Compass

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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.

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OBJECTIVES5. 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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AGENDAEarth’s MagnetismBearings Magnetic CompassConversion of Bearings Angles and bearingsLocal AttractionMagnetic DeclinationPlottingPrecision

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EARTH’S MAGNETISM

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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 typesi) Edge bar needleii) Broad form needleThe 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 PRINCIPLE1. 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

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MAGNETIC BEARINGBearing is the angle to a

line from any reference direction

• When the reference direction is the magnetic north-south, the angle is magnetic bearing

Reference direction

Line

B

A

θB

A

N

S

θ

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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.

1st quadrant2nd quadrant

3rd quadrant 4th quadrant

N

S

EW

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MAGNETIC BEARINGS

Whole Circle bearing is measured from the North direction, clockwise.

North

θ θ

Line in 4th quadrantLine in 1st quadrant

North

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WHOLE CIRCLE BEARINGS

Whole circle bearing can have a value from 0 to 360 degrees

North

North

θθ

Line in 2nd quadrantLine in 3rd quadrant

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REDUCED BEARINGS

Reduced bearing is the acute angle measured from North or South directions. It has a value from 0 to 90 degrees

North

South

W E

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REDUCED BEARINGSReduced 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 BEARINGSWhole circle bearing Reduced bearing

NN

S(180 – θ)E

S

EWθ

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DESIGNATION OF RB

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DESIGNATION OF RBReduced bearing Whole circle bearing

N

180+θSθW

N

E

S

W

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PRISMATIC COMPASS

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PRISMATIC COMPASS

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PRISMATIC COMPASSMain Parts:1. Magnetic needle – broad form, symmetrical,

with rider weight to remain horizontal2. Graduated ring – aluminium, graduated to half

minute, with zero on south end3. Object vane – metal frame, hinged for folding,

with a fine vertical thread or wire.4. Prism and eye vane – for taking reading of the

ring5. Agate cap and pivot- on which the ring and

needle move.

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PRISMATIC COMPASSMain 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

needle9. 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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COMPARISONPrismatic Compass1. Broad needle2. Ring moves with needle3. Graduations 0 to 360

clockwise4. Whole circle bearings5. Numbering inverted6. Eye vane and prism used to

read7. Reading taken at south end8. Can be used hand-held9. Sighting and reading

simultaneous

Surveyor’s Compass1. Edge-bar needle2. Ring fixed to box3. 0°at N and S to 90° at E and

W in four quadrants; E and W interchanged.

4. Measures RB5. Numbering erect6. Eye vane not used for

reading7. Reading taken at north end8. Has to be used with tripod9. Object sighted first; then

move around to take reading

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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 EWCB 90 to 180 – RB S (180 – θ) EWCB 180 to 270 – RB S (θ – 180) WWCB 270 to 360 – RB N(360 – θ) W

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INTERCONVERSION

Reduced bearing to whole circle bearing

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INTERCONVERSIONReduced 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)

Bearing SθW

WCB = 180 + θ

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ANGLES AND BEARINGS

Interior and exterior angles

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ANGLES AND BEARINGS

Forebearing and back bearing

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ANGLES AND BEARINGSForebearings and back bearingsFor a line AB the bearing AB measured at A is the

forebearing and the bearing measured at B of BA is the back bearing.

In WCB system, the fore- and back bearings differ by 180 degrees.

In the RB system, the fore- and back bearing has the same value bu the designating letters change from N to S and E to W and vice versa.

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ANGLES AND BEARINGS

Reduced bearings – Fore- and back bearings

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ANGLES AND BEARINGSReduced bearings –

Fore- and back bearings

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INCLUDED ANGLES

Included angles from WCBs

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INCLUDED ANGLES

If whole circle bearings are given,Included angle = Difference in the bearingsIf the difference is more than 180 degrees

then included angle is (360 - difference ).Included angles are generally acute angles

less than 180 degrees.

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INCLUDED ANGLES

Included angles from RBs

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INCLUDED ANGLESIncluded angles from RBsTo calculate included angles from bearings, draw a sketch

and then decide the procedure to calculate the angle.i) Both angles measured from N or S to E or W. (α- β)ii) Both angles measured from N or S but in different

directions (α+β)iii) Measured from N and S but in the same direction (180

– (α+β); in different directions (180 – (α- β).

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INCLUDED ANGLESIncluded angles from RBs

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LOCAL ATTRACTION

Presence of any magnetic materials like overhead electric cables, iron ore, metallic objects affect the movement of the needle. This is called local attraction.

If the forebearing and back bearing of a line do not differ by 180 degrees, even after repeated measurements, then, one may suspect the presence of local attraction.

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LOCAL ATTRACTIONCorrections to set of bearings with local attraction* Identify the line with no local attraction• Or identify the line with least difference• First method – based on principle that internal

angles between lines not affected by local attraction.

• Calculate internal angles and starting from bearing of a line not affected, calculate other bearings from angles.

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LOCAL ATTRACTION

Correction to local attraction• Second method – based on principle that

bearings measured at a station are equally affected by local attraction; the needle deflects equally while measuring the bearings at a station.

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Compass traversing: Important Definition• True meridian: Line or plane passing

through geographical north pole and geographical south pole

• 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.

North pole

True meridian

Magnetic meridian

True bearing

Magnetic bearing

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MAGNETIC DECLINATION

Angle between true meridian and magnetic meridian

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MAGNETIC DECLINATION

True meridian remains the same.Magnetic meridian changes due to many

causes. Thus magnetic declination changes;

If magnetic meridian is to the east of true meridian, it is called East declination (θ E) and if to the west, it is west declination

(θ W).

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VARIATIONS1. Diurnal Variation – Daily variations of about

12’; locality, season and time2. Annual Variation – Magnetic meridian changes

position over one year period.3. the swinging motion of meridian; In one

direction for 200 years and then reverses.4. Irregular variation – due to magnetic

disturbances, Sun storms, volcanic eruptions, Seismic shocks.

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MAGNETIC DECLINATIONIn important maps magnetic bearing and

declination are mentioned.This helps to locate the line, even if

magnetic meridian has changed over time.The true bearing calculated from bearing

and declination as given in the map.The present bearing calculated from true

bearing and present declination.

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PLOTTING A TRAVERSETraverse Data

343° 38’274.5EA

278° 15’490DE

202° 30’430CD

190° 30’398BC

60° 45’402AB

BearingLengthLine

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PLOTTING

1. Plotting by bearings

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PLOTTING

2. Plotting by included angles

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Problems

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Contd…

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PRECISION AND CARE1. Prismatic compass is graduated to half

degree and by judgment can be read up to 15 minutes.

2. Ensure that magnetic materials like key chains, watch etc are not affecting the compass movement.

3. Take reading without parallax error.4. Bisect targets like ranging rod at its

pointed base.

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PRECISION AND CARE

4. Centre and level the compass over the station. Levelling is for convenience. Centering can be done with a plumb bob and by adjusting the legs of tripod.

5. Take double readings for important lines.6. Take back bearings as a check. If there is

discrepancy, repeat.7. The graduated ring should move freely.