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SURVEYING Preamble Surveying 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.
Definition : Operation of making such measurements that the relative position of various 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.
Purpose : The main object of surveying is the preparation of maps or plans which are the basis in planning and design of engineering project such as route location of railway line, roads and water supply scheme.Basic Principles in Surveying : Ruling principle 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 further divided into triangle, which are surveyed with less accuracy.ii) to fix the position of new stations by atleast two independent processes – By linear and Angular
Classification of Surveys : Surveying is divided into two main categories-i) Geodetic Surveyii) Plane surveyi) Geodetic Survey :-When survey extends over a large areas more than 200 sq. km. and degree of accuracy is also great. The curvature of earth is also taken into account. Geodetic survey is used to provide control points to which small surveys can be connected.ii) Plane Survey :-For small projects covering Area less 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).
Classification based upon equipment used:i) Chain surveyii) Compass surveyiii) Theodolite surveyiv) Plane Table surveyv) Tachometric surveyvi) Aerial Photographic Surveyvi) Remote sensing.B) Based upon Method Employed :i) Triangulation : Control points are established through a net-work of triangles.ii) Traversing: Scheme of control points consisting of a series of connected lines.iii) Trilateration: Distances are measured for exercising the control.
Levelling :The art of determining relative altitudes of 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 of different points along the alignment of proposed project.
Other applications are :i) Taking rail levels existing before track renewals to finalise 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) DATUM – or Datum plane is an arbitrarily assumed level surface or line with reference to which level of other line or surface are calculated.
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 elevation above MSL is given in a special catalogue known as GTS Maps ( 100 km. interval).
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 are reference points whose elevations are arbitrarily assumed. In most of Engineering projects, the difference in elevation is more important than their reduced levels with reference to MSL as given in a special catalogue known as GTS Maps ( 100 Km. interval).
Mean Sea Level (M.S.L.) :M.S.L. is obtained by making hourly observations of the tides at any place over a period of 19 years. MSL adopted by Survey of India is now Bombay which was Karachi earlier.e) Level Surface :The surface which is parallel to the mean sphereoidal surface of the earth is known as level surface.f) Line of Collimation :It is the line joining the intersection of the cross hair and the optical center of the objective and its extensions, it is also called line of sight or collimation.
Height of Instrument (HI) :The elevation of the line of sight with respect to assumed datum is known as HI.h) Back sight : (B.S.) -The first sight taken on a levelling 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 at a 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.
k) Change Point (CP) :The point on which both the foresight and back sight are taken during 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, to determine the elevation of that point, is known as intermediate sight.
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 Levelling Equipments:i) Dumpy levelii) Tilting leveliii) Automatic leveliv) Digital Auto levelDumpy level :It is simple compact and stable. The telescope is rigidly fixed to its support therefore cannot be rotated about its longitudinal axis. A long bubble tube is attached to the top of telescope. Dumpy literally means short and thick.
Tilting level :It consists of a telescope attached 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 the vertical axis need not be truly vertical, since the line of collimation is not perpendicular to it. The line of collimation, is, however, made horizontal for each pointing of telescope by means of tilting screw. It is mainly designed for precise levelling work.
The Automatic level : Also termed as self aligning level. The fundamental difference 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 levelled automatically within a certain tilt range. This is achieved by compensator in the telescope.
Advantage of automatic leveli) Much simpler to useii) High precision – Mean elevation error on staff graduated to 5mm division varies between +0.5 to 0.8 mm per km of forward and backward levelling.iii) High speed : For fly levelling the progress achieved by various level-wise compared.
Type of level D(m) 20m 40 60 80 100 120Automatic 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 levelling in km/hr.D = Sighting distance in meter.The speed of Dumpy level is about 25% lower than tilting level.iv)Freedom from errors – Accuracy is increased by an errect telescope image.v) Range of application – level can be used on medium and large sized projects and setting bench marks.
Basic components of level :1. Telescope – to provide a line of sight2. Level Tube – to make line of sight horizontal3. Levelling head – to bring the bubble of tube level at the centre of its run.4. Tripod – to support the above three parts of the level.1. TELESCOPE : Telescope is an optical instrument used for magnifying and viewing the images of distant objects. It consists of two lenses. The lens fitted near the eye is called the eye piece and the other fitted at the end near to the object is called the objective lens.
The objective provides a real inverted image infront of the eye piece at a distance lesser than its focal distance.Two essential conditions are involved. : i) The real image of the object, must be formed.Ii) the plane of image must coincide with that of cross hairs.Focusing of Telescope : The operation of obtaining a clear image of the object in the plane of cross hairs is known as focusing.
