CiIvil Surveying - Levelling and Applications
LEVELLING AND APPLICATIONS
1. Define Levelling. What are the uses of leveling?
Levelling is a branch of surveying, the object of which is; (i). To find the elevations of given points with respect to a given or assumed datum, and (ii). To establish points at a given elevation or at different elevations with respect to a given or assumed datum.
2. Define benchmark and reduced level. Benchmark:
Benchmark is a relatively permanent point of reference whose elevation with respect to some assumed datum is known.
Reduced level or Elevation:
The vertical distance of a point above or below the datum is known as the elevation or R.L of that point. R.L of a point may be positive or negative according as the point is above or below the datum.
3. What are the different kinds of bench marks?
A BM is the reference point of known elevation. It may be classified into following types.
(i).G.T.S Bench Mark: The great trigonometrical survey (G.T.S) bench marks are established by the survey of India throughout the country. The levels of this bench marks are established very accurately at a large interval with respect to the mean sea level at Bombay port.
(ii). Permanent Bench Mark: These are established by different Government departments like PWD, Railways, Irrigation etc,. The RL of these points are determined with reference to the G.T.S Bench Marks. Points on rocks, culvert, gate pillars, etc,.
(iii).Temporary Bench Mark: These are established temporarily whenever required. These are generally chosen to close the day’s work and to start the next days. Points on roofs, walls, basements, etc,.
(iv). Arbitrary Bench Mark: When the RL of some fixed points are assumed, they are termed arbitrary benchmark.
4. What do you mean by datum surface?
It is any surface, to which elevations are taken as a reference for the determination of elevations of various points. In India the datum adopted for the great trigonometrical survey (G.T.S) is the mean sea level at Bombay port.
5. What is mean by line of collimation and height of collimation?
Line of collimation (Line of sight): It is an imaginary line passing through the intersection of the cross hairs at the diaphragm and the optical centre of the object glass and its continuation.
Height of collimation(HOC): The elevation of the Line of collimation (Line of sight) is known as Height of collimation. HOC = RL of BM + BS.
6. Write the different types of levels.
1. Dumpy level
2. Tilting level
3. Quick setting level.
5. Reversible level.
6. Automatic level
7. Lazer level.
7. List the essential parts of a Level.
Telescope, Eyepiece, Objective
Focusing screws. Longitudinal bubble
Foot screws, Upper parallel plate ( Tribrach), Foot plate( Trivet).
Diaphragm adjusting screws, Bubble adjusting screws, Tripod.
8. What are the different type’s leveling staffs?
1. Target Staff
2. Self reading Staff.
b. Folding Staff
c. Telescopic Staff.
9. Define and distinguish between ‘Back sights’ and ‘Fore sight’ in the process of fly Levelling.
Back sights (BS)
1.This is the first staff reading taken in any setup of the instrument. It is always taken on a point of known elevation( BM).
2.It is used to determine the height of the instrument.
HI = known RL + BS
Fore sight (FS)
1.This is the last staff reading taken in any setup of the instrument and, after that instrument is shifted.
2.It is used to determine the elevation (RL) of the staff station.
RL = HI – FS.
10. Explain the theory of direct leveling.
It is the branch of leveling in which the vertical distances with respect to a horizontal line may be used to determine the relative difference in elevation between two adjacent points. Steps involved:
HI = known RL(BM) + BS RL = HI – FS.
11. Distinguish between differential levelling and reciprocal levelling.
1.Difference in elevation between two or more points is determined by without any regard to the alignment of the points is called differential leveling.
2.It is used when:
(i). two points are a large distance apart.
(ii). The difference in elevation between two points is large.
(iii). Some obstacles intervenes between the points.
1.Difference in elevation between two points is accurately determined by two sets of reciprocal observations..
2.It is used when:
(i). The instrument cannot be setup between the two points due to an obstruction such as a valley, river, etc,.
12. Reduced level of Bench Mark A - 50.000m
Reading on staff held at A - 2.435m
Reading on staff held at station point B - 1.650m
Find: (a) Height of collimation.
(b) Reduced level of station point B.
(c) Rise/fall of B with respect to A.
(a). Height of collimation = RL of BM A + BS
= 50.000 + 2.435
(b) Reduced level of station point B.
= HOC – FS.
= 52.435 - 1.650
= 50.785 m
( c). Rise/fall of B with respect to A.
= 2.435- 1.65 ( Lower staff reading being higher)
= with compare to A, the station point B being 0.785m higher.
13. Compare height of collimation method and rise and fall method.
a. Height of collimation method
b.Rise and fall method
a.It is more rapid, less tedious and It is as it involves few calculation. involving
b.more laborious and tedious , simpler several calculations.
There is no check on the RL of the intermediate points.
There is a check on the RL of the intermediate points.
Errors in intermediate RL’s cannot be detected.
Errors in intermediate RL’s can be detected.
