Gandhinagar Institute of Technology(012)

Types of Compass, Types of Meridians,& Types of Bearing.

Element Of Civil Engineering

Prepared by :- Kundariya Ankit Patel Avi Patel Dharmik

Prof. Saurabh ShahGuided by :-

• A Compass is an instrument containing a magnetized pointer which shows the direction of magnetic north and bearings from it.

What is Compass?

Types of Compass: • Prismatic Compass• Surveyor’s Compass• A Prismatic compass is a navigation and

surveying instrument which is extensively used for determining course, waypoints and direction, and for calculating bearings of survey lines and included angles between them.

• A Surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century.

Prismatic Compass Surveyors' Compass

Prismatic Compass Surveyor’s CompassThe Graduated ring attached to the

magnetic needle.The Graduated ring and needle are free to

move independently wrt each otherGraduated ring remains stationary while box is prism and object vane rotates as the ring attach with needle is not attach

with the box

Graduated ring rotates with rotation of box ,eye vane & object vane as the ring is attach with the box of the compass & only

needle remains stationaryPrism is provided to take reading The graduated ring is graduated with ejected

figures and no prism is provided to take the reading

Graduation are marked 0 to 360 in clockwise direction

Graduations are marked 0 to 90 in each quadrant

Tripoid may or may not be provided. The instrument cannot be used without tripoid

It measures or gives WCB of a line It measures or gives Q.B. of a line.

Compass Surveying

• Chain Surveying can be used when the Area to be surveyed is comparatively small and is fairly flat.

• But when the area is large, undulated and crowded with many details, triangulation(which is the principle of chain survey) is not possible. In such an area, the method of traversing is adopted.

Principle of Compass Surveying:

• The Principle of compass survey is Traversing; which involves a series of connected lines the magnetic bearing of the lines are measured by prismatic compass and the distance (lengths) are measured by chain.

• Such survey does not require the formulation of a network of triangle.

• Compass surveying is recommended when the area is large, undulating and crowded with many details.

• Compass surveying is not recommended for areas where local attraction is suspected due to the presence of magnetic substances like steel structures, iron ore deposits, electric cables conveying currents.

• In traversing , the frame work consist of connected lines.• The length are measured by a chain or a tape and the

direction measured by angle measuring instruments.• Hence in compass surveying direction of survey lines

are determined with a compass and the length of the lines are measured with a tape or a chain. This process is known as compass traversing.

Traversing

Types of Bearing:-The bearing of a line is the horizontal angle which it makes with a reference line(meridian).there are four type of bearings :-1) True Bearing: The true bearing of a line is the horizontal

angle between the true meridian and the survey line. The true bearing is measured from the true north in the clockwise direction.

2) Magnetic Bearing: the magnetic bearing of a line is the horizontal angle which the line makes with the magnetic north.

3) Grid Bearing: The grid bearing of a line is the horizontal angle which the line makes with the grid meridian.

4) Arbitrary Bearing: The arbitrary baring of a line is the horizontal angle which the line makes with the arbitrary meridian.

BEARINGS

TRUE MERIDIAN

MAGNETIC MERIDIAN

TRUE BEARING

MAGNETIC BEARING

A

B

MNTN

The bearing are designated in the following two system:-

1) Whole Circle Bearing System.(W.C.B)2) Quadrantal Bearing System.(Q.B)

Measurement of Bearings :

• The bearing of a line measured with respect to magnetic meridian in clockwise direction is called magnetic bearing and its value varies between 0ᴼ to 360ᴼ.• The quadrant start from north an progress in a clockwise direction as the first quadrant is 0ᴼ to 90ᴼ in clockwise direction , 2nd 90ᴼ to 180ᴼ , 3rd 180ᴼ to 270ᴼ, and up to 360ᴼ is 4th one.

Whole Circle Bearing System.(W.C.B)

• In this system, the bearing of survey lines are measured wrt to north line or south line which ever is the nearest to the given survey line and either in clockwise direction or in anti clockwise direction

Quadrantal bearing system(Q.B.)

