

3. 0°at N and S to 90° at E and W in four quadrants; E and W interchanged.

4. Measures RB

5. Numbering erect

6. Eye vane not used for reading

7. Reading taken at north end

8. Has to be used with tripod

9. Object sighted first; then move around to take reading

47 BACK

INTERCONVERSION

Whole circle to reduced bearing

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INTERCONVERSION

Whole circle bearing (WCB), θ, to reduced

bearing (RB)

WCB 0 to 90 – RB N θ E

WCB 90 to 180 – RB S (180 – θ) E

WCB 180 to 270 – RB S (θ – 180) W

WCB 270 to 360 – RB N(360 – θ) W

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INTERCONVERSION

Reduced bearing to whole circle bearing

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INTERCONVERSION

Reduced bearing to Whole circle bearing

Reduced bearing is θ

When RB is N θ E, WCB is equal to θ.

When RB is S θ E, WCB is (180 – θ)

When RB is S θ W, WCB is (180 + θ)

When RB is N θ W, WCB is (360 – θ)

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BACK

Bearing

SθW

WCB = 180 + θ