Surveying - Module iii-levelling only note

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  • OPTC Edavanna Page 1

    MODULE III : Levelling

    1. Define levelling?

    It is a branch of surveying the object

    (i) To find the elevations of points

    (ii) To establish points based on assumed datum

    or is defined as art/science of determining relative heights of different points on,

    above, or below earth surface.

    2. Define the following terms used in levelling:

    (i) Datum (ii) Mean sea level (iii) Bench mark

    (iv) Level surface (v) Level line

    Datum: It is any surface to which elevations (heights) are referred. or the imaginary

    surface with reference to which vertical distances of points (on, above, or below) are

    measured. eg: MSL

    Mean Sea Level (MSL): It is the datum adopted by GTS department of India

    Bench mark:It is fixed reference point of known elevation w.r.t any datum

    Level Surface: It is a curved surface which at each point is perpendicular to the

    direction of gravity at that point.

    It is perpendicular to the plumb line

    It is parallel to the earth surface

    Level line:

    It is a line lying on level surface

    It is perpendicular to the plumb line (direction of gravity)

    3. Write a note on levelling instruments? Explain.

    Which is the smallest division of the levelling staff?

    Differentiate between telescopic levelling staff and target staff?

    Orphanage Polytechnic College, Edavanna REVISION PACKAGE

    Surveying - I ( Revision 2015)

  • OPTC Edavanna Page 2

    LEVEL: Instrument used to furnish horizontal line of sight for observing staff

    readings and determining reduced levels

    1. Dumpy level:

    Simple, compact and stable instrument

    Telescope rigidly fixed to its supports

    It cannot be rotated about horizontal axis

    2. Wye (Y) level: Telescope is carried by two vertical wye supports

    3. Tilting level: Telescope tilt about its horizontal axis. It is also called as IOP

    (Indian Office Pattern) level

    4. Automatic level: Telescope tilt about its horizontal axis

    It is also called as IOP (Indian Office Pattern) level

    LEVELLING STAFF:

    Levelling staffs are scales on which the distances are measured

    Straight rectangular rod having graduations

    Bottom of levelling staff represents zero reading

    The smallest division of a levelling staff is 5 mm

    A. Self-reading staff: A staff on which readings are directly read by the observer

    through the telescope. They are three types

    1. Solid staff

    2. Folding staff

    3. Telescopic staff

    1. Solid staff:

    Made of well-seasoned wood

    3 m length

    Greater accuracy in reading, so used in precise levelling

    No Hinge joints

    Difficult to carry to field

    2. Folding staff

    Also called as Hinged staff

    Hinge joints present

    Less accuracy

    4 m length

    Can be folded into 2 m length

  • OPTC Edavanna Page 3

    Thickness of graduations = 5 mm

    3. Telescopic staff

    Three lengths placed one into the other

    Extended to full length by pulling

    Smallest division is 5 mm

    4 m length

    Values in meter (Red - left side) and decimeter (Black - right side) are

    marked on it.

    B. Target Staff:

    A staff which contains a target is called target staff

    For taking readings, the level man directs the staff

    man to raise or lower the target till it is bisected

    by line of sight.

    The staff man clamps the target and takes reading.

    4. What are the advantage sof dumpy level over wye level?

    Simpler construction with few movable parts

    Fewer adjustments to be made

    Longer life of the adjustments

    5. What are the merits and demerits of self-reading staff?

    Merits:

    Easier and Quicker to read a self-reading staff

    The reading on a self-reading staff are taken by the leveler himself

    For holding the staff, no experience required, no knowledge required

    Demerits:

    Accuracy is less than that of target staff.

    The leveler have all responsibilities in taking reading

    6. What are the merits and demerits of target staff?

    Merits:

    The reading with target staff can be taken with greater fineness (more

    accurate)

    The leveller and staff man have the responsibility in taking the reading

    Vernier is provided on target staff for accurate reading

    Demerits:

    It is difficult to adjust the target, so that the line of sight bisects the target

    The staff man is responsible in marking the reading

    So, for holding the staff, an experienced staff man is required

  • OPTC Edavanna Page 4

    7. What is mean by curvature and refraction correction?

    For long sights and accurate levelling work, the effects of the line of sight shall have

    to be taken into consideration. Due to curvature of earth, the points appear to be

    lower than they actually are. While due to refraction, they appear to be higher than

    they actually are. The effects of curvature being greater than that of refraction, the

    combined effect causes the points to appear to be lower than they actually are.

