COMPASS AND PLANE TABLE SURVEYING I/Unit 2...Systems and conversions- Sources of errors - Local ......
Transcript of COMPASS AND PLANE TABLE SURVEYING I/Unit 2...Systems and conversions- Sources of errors - Local ......
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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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.
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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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AGENDA
Earth’s Magnetism
Bearings
Magnetic Compass
Conversion of Bearings
Angles and bearings
Local Attraction
Magnetic Declination
Plotting
Precision
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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 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
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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
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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 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