important Questions and Answers: Compass Surveying And Plane Table Surveying

COMPASS SURVEYING AND PLANE TABLE SURVEYING

1. Define:  Compass surveying. What are the objects of compass surveying?

Compass surveying is the type of surveying in which the direction of the survey lines are measured with a compass and the length of the survey lines are measured with a tape or chain in the field.

2. Write the names of the instruments used in chain surveying.

(i). Instruments for the direct measurement of directions:

1. Surveyor's compass.

2. Prismatic compass.

(ii).Instruments for the measurement of angles:

1. Sextant.

2. Theodolite.

3.         Define: (a). True meridian and bearing. True meridian:

The line or plane passing through the geographical North Pole, South Pole and any point on the surface of the earth, is known as true meridian or geographical meridian. True meridian at a point is constant.

True bearing:

The angle between the true meridian and a survey line is known as true bearing or Azimuth of the line.

(b). Magnetic meridian and Bearing. Magnetic meridian :

Magnetic meridian at a point is the direction indicated by freely suspended,

properly balanced and unaffected magnetic needle at that point.

Magnetic Bearing:

The angle between the magnetic meridian and a survey line is known as magnetic bearing or bearing .of the line. It changes with time.

4. What do you understand by Whole circle bearing and quadrantal bearing of a line? Magnetic Bearings are designated by Whole circle bearing system and

quadrantal bearing system.

In Whole circle bearing system (WCB), the bearing of the line is measured with magnetic north in clockwise direction. It varies from 0⁰ to 360⁰.

In quadrantal bearing system (Q.B or R.B) the bearing of the line is measured eastward or westward from north or south, whichever is nearer. The directions can be either clockwise or anticlockwise. It varies from 0⁰ to 90⁰.

5       Convert the whole circle bearing into reduced bearing: 50^0, 176⁰, 210⁰, 232⁰, 150^0, 76⁰, 310⁰ ,242⁰.

Whole circle bearing WCB

500

1760

2100

2320

1500

760

3100

2420

Reduced bearing

RB

N  500  E.

S (1800 - 1760)E  = S  40 E

S (2100 - 1800)W = S  300 W

S (2320 - 1800)W = S  520 W

S (1800 - 1500) E = S  300 E

N  760  E

N (3600 - 3100) W = N 50 0 W

S (2420 - 1800)W  = S  620 W

1.     Differentiate between Prismatic compass and Surveyor's compass with reference to reading and tripod.

2.     The fore bearing of a line PQ is N 28⁰ W. What is its back bearing?

1. Reading 

Prismatic compass:

(i). The reading is taken with a help of prism provided at the eye slit.

(ii). Sighting and reading taking can be done simultaneously from one position of the observer.

Surveyor's compass:

(i). The reading is taken by directly seeing through the top of the glass.

(ii). Sighting and reading taking cannot be done simultaneously from one position of the observer.

2.Tripod    

Prismatic compass: Tripod may or may not be provided.

Surveyor's compass: The instrument cannot be used without a tripod.

In quadrantal bearing (RB) system, the FB and BB are numerically equal but the quadrants are just opposite.

The FB of a line PQ is N 28⁰ W, Then its BB is S 28⁰ E .

8. Define: Fore and Back bearing.

The bearing of a line is measured in the direction of the progress of the survey is called the fore bearing of the (FB) line.

The bearing of a line is measured in the direction opposite to the survey is called the back bearing of the (BB) line.

BB = FB + 180 ⁰ . (FB greater than 180 ⁰, use - sign) (FB smaller than 180 ⁰, use + sign)

9. The fore bearing of line AB is 155⁰25'20'. Identify the back bearing of the line AB in quadrantal system.

The fore bearing of line AB = 155⁰25'20'. The back bearing of line AB , BB = FB + 180 ⁰

= 155⁰25'20' + 180 ⁰

= 335⁰25'20'  (WCB)

=  N (360⁰ - 335⁰25'20') W

= N 24⁰34' 40'' W

10.    Define and distinguish between magnetic dip and magnetic declination. Magnetic dip:

Due to the magnetic influence of the earth, the needle does not remain in the balanced position. This inclination of the needle with the horizontal is known as the dip of the magnetic needle. To balance the dip of the needle, a rider (brass or silver coil) is provided along with it.