1. Diaphram :A frame carrying cross hairs usually made of either silk thread or platinum wire and placed at the plane at which vertical image of the object is formed by the objective. Vertical hair of the diaphram enables the surveyor to check the verticality of levelling staff whereas horizontal hairs are used to read the staff graduations.2.Level Tube :Also known as Bubble Tube consists of a glass tube placed in a brass tube which is sealed with plaster of paris. The whole of the interior surface or the upper half is accurately ground so that its longitudinal section, is an arc of a circle. Level tube is filled with either or alcohol, the remaining space is occupied by an air bubble. The centre of air bubble always rest at the highest point of the tube.
Outer surface of the bubble tube is graduated in both the directions from the centre.The line tangential to the circular are at its highest point i.e. the middle of tube is called the axis of bubble tube. When the bubble is central the axis of bubble becomes Horizontal.The level tube is attached on the top of Telescope by means of capstan headed nuts.
Levelling head :Levelling head generally consists of two parallel plates with 3 foot screws. Upper plate is known as Tribrach and lower plate is trivet which can be screwed on to the tripod. Levelling head has to perform 3 distant functions :i) to support the telescopeii) to attach the level to the tripodiii) to provide a means for level (foot screws)
Adjustment of level :i) Temporarily Adjustments – adjustments which are made for every setting of a level.ii) Permanent adjustments- required if some error is there in instrument. i) Temporary Adjustments : includes a) setting up the level b) levelling up c) elimination of parallaxa) Setting up the level :This operation includes fixing the instrument on the tripod and also approximate levelling by leg adjustment.
b) Levelling up :Accurate levelling is done with the help of foot screws and by using plate levels. The object of levelling up the instrument is to make its vertical axis truly vertical.c) Elimination of parallax :If the image formed by the objective does not lie in the plane of the cross hairs, there will be a shift in the image due to shift of the eye. Such displacement of image is termed as parallax. Parallax is removed in two stages.1) Focusing the eye for distinct vision of cross hairs.2) Focusing the objective so that image is formed in the plane of cross hairs.
Principles of levelling :a) Simple levelling : The operation of levelling for determining the difference in elevation, if not too great between two points visible from single position of the level is known as simple levelling.PROCEDURE : Following steps are involved.
Level the instrument correctly.2. Direct the telescope towards the staff held3. Take the reading of Central, horizontal hair of the diaphram, where it appears to cut the staff ensuring that the bubble is central.4. Send the staff to next point5. Direct the telescope towards C and focus it again6. Check up the bubble if central, if not bring it to the Central position by the foot screw nearest to the telescope.7. Take the reading of Central Horizontal cross hair.
b) Differential levelling or fly levelling :This method is used in order to find the difference in elevation between two points.i) If they are too far apartii) if the difference in elevation between them is too great.iii) If there are obstacles intervening. In such case it is necessary to set up the level in several positions and to work in series of stages.
The difference of level of the points A&B is equal to the algebraic sum of these difference between the sum of back sights and sum of the fore sights i.e. ΣBS - Σ FSBooking and reduction of the levels may be done by following 2 methods.i) Rise and fall methodii) Height of collimation method
RISE AND FALL METHOD –In this method, the difference of level between two consecutive points for each setting of the instrument is obtained by comparing their staff readings. The difference between their staff readings indicates a rise if back sight is more than foresight and a fall if it is less than foresight. The Rise and Fall worked out for all the points given the vertical distances of each point relative to the proceeding one. If the RL of the Back staff point is known, then RL of the following staff point may be obtained by adding its rise or substracting fall from the RL of preceding point.
Height of Collimation Method:In this method Height of Instrument (H.I.) is calculated for each setting of the instrument by adding the back sight (B.S.) to the elevation of B.M.Height of instrument (H.I.) = R.L. of the plane of collimation = R.L. of B.M.+B.S.
RL of a point = H.I. – FSOr = H.I. – IS - After every back sight, there may be many intermediate sights but there must be only one foresight.- The B.S. & F.S. forms the two ends of one stage in levelling.- Levelling should always commence from a permanent B.M. and end on a permanent B.M.
THEODOLITE - An instrument used for measuring horizontal and vertical angles accurately is known as theodolite.Uses of Theodolitei) Measurement of Horizontal and vertical angles.Ii) Setting out lines and anglesiii) Optical distance measurementiv) Plumbing tall buildingv) Setting out of Railway curvesvi) Locating the position of piers for Bridge etc.vii) Geographical position fixing from observation of sun and stars.viii) Alignment control in tunnel construction.
Types of Theodolite : Transit theodolites are categorised into 3 types :1. Vernier theodolite2. Optical Reading Theodolite3. Digital Theodolite/Electronic TheodoliteBasically Transit Theodolite are those in which the telescope can revolve through a complete revolution about its Horizontal axis in vertical plane.