    Curvature correction:

    PC = apparent staff reading

    PB = True staff reading

    BC = correction for curvature

    R = Radius of earth = 6370 km

    Error due to curvature of earth

    is considered when the disyance

    is large

    The line of sight is horizontal, but the level line is curved and parallel to the

    earth surface

    The vertical distance b/w the line of sight and level line at a particular point is

    called curvature correction

    Effect of curvature is to cause the object sighted to appear lower than they

    actually are

    curvature correction is always negative

    OC2 = OA

    2 + AC

    2

    (R+CC)2 = R

    2 + d

    2

    R2 + 2.R.CC +CC

    2 = R

    2 + d

    2

    2.R.CC = d2

    CC =

    =

    Refraction correction:

    Ray of light passes through air layers of different densities. They donot

    remain straight, but are refracted and bend downward towards denser medium.

    Refraction correction is assumed as 1/7 times the curvature correction.

    Curvature correction, CC =

    (subtractive) = 0.0784 d2

    Cr =

    = 0.01121d2

  • OPTC Edavanna Page 5

    Combined correction:

    Combined correction = Curvature correction (-ve) + Refraction correction

    Cr =

    +

    =

    (subtractive) = 0.06728d2

    Combined correction is subtractive

    8. An observer standing on the deck of a ship just sees the top of a light house. The

    top of the light house is 30 m above sea level and the height of the observers

    eye is 5 m above the sea level. Find the distance of the sailor from the light

    house?

    d = d1 + d2

    =

    +

    =

    +

    = 29.60 m

    9. Explain errors in levelling?

    The errors in levelling are mainly 3 types,

    (i) Instrumental error

    (ii) Personal error

    (iii) Natural error

    Instrumental error Personal error Natural error Error due to imperfect

    adjustment

    Mistakes in manipulation Earths curvature

    Error due to sluggish

    bubble

    Mistake in rod handling Atmospheric refraction

    Error due to movement of

    objective slide

    Mistakes in reading the rod Variations in temperature

    Rod not of standard length Errors in sighting Settlement of tripod or

    turning points

    Error due to defective joint Mistakes in recording Wind vibrations

    10. Define sensitivity of bubble tube. How the sensitiveness of bubble tube can be

    increased?

    It is the capacity of a level tube to exhibit small deviation from the horizontal

    Also defined as the angular value of one division of the bubble tube.

    The sensitivity of a bubble tube can be increase by

    o Increasing the internal radius of the tube

    o Increasing the diameter of the tube

    o Increasing the length of the bubble

    o Decreasing the roughness of the wall

    o Decreasing the viscosity of the liquid

  • OPTC Edavanna Page 6

    11. The following readings were observed with a level 1.150 (BM 37.430), 1.930,

    2.810, 4.000, CP 1.520, 2.440, 0.730, 0.490, CP 4.000, 3.160, 1.800, 0.7000.

    Reduce the levels by height of collimation method. Apply check.

    Method I: Rise and Fall method

    Check:

    Last RL First RL = 38.91 37.430 =1.48

    BS - FS = 6.67 5.19 = 1.48

    Rise - Fall = 5.25 3.77 = 1.48

    Method II: Height of Instrument method (HI or HC method)

  • OPTC Edavanna Page 7

    Check:

    Last RL First RL = 38.91 37.430 =1.48

    BS - FS = 6.67 5.19 = 1.48

    12. Following series of readings of BS and FS was taken in a fly levelling. The first

    reading was taken on a point of RL 100.000 m. Draw a page of levelling field-

    book and enter readings in it. Find the RLs of all points. Apply check.

    1.235, 1.396, 2.345, 1.986, 2.148, 3.755, 0.325, 2.568, 1.465, 2.435, 1.356, 0.768,

    1.985, 2.655

    Last RL First RL =

    BS - FS =

    Rise - Fall =

    13. The following staff readings were taken with a level. The instrument was moved

    after the fourth and the seventh readings. Calculate the reduced levels of the

    points and find level difference between the first and last points. The first

    reading was taken on a BM of +100.000 m. Use HC method.

    0.875, 2.625, 1.050, 2.185, 1.850, 0.885, 1.375, 2.180, 0.855, 2.200 [refer note]

    14. The following staff readings were recorded for a certain work of levelling 1.810,

    2.110, 1.225, 1.455, 0.905, 2.435, 2.810, 2.675, and 1.765. The level was shifted

    after 4th

    and 7th

    readings. The first reading was taken on a BM of RL 50.000 m.

    Rule out a page of level book and enter the readings

    (i) Work out the RL of all stations

    (ii) If the staff were held inverted and readings on a ceiling from last instrument

    position was 3.500 m, Find the RL of ceiling. [refer note]

  • OPTC Edavanna Page 8

    15. During a construction work, the bottom of an RC sunshade A was taken as a

    temporary BM (RL 63.120). The following notes were recorded.

    Reading of inverted staff on BM No.A = 2.232

    Reading on peg P on ground = 1.034

    Change of instrument

    Reading of peg P on ground = 1.328

    Reading an inverted staff on bottom of cornice B = 4.124

    Enter the readings in a level book page and calculate the RL of cornice B.