There are two arithmetic checks on the accuracy of RL calculation. ?BS -?FS = Last RL –First RL.
There are three arithmetic checks on the accuracy of RL calculation. ?BS -?FS = ?Rise -?Fall =Last RL –First RL.
It is suitable in the case of L.S and C.S, Contour etc.
It is suitable in fly leveling where I intermediate sights are less.
14. Write the formula for curvature correction, refraction correction and combined correction.
Curvature correction CC = 0.07849 d2 ( negative) m Refraction correction Cr = 0.01121 d2 (positive) m Combined correction. C = CC – Cr = 0.06728 d2 (negative) m.
Note: ‘d’ is to be substituted in Km, while the corrections will be in m.
15. List out the various sources of errors in levelling.
Three principal sources:
(i). Instrumental error
a. Error due to imperfect adjustment
b. Error due to sluggish bubble.
c. Error due to movement of objective slide.
d. Error due to defective joint.
e. Error due to incorrect length of staff.
(ii). Natural error.
a. Earth’s curvature.
b. Atmospheric refraction.
c. Variations in temperature.
d. Settlement of tripod.
e. Wind vibrations.
(iii). Personal errors.
a. Mistakes in manipulation.
b. Mistake in staff handling
c. Mistake in reading the staff.
d. Error’s in sighting.
e. Mistakes in recording.
16. List out the leveling problems.
1. Levelling on Steep Slope.
2. Levelling on Summits and Hollows. 3.Taking Level of an Overhead Point.
4. Levelling Ponds and Lakes too Wide to be Sighted across.
5. Levelling across River.
6. Levelling on Past High Wall.
17. Define sensitivity of a bubble. State any two factors affecting the same.
The sensitiveness of a bubble is defined the angular value of one division of the bubble tube. It means the capability of showing small angular movements of the tube vertically. It can be increased by:
1. Increasing the internal radius of the tube.
2. Increasing the diameter of the tube.
3. Increasing the length of the tube.
4. Decreasing the roughness of the walls.
5. Decreasing the viscosity of the liquid.
18. What is a spire test?
It is used to make the horizontal axis perpendicular to the vertical axis. This test is also known as the test for the adjustments of the standards. It is done by means of the adjustments of the vertical hair. It is one of the permanent adjustment of the level and theodolite.
19. Define Contour, contour interval and, horizontal equivalent.
Contour: A contour is an imaginary line on the ground joining the points of equal elevation.
Contour interval: It is the vertical distance between any two consecutive contours. It depends upon the nature of the ground, the scale of the map and the purpose of the survey.
Horizontal equivalent: It is the horizontal distance between any two consecutive contours. It varies according to the steepness of the ground.
20. What are the different Characteristics of contour?
1. Contour lines are closed curves. They may either within the map itself or outside the map depending upon the topography.
2. Uniformly spaced, contour lines indicate a uniform slope.
3. A series of closed contours with increase in elevation from outside to
inside in plan denotes a hill.
4. A series of closed contours with increase in elevation from inside to outside in plan denotes a depression.
5. The spacing between the contour lines depends upon the slope of the ground. In steep slopes, the spacing is small and for gentle slope, the spacing is large.
21. What are the uses of contours?
1. Volume of earthwork for any work can be estimated.
2. The capacity of the reservoir or the area of the catchments can be
3. Very useful in military operations to decide the position of the guns, the line of march.
4. Longitudinal and cross section can be drawn along any direction to know the nature of the ground.
22. Write the different formulae to calculate the area of the irregular plate.
1. By computations based directly on the field measurements:
(i). By dividing the area into number of triangles. (ii). By offsets to base line.
a.Mid ordinate rule = ?O .d
b. Average ordinate Rule = nd. ?O
c. Trapezoidal rule = ( Oo + On + Ol +O2+O3+……On-1) d 2
d. Simpson’s rule = [( Oo + On )+ 4( O1+ O3+ …. On-1) + 2( O2+ O4+ …. On-2 ) ] (d/3)
Oo = Ordinate at one end.
On = Ordinate at other end.
Ol +O2+O3+……On-1 = Ordinate at end of each divison. d = Length of the base
(iii). By latitudes and Departures:
a.By double meridian distance (D.M.D. method).
b. By double parallel distance (D.P.D. method). (iv). By co- ordinates.
2. By computations based on measurements scaled from a map.
3. By mechanical method: By means of planimeter.
23. How do you calculate the capacity of the reservoir from the contour map?
From the contour map, the capacity of the reservoir is calculated by the
1. Trapezoidal rule = ( Ao + An + Al +A2+A3+……An-1) 2
2. Prismoidal rule = [( Ao + An )+ 4( A1+ A3+ …. An-1) +
2( A2+ A4+ …. … An-2 ) ] (d/3)
Ao, A1, A2, A3,……An = Areas enclosed by successive contours. d = contour interval.