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 f