• When the whole circle bearing is converted into Quadrantal bearing , it is termed as “REDUCED BEARING”.Thus , the reduced bearing is similar to the Quadrantal bearing.• Its values lies between 0ᴼ to 90ᴼ, but the quadrant should be mentioned for proper designation.

Reduced bearing (R.B)

W.C.B OF ANY LINE

QUADRANT IN WHICH IT LIES

RULES FOR CONVERSION

QUADRANT

0 TO 90 I RB=WCB N-E

90 TO 180 II RB=180-WCB S-E

180 TO 270 III RB =WCB-180ᴼ S-W

270 TO 360 IV RB=360ᴼ - WCB N-W

Fore bearing and Back bearing

The bearing of a line measured in the forward direction of the survey lines is called the ‘fore bearing’(F.B.) of that line.

The bearing of a line measured in direction backward to the direction of the progress of survey is called the ‘back bearing’(B.B.) of the line.

Types of Meridians:

•Magnetic meridian :- The direction shown by a freely suspended needle which is magnetised and balanced properly without influenced by other factors is known as magnetic meridian •True meridian :- True meridian is the line which passage through the true north and south. The true meridian at any point can be determinate by either observing the bearing of the sun at 12 noon or by sun’s shadow.

•Arbitrary meridian :- In case of small work or in places were true meridian or magnetic meridian cannot be determined, then any direction of prominent object is taken as a reference direction called as arbitrary meridian.

Example of WCB into Reduced Bearings:-• Convert the following WCB into RB

• S 43 °15’ E

Example :- 1

Note :- If FB > 180 then subtraction is required And if FB<180 then addition is required

Example 2 :-

The following bearings were taken of a closed traverse ABCD.

Line FB BBAB 45°00’ 225°00’BC 123°30’ 303°00’CD 181°00’ 1°00’DA 289°00’ 109°00’

Calculate the interior angels of the traverse.

Station Line FB Line BB Diff of FB & BB

Exterior Angle

Interior Angle

A AB 45°00’ DA 109°00’ 64°00’ 64°00’B BC 123°3’ AB 225°00’ 101°00’ 101°30’C CD 181°0’ BC 303°30’ 122°30’ 122°30’D DA 289°0’ CD 1°00’ 288° 288° 72°00’

Interior angle A = BB of DA – FB of ABInterior angle B = BB of AB – FB of BCInterior angle C = BB of BC – FB of CDExterior angle D = FB of DA – BB of CDSo the Interior angle D = 360° – Interior angle DSum of angles = 64°+101°30’+122°30’+72° = 360 °

Check = (2N – 4 ) * 90° = (2*4 – 4 )*90 ° = 360 °

Example 3 :- The Details of observed bearings are mentioned below.Find out the included angles and correct the angles if needed to be corrected.

LINE FB BBAB 20°30’ 200°00’BC 110°00’ 290°30’CD 195°00’ 15°00’DA 286°30’ 106°00’

Station Line FB Line

BB Diff. of FB & BB

Exterior Angle

Interior Angle

A AB 20°30’ DA 106°0’ 85°30’ 85°30’B BC 110°00’ AB 200°00’ 90°00’ 90°00’C CD 195°00’ BC 290°30’ 95°30’ 95°30’D DA 286°30’ CD 15°00’ 271°30’ 271°30’ 88°30’

Sum = 359°30’

Interior angle A = BB of DA – FB of ABInterior angle B = BB of AB – FB of BCInterior angle C = BB of BC – FB of CDExterior angle D = FB of DA – BB of CDSo the Interior angle D = 360° – Interior angle DSum of angles = 85°30’+90°00’+95°30’+88°30’= 359°30’

Check = (2N – 4 ) * 90° = (2*4 – 4 )*90 ° = 360 ° Total Error = 359°30’ – 360° = – 30°

∴

Total Correction = +30’ ∴

∴ Correction per angle = + 30’/4 = + 7’30”

Corrected value :-

Angle Calculated value Correction Corrected included Angle

A 85°30’ + 7’30” 85°37’30”B 90°00’ + 7’30” 90°07’30”C 95°30’ + 7’30” 95°37’30”D 88°30’ + 7’30” 88°37’30”

Sum 359°30’ + 30’00” 360°00’00”