basics of surveying ii

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    SURVEYING

    PreambleSurveying is involved in a project from conceptual

    stage to construction and afterwards in

    maintenance also. Depending on the stage at which

    surveying is carried out it can be called.

    i) Pre construction

    survey

    Feasibility survey (RECT, PECT)

    ii) Constructionsurvey

    To maintain alignment and Geometrycontrol during construction. (FLS

    and other )

    iii) Post

    construction

    Maintenance survey

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    Definition : Operation of making such

    measurements that the relative position ofvarious features, natural or Artificial on the

    surface of the earth can be exhibited in their

    correct Horizontal and vertical relationship.

    Normally determining position in Horizontal

    plane is called surveying.

    Determining relative heights or depth is called

    levelling.

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    Purpose : The main object of surveying is thepreparation of maps or plans which are the basis in

    planning and design of engineering project such asroute location of railway line, roads and water supply

    scheme.

    Basic Principles in Surveying : Rulingprinciple of survey is :

    i) to work from whole to part. For surveying

    Establish control points with high precision by use of

    Triangulation and precise levelling. Area is furtherdivided into triangle, which are surveyed with less

    accuracy.

    ii) to fix the position of new stations by atleast two

    independent processesBy linear and Angular

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    Classification of Surveys : Surveying is divided intotwo main categories-

    i) Geodetic Surveyii) Plane survey

    i) Geodetic Survey :-When survey extends over alarge areas more than 200 sq. km. and degree of

    accuracy is also great. The curvature of earth is alsotaken into account. Geodetic survey is used to provide

    control points to which small surveys can be connected.

    ii) Plane Survey :- For small projects covering Arealess than 200 sq.km. Earth curvature is not counted for

    in distances. Earth surface is considered as plane.

    (Angular error of 1 in 200 sq. km. area by assuming

    plane).

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    A) Classification based upon equipment used:i) Chain survey

    ii) Compass survey

    iii) Theodolite surveyiv) Plane Table survey

    v) Tachometric survey

    vi) Aerial Photographic Survey

    vi) Remote sensing.B) Based upon Method Employed :

    i) Triangulation : Control points are established

    through a net-work of triangles.ii) Traversing: Scheme of control pointsconsisting of a series of connected lines.

    iii) Trilateration: Distances are measured for

    exercising the control.

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    Leveling :

    The art of determining relative altitudesof points on the surface of the earth of

    beneath the surface of earth is called

    LEVELLING.

    For execution of Engineering Projects it is

    very necessary to determine elevations ofdifferent points along the alignment of

    proposed project.

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    Other applications are :

    i) Taking rail levels existing before track renewals to

    finalize final rail level profile including vertical curves.

    ii) Initial ground levels for earthwork calculations.

    iii) Levels for measurement of earthwork.

    iv) Measurement of ballast etc.

    Terms used in Levelling

    a) DATUMor Datum plane is an arbitrarily assumedlevel surface or line with reference to which level of

    other line or surface are calculated.

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    b) REDUCED LEVEL (RL)Height or depth of a

    point above or below the assumed datum is called

    Reduced level.

    c) BENCH MARK(BM)B.M. is a fixed reference

    point of known elevation. It may be of the following

    types.i) GTS Bench mark (Geodetic Triangulation Survey) :

    These Bench marks are established by national

    agency like Survey of India. They are established

    with highest precision. Their position and elevationabove MSL is given in a special catalogue known as

    GTS Maps ( 100 km. interval).

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    ii)Permanent Bench Mark : They are fixed

    points of reference establish with reference to GTS

    Bench mark (10 km. interval).

    iii) Arbitrary Bench mark : These arereference points whose elevations are arbitrarilyassumed. In most of Engineering projects, the

    difference in elevation is more important than their

    reduced levels with reference to MSL as given in aspecial catalogue known as GTS Maps ( 100 Km.

    interval).

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    d)Mean Sea Level (M.S.L.) : M.S.L. is obtained bymaking hourly observations of the tides at any place

    over a period of 19 years. MSL adopted by Survey ofIndia is now Bombay which was Karachi earlier.

    e) Level Surface : The surface which is parallel tothe mean sphereoidal surface of the earth is known aslevel surface.

    f) Line of Collimation : It is the line joining theintersection of the cross hair and the optical center of

    the objective and its extensions, it is also called line of

    sight or collimation.

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    g) Height of Instrument (HI) : The elevation of the lineof sight with respect to assumed datum is known as HI.

    h) Back sight : (B.S.) - The first sight taken on alevelling staff held at a point of known elevation. B.S.

    enables the surveyor to obtain HI +sight i.e. Height of

    Instrument or line of sight.

    i) Fore Sight : (F.S.)It is the last staff reading taken

    from a setting of the level. It is also termed as minus

    sight.

    Fore sight is the sight taken on a levelling staff held ata point of unknown elevation to ascertain the amount

    by which the point is above or below the line of sight.

    This is also called minus sight as the foresight reading

    is always subtracted from height of Instrument.

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    k) Change Point (CP) : The point on which

    both the foresight and back sight are takenduring the operation of levelling is called

    change point.

    l) Intermediate Sight (IS) :

    The foresight taken on a levelling staff held

    at a point between two turning points, todetermine the elevation of that point, is

    known as intermediate sight.

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    It may be noted that for one setting of a level, there

    will be only one back sight and one foresight but there

    can be any number of intermediate sights.Type of Leveling Equipments:i) Dumpy level

    ii) Tilting level

    iii) Automatic leveliv) Digital Auto level

    Dumpy level : It is simple compact and stable. The

    telescope is rigidly fixed to its support thereforecannot be rotated about its longitudinal axis. A long

    bubble tube is attached to the top of telescope.

    Dumpy literally means short and thick.

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    Tilting level : It consists of a telescopeattached with a level tube which can be tilted

    within few degrees in vertical plane by a tilting

    screw.

    The main peculiarity of this level is that thevertical axis need not be truly vertical, since the

    line of collimation is not perpendicular to it. The

    line of collimation, is, however, made horizontalfor each pointing of telescope by means of tilting

    screw. It is mainly designed for precise levelling

    work.

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    The Automatic level : Also termed as self

    aligning level. The fundamentaldifference between automatic and the

    classic spirit level is that in the former

    the line of sight is no longer levelled

    manually using a tubular spirit level, but

    is leveled automatically within a certain

    tilt range. This is achieved by

    compensator in the telescope.

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    Advantage of automatic level

    i) Much simpler to use

    ii) High precisionMean elevation error

    on staff graduated to 5mm division varies

    between +0.5 to 0.8 mm per km of forward

    and backward leveling.

    iii) High speed : For fly leveling the

    progress achieved by various level-wise

    compared.

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    Type of level D(m) 20m 40 60 80 100 120

    Automatic level speed 1.2 2 2.4 2.5 2.6 2.7

    S(Km/hr)

    Tilting level speed 0.6 1.1 1.5 1.7 1.8 1.9

    S(Km/hr)

    S = Speed of leveling in km/hr.

    D = Sighting distance in meter.The speed of Dumpy level is about 25% lower than

    tilting level.

    iv) Freedom from errorsAccuracy is increased by

    an erect telescope image.v) Range of applicationlevel can be used onmedium and large sized projects and setting bench

    marks.

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    Basic components of level :

    1. Telescopeto provide a line of sight

    2. Level Tubeto make line of sight horizontal3. Leveling headto bring the bubble of tube levelat the centre of its run.

    4. Tripodto support the above three parts of thelevel.

    1. TELESCOPE : Telescope is an opticalinstrument used for magnifying and viewing the

    images of distant objects. It consists of two lenses