Magnetic declination:

The magnetic meridian of a place is variable one due to some local attractive forces. Therefore, the true meridian and the magnetic meridian at a place do not coincide.

The horizontal angle between the magnetic meridian and the true meridian is known as magnetic declination.

11. The magnetic bearing of a line is 48⁰24'. Calculate the true bearing if the magnetic declinations are 5⁰38' East and 5⁰ 38' West.

True bearing = Magnetic bearing + Eastern magnetic declination

= 48⁰24' +  5⁰38'

= 54⁰02'.

True bearing = Magnetic bearing - Western magnetic declination

= 48⁰24' -  5⁰38'

= 42⁰46'.

12.         The magnetic bearing of a line is S 28⁰30' E. Calculate the true bearing if the magnetic declinations are 5⁰38' East and 5⁰ 38' West.

Convert the reduced bearing into Whole circle bearing.

Magnetic bearing = S 28⁰30' E.  (RB)

= 180⁰ - 28⁰30'

= 151⁰ 30⁰ .

True bearing = Magnetic bearing + Eastern magnetic declination

= 151⁰30' +  7⁰30'

= 159⁰00'. (WCB)

= S(180⁰-159⁰00')E   (RB)

=   S 21 ⁰ E .

True bearing = Magnetic bearing - Western magnetic declination

= 151⁰30' -  7⁰30'

= 144⁰00'. (WCB)

= S(180⁰-144⁰00')E  (RB)

=   S 36 ⁰ E.

13.         What is local attraction? What are the sources of local attractions?

Local attraction is a term used to denote any influence, such as magnetic substances, which prevents the needle from pointing to the magnetic north in a given locality.

The sources of local attractions are : magnetite in the ground, wire carrying electric current, steel structures, rails, underground iron pipes, chain etc.,

14. Distinguish between closed traverse and open traverse.

Closed traverse

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

It is suitable for locating the boundaries of lakes, woods etc,.

Check: Sum of interior angles = (2n - 4) 90 Sum of exterior angles = (2n +4) 90 Where, n= number of sides of the traverse.

Open traverse

If the circuit ends elsewhere, it is known as open traverse.

It is suitable for surveying a long narrow strip of land required for road canal etc,.

No direct check on angular measurements.

15. What is plane table surveying? When is it preferred? Write its principle.

Plane tabling is the graphical method of surveying in which the field observations and plotting proceed simultaneously.

It is mainly suitable for filling the interior details between the control stations and also in magnetic areas.

The main principle of plane table surveying is that the lines joining the points on the plane table are made to lie parallel to the corresponding lines joining the points on the ground while working at each station.

16.         Name four methods of plane surveying.

1. Radiation

2. Intersection.

3. Traversing

4. Resection.

(i). Resection after orientation by compass.

(ii). Resection after orientation by back sighting.

(iii). Resection after orientation by three point problem.

a. Mechanical method.(Tracing paper method)

b. Graphical method. ( Bessel's method)

c.           Lehman's method. (Trial and error method) (iv). Resection after orientation by two point problem

17. When a three- point problem resorted to in plane table surveying?

It is the method of orientation when the table occupies a position not yet located on the drawing sheet.

This method is employed when during surveying the surveyor feels that some important details can be plotted easily by choosing any stations.

18. State the First and second Lehman's rule.

First  rule:  The  distance  of  the  point  'p' (position  of  plane  table  on  the

drawing sheet) to be fixed from each rays Aa (paper a, to ground A), Bb (paper ,b to ground B) and Cc (paper c, to ground C) is proportional to the respective distances of the stations A, B and C from the ground station P.

Second rule: While looking towards the stations the point 'p' to be fixed, will be either to the left or to the right of each of the rays.

19.  What are the Advantages of plane table surveying?

1. It is useful in magnetic areas.

2. It is cheaper than the theodolite surveying.

3. It is most suitable for small-scale maps.

4. The surveyor can compare plotted work with the actual features.

5. No skill is required .

20.Name some of the errors in plane tabling.

1. Instrumental error

2. Errors of plotting

3. Errors due to manipulation and sighting.

1. Non horizontality of the board.

2. Defective sighting.

3. Defective orientation.

4. Movement of board between sights

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