    16. What is a bench mark? What are the different types of BM?

    Bench mark: It is fixed reference point of known elevation w.r.t any datum. There

    are four types of bench marks.

    1. GTS (Great Trignometrical Survey) BM

    2. Permanent BM

    3. Arbitrary BM

    4. Temperory BM

    GTS (Great Trignometrical Survey) BM

    Established by Survey of India

    Established with higher accuracy

    Based on MSL at Bomabay High

    Permanent BM

    Fixed in b/w GTS bench marks

    By govt. agencies like railways, PWD

    Written on permanent objects like milestones, railways, bridges, etc.

    Value clearly written

    Arbitrary BM

    These are reference points whose RLs are arbitrarily assumed

    Used for small works

    Temperory BM

    Reference points established temporarily whenever required

    Generally chosen to close days work and to start next days.

    Also if there is a break in work

    Marked on objects like stone, trees,..

  • OPTC Edavanna Page 9

    17. The following staff readings were taken with a level. The instrument was moved

    after third, sixth, and eighth readings. Calculate the RL of points, if the first

    reading was taken by a staff held on a BM 75.000 m. Use HC method.

    0.765, 1.825, 1.050, 1.885, 1.850, 0.995, 1.375, 2.250, 0.765, 2.200

    18. The staff readings recorded for a work are 1.205, 1.710, 2.500, 0.230, 0.910,

    2.100, 2.800, 0.310, 1.820, 2.915. The first reading was taken at BM 140.000.

    Rule out a page of field book in rise and fall method and find the average slope

    between the points. The instrument was shifted after the fourth and eighth

    reading. The distance between first and last station is 1500 m.

    19. What are the adjustments done at every set-up of levelling instruments?

    20. What are the temporary adjustments in levelling?

    21. What are the points should be observed while conducting levelling work?

    Adjustments done at every setting of levelling instrument are called temperory

    adjustments. They are

    (i) Setting out: Fixing the instrument on tripod and levelling it approx by tripod

    (ii) Levelling up: Accurate levelling using foot screws and to make the vertical axis

    truly vertical. It is done as below,

    Loose the clamp and turn the instrument such that telescope is

    parallel to two of the foot screws. Turn level screws and level it.

    Turn the telescope 900 so that telescope is parallel to the third

    screw. Turn the level screw until the bubble comes in the center of

    level tube.

    (iii) Elimination of parallax: Parallax is a condition arising when the image formed

    by the objective is not in the plane of cross hairs. It is eliminated by focusing

    the objective and eyepiece.

    22. What is balancing of sights? What is its importance in the field?

  • OPTC Edavanna Page 10

    In the above fig, let the observations be made with a level in which the line of

    collimation is inclined upward by an amount from horizontal. When the bubble is

    in the centre of its run, the level being kept exactly mid-way b/w the two points A &

    B. The observed back sight and fore sight are x1 and x2. The correct BS on A will be

    equal to x1 y1, where y1 = D1tan . The correct fore sight on B will be equal to

    x2 y2, where y2 = D2tan . Hence the correct difference b/w A & B is

    = (x1 y1) (x2 y2)

    = (x1 y1 ) + (y2 y1)

    = (x1 x2) + ( D2tan - D1tan )

    = (x1 x2), if D1 =D2

    Errors eliminated by balancing:

    The error due to non-parallelism of line of collimation and axis of bubble tube

    The error due to curvature and refraction may be eliminated if the length of two

    sights can be made equal.

    23. Distinguish between two methods of booking and reducing the elevation of

    points from the observed staff readings.

    24. Describe the HI method and rise and fall method of computing the levels.

    Discuss the merits and demerits of each.

    25. What are the two systems of reduction of levels? Explain.

    The observed staff readings as noted in a level book are further required to be

    manipulated to find out the elevation of points. The operation is known as reduction

    of level. There are two methods for reduction of levels:

    HI = BM + BS

    RL of A = HI - FS (or IS)

  • OPTC Edavanna Page 11

    26. Draw a neat diagram of component parts of a levelling instrument? Describe

    the function of its salient parts?

    Telescope : Used to sight and read staff readings

    Diaphragm : Holds the cross hairs (fitted with telescope)

    Focussing screws : Separate focussing screws for eye piece and objective.

    Eye piece : Magnifies the image formed in the plane of the diaphragm and thus to

    read staff during leveling.

    Level Tube : Used to make the axis of the telescope horizontal and thus the

    line of sight.

    Levelling screws : To adjust instrument (level) so that the line of sight is horizontal

    for any orientation of the telescope.

    Tripod stand : To fix the instrument (level) at a convenient height of an observer.