Components of Transit theodolite –Transit theodolite consists of the following parts :1. Levelling Head2. Lower Plate or Scale Plate3. Upper Plate or Vernier Plate4. The standard or A Frame5. T-Frame or Index Bar.6. Plate Levels7. Telescope
Levelling Head -Levelling Head consists of upper tribrach and lower Tribrach. Upper tribrach has three arms, each arm carries a levelling screw for levelling the equipment. Lower tribrach has got a circular hole through which a plumb bob may be suspended for centering.Three distinct functions of levelling head are:i) to support the main part of the instrumentii) to attach the Theodolite to the Tripodiii) to provide a means for levelling the theodolite
Lower Plate (Scale Plate) : Lower Plate which is attached to outer spindle, carries a horizonta graduated circle, it is graduated from 0-360. Each degree is further divided into 10 minutes or 20 minutes. Scale plate can be clamped to any position by a clamping screw and a corresponding slow motion screw.When the lower plate is tightened, the lower plate is fixed to the upper tribrach of the levelling head. The size of the Theodolite is determined by the size of the diameter of this lower plate.
Upper plate or Vernier Plate :Upper plate is attached to Inner spindle axis. Two verniers are screwed to the upper plats. It carries an upper clamp screw and tangent screw. On clamping the upper clamp and unclamping the lower clamp, the instrument may be rotated on its outer spindle without any relative motion between the two plates. On the other hand if lower clamp screw is tightened and upper clamp screw is unclamped, the instrument may be rotated about its inner spindle with a relative motion between the vernier and graduated scale of the lower plate. This property is utilised for measuring angles.
4. Plate Levels -Upper plates carries two platelevels placed at right angles to each other. One of the plate bubble is kept parallel to the trunion axis. Plate levels can be centred with the help of foot screws.5. Telescope –Telescope is supported on the pivots of the trunion axis which affords its movement in the vertical plane.IMPORTANT DEFINITIONS –I) Line of Collimation - the line which passes through the Intersection of the cross hairs of the eye piece and optical centre of the objective and its continuation is called as line of collimation. This is also known as line of sight.
Transiting -The process of turning the telescope in vertical plane through 180 deg. about its horizontal axis is known as transiting.iii) Swing -A continuous motion of telescope about the vertical axis in horizontal plane is called swing. The swing may be in either direction i.e. Right swing or left swing.
iv) Face left observation –When vertical circle is on the left of the telescope at the time of observation, the observations are called face left observation.v) Face right observation –When vertical circle is on the right of the telescope at the time of observation.TEMPORARY ADJUSTMENT OF THEODOLITE1) Setting up the Theodolite over the station2) Levelling up the theodolite3) Elimination of the parallax
Setting up :Operation of setting up a theodolite includes:a) centering the theodolite over the ground markb) approximate levelling with the help of tripod legs.2. Levelling up of theodoliteThe operation of making the vertical axis truly vertical is known as levelling of Theodolite.i) Turn the horizontal plate until the longitudinal axis of the plate level is approximately parallel to a line joining any two levelling screws.
ii) Bring the bubble to the centre of its run by turning both foot screws simultaneously in opposite directions either inwards or outwards. The movement of the left thumb indicates the direction of movement of bubble.iii) Turn the instrument through 1800 in azimuth.iv) Note the position of the bubble. If it occupies a different position, move it by means of the same two foot screws to the approx. mean of the two positions.v) Turn the theodolite through 90 in azimuth so that the plate level becomes perpendicular to the previous position.vi) With the help of third floor screw, move the bubble to the approx. mean position already indicated.
vii) Repeat the process until the bubble, retains the same position for every setting of the instrument.3. Elimination of Parallax :Elimination of parallax may be done by focusing the eye piece for distinct vision of cross hairs and focusing the objective to bring the image of the object in the plane of cross hairs.Measurement of Horizontal AngleProcedure : to measure a Horizontal Angle ABC between BA & BC the following procedure is followed.
Set up, Centre and level the theodolite over the ground point B.2. Loosen the upper plate, set the vernier to read zero and clamp the upper clamp.3. Loosen the lower plate and swing the telescope until the left point A is sighted. Tighten the lower clamp. Accurate bisection of the arrow held on the Point A is done by using the lower tangent screw. Read both the vernier and take the mean of the reading.
4. Unclamp the upper plate and swing the telescope in clockwise direction until point C is brought in the field of view. Tighten the upper clamp and bisect the mark of C accurately, using the upper clamp tangent screw.5. Read both the verniers and take the mean of readings. The difference of the means of the reading to C to A is the required angle ABC.
6. Change the face of the instrument and repeat the show procedure, the measure of the angle is again obtained by taking the difference of the means of the readings to C&A on face right.7. The mean of the two measures of the angle ABC on two faces is the required value of the angle ABC.
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