Levelling :The art of determining relative altitudes ofpoints on the surface of the earth of beneaththe surface of earth is called levelling.It deals with angular and linear measurement ina vertical plane.NEED:For execution of Engineering Projects it is verynecessary to determine elevations of differentpoints along the alignment of proposed project.
APPLICATIONS:i) Taking rail levels .ii) Initial ground levels for earthworkcalculations.iii) Levels for measurement of earthworketc.
BASIC COMPONENTS OF LEVEL:1. Telescope – to provide a line of sight2. Level Tube – to make line of sighthorizontal3. Levelling head – to bring the bubble of tubelevel at the centre of its run.4. Tripod – to support the above three parts ofthe level.1. TELESCOPE : Telescope is an opticalinstrument used for magnifying and viewing theimages of distant objects. It consists of twolenses. The lens fitted near the eye is called theeye piece and the other fitted at the end near tothe object is called the objective lens.
The objective provides a real inverted imageinfront of the eye piece at a distance lesserthan its focal distance.Two essential conditions are involved. :i) The real image of the object, must beformed.Ii) the plane of image must coincide with thatof cross hairs.Focusing of Telescope : The operation ofobtaining a clear image of the object in theplane of cross hairs is known as focusing.
2.Level Tube : Also known as Bubble Tube consistsof a glass tube placed in a brass tube which is sealedwith plaster of paris. Level tube is filled with either oralcohol, the remaining space is occupied by an airbubble. The centre of air bubble always rest at thehighest point of the tube.Outer surface of the bubble tube is graduated in boththe directions from the centre. The line tangential to the circular are at its highestpoint i.e. the middle of tube is called the axis of bubbletube. When the bubble is central the axis of bubblebecomes Horizontal.The level tube is attached on the top of telescope bymeans of capstan headed nuts.
3)Levelling head : Levelling head generallyconsists of two parallel plates with 3 footscrews. Upper plate is known as Tribrach andlower plate is trivet which can be screwed onto the tripod. Levelling head has to perform 3distant functions :i) to support the telescopeii) to attach the level to the tripodiii) to provide a means for level (foot screws)
IMPORTANT TERMS:a) DATUM – or Datum plane is an arbitrarily assumed levelsurface or line with reference to which level of other line orsurface are calculated.b) LINE OF COLLIMATION- - the line which passes throughthe Intersection of the cross hairs of the eye piece and opticalcentre of the objective and its continuation is called as line ofcollimation. This is also known as line of sight.c) REDUCED LEVEL (RL) – Height or depth of a point above orbelow the assumed datum is called Reduced level.
d) BENCH MARK – (BM) – B.M. is a fixed referencepoint of known elevation. It may be of the followingtypes.i) GTS Bench mark (Geodetic Triangulation Survey) :These Bench marks are established by national agencylike Survey of India. They are established with highestprecision. Their position and elevation above MSL isgiven in a special catalogue known as GTS Maps ( 100km. interval).
ii) Permanent Bench Mark : They are fixedpoints of reference establish with reference to GTSBench mark (10 km. interval).iii) Arbitrary Bench mark : These are referencepoints whose elevations are arbitrarily assumed. Inmost of Engineering projects, the difference inelevation is more important than their reduced levelswith reference to MSL as given in a special catalogueknown as GTS Maps ( 100 Km. interval).
e)Mean Sea Level (M.S.L.) : M.S.L. is obtained bymaking hourly observations of the tides at any place overa period of 19 years. MSL adopted by Survey of India isnow Bombay which was Karachi earlier.f) Level Surface : The surface which is parallel to themean sphereoidal surface of the earth is known as levelsurface.g) Line of Collimation : It is the line joining theintersection of the cross hair and the optical center ofthe objective and its extensions, it is also called line ofsight or collimation.
h) Height of Instrument (HI) : The elevation of the line ofsight with respect to assumed datum is known as HI.i) Back sight : (B.S.) - The first sight taken on a levellingstaff held at a point of known elevation. B.S. enables thesurveyor to obtain HI +sight i.e. Height of Instrument orline of sight.j) Fore Sight : (F.S.) – It is the last staff reading taken froma setting of the level. It is also termed as minus sight.Fore sight is the sight taken on a levelling staff held at apoint of unknown elevation to ascertain the amount bywhich the point is above or below the line of sight. Thisis also called minus sight as the foresight reading is alwayssubtracted from height of Instrument.
k) Change Point (CP) : The point on whichboth the foresight and back sight are takenduring the operation of levelling is calledchange point.l) Intermediate Sight (IS) :The foresight taken on a levelling staff held at apoint between two turning points, todetermine the elevation of that point, is knownas intermediate sight.
TYPES OF LEVELS:i) Dumpy levelii) Tilting leveliii) Automatic leveliv) Digital Auto level
DUMPY LEVELThis are the basic levels used in constructionwork.The telescope is attached to a single bubbleand the assembly is adjusted either by meansof a screwed ball-joint or by footscrewswhich are adjusted first in one direction thenat 90 degrees.
ADVANTAGES:Itis simple compact and stable.Fewer adjustments to be made.Simple construction with fewer movable parts.Longer life of the adjustments.DISADVANTAGE:The telescope is rigidly fixed to its support therefore cannot be rotated about its longitudinal axis
TILTING LEVEL It consists of a telescope attached with a leveltube which can be tilted within few degrees invertical plane by a tilting screw.The main peculiarity of this level is that thevertical axis need not be truly vertical, sincethe line of collimation is not perpendicular toit. The line of collimation, is, however, madehorizontal for each pointing of telescope bymeans of tilting screw. It is mainly designedfor precise levelling work.
ADVANTAGES :Helpful in quick levelling.Ball and socket arrangement permits the head to be tilted and quickly locked nearly level.
The Automatic level : Also termed as selfaligning level. It has an compensator whichconsists of an arrangement of three prisms.Thetwo outer ones are attached to the barrel ofthe telescope.The middle prism is suspended byfine wiring and reacts to gravity .The instrumentis first levelled appproximately by the circularbubble ,the compensator then deviate the lineof sight by the amount that the telescope is outof sight.
ADVANTAGES:i) Much simpler to useii) High precision – Mean elevation error on staffgraduated to 5mm division varies between +0.5to 0.8 mm per km of forward and backwardlevelling.iii) High speed : The speed of Dumpy level isabout 25% lower than tilting level. iv) Freedom from errors – Accuracy isincreased by an errect telescope image.v) Range of application – level can be used onmedium and large sized projects and settingbench marks.
DIGITAL LEVEL:It uses digital-electronic image processorusing a charged-coupled device(CCD) fordetermining heights and distances.Its accuracy in rod reading is .5mm and themaximum range is 100m.
ADVANTAGES :Data can be transferred to computer directly.Error in writing of data and calculations can be minimised.DISADVANTAGE:It needs clear and better illuminated area for levelling hence not useful in cloudy area.