TYPES OF LEVELSTYPES OF LEVELS

BY SUMIT M.

MESHRAM

Levelling :Levelling :The art of determining relative The art of determining relative altitudes of points on the surface of altitudes of points on the surface of the earth of beneath the surface of the earth of beneath the surface of earth is called levelling.earth is called levelling.

It deals with angular and linear It deals with angular and linear measurement in a vertical plane.measurement in a vertical plane.

NEED:NEED:For execution of Engineering Projects it For execution of Engineering Projects it is very necessary to determine is very necessary to determine elevations of different points along the elevations of different points along the alignment of proposed project.alignment of proposed project.

APPLICATIONS:i) Taking rail levels .ii) Initial ground levels for earthwork calculations.iii) Levels for measurement of earthwork etc.

COMPONENTS OF LEVELCOMPONENTS OF LEVEL

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.

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. 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.

3)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)

IMPORTANT TERMS: IMPORTANT TERMS: a) DATUM – a) DATUM – or Datum plane is an arbitrarily assumed or Datum plane is an arbitrarily assumed level surface or line with reference to which level of level surface or line with reference to which level of other line or surface are calculated.other line or surface are calculated.

b) LINE OF COLLIMATIONb) LINE OF COLLIMATION-- - the line which passes - the line which passes through the Intersection of the cross hairs of the eye through the Intersection of the cross hairs of the eye piece and optical centre of the objective and its piece and optical centre of the objective and its continuation is called as line of collimation. This is continuation is called as line of collimation. This is also known as line of sight. also known as line of sight.

cc) REDUCED LEVEL (RL) –) REDUCED LEVEL (RL) – Height or depth of a point Height or depth of a point above or below the assumed datum is called above or below the assumed datum is called Reduced level. Reduced level.

d) BENCH MARK –d) BENCH MARK – (BM) – (BM) – B.M. is a fixed B.M. is a fixed reference point of known elevation. It may reference point of known elevation. It may be of the following types.be of the following types.i) GTS Bench mark (i) GTS Bench mark (Geodetic Triangulation Geodetic Triangulation Survey) :Survey) :These Bench marks are established by These Bench marks are established by national agency like Survey of India. They national agency like Survey of India. They are established with highest precision. Their are established with highest precision. Their position and elevation above MSL is given in position and elevation above MSL is given in a special catalogue known as GTS Maps a special catalogue known as GTS Maps ( 100 km. interval).( 100 km. interval).

ii) ii) Permanent Bench Mark :Permanent Bench Mark : They They are fixed points of reference establish with are fixed points of reference establish with reference to GTS Bench mark (10 km. reference to GTS Bench mark (10 km. interval).interval).

iii) Arbitrary Bench mark :iii) Arbitrary Bench mark : These are These are reference points whose elevations are reference points whose elevations are arbitrarily assumed. In most of Engineering arbitrarily assumed. In most of Engineering projects, the difference in elevation is more projects, the difference in elevation is more important than their reduced levels with important than their reduced levels with reference to MSL as given in a special reference to MSL as given in a special catalogue known as GTS Maps ( 100 Km. catalogue known as GTS Maps ( 100 Km. interval).interval).

e)Mean Sea Level (M.S.L.) :e)Mean Sea Level (M.S.L.) : M.S.L. is M.S.L. is obtained by making hourly observations of obtained by making hourly observations of the tides at any place over a period of 19 the tides at any place over a period of 19 years. MSL adopted by Survey of India is now years. MSL adopted by Survey of India is now Bombay which was Karachi earlier.Bombay which was Karachi earlier.

f) Level Surface :f) Level Surface : The surface which is The surface which is parallel to the mean sphereoidal surface of parallel to the mean sphereoidal surface of the earth is known as level surface.the earth is known as level surface.

g)g) Line of Collimation : Line of Collimation : It is the line joining It is the line joining the intersection of the cross hair and the the intersection of the cross hair and the optical center of the objective and its optical center of the objective and its extensions, it is also called line of sight or extensions, it is also called line of sight or collimation.collimation.

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

i) Back sight : (B.S.) -i) Back sight : (B.S.) - The first sight taken on The first sight taken on a levelling staff held at a point of known a levelling staff held at a point of known elevation. B.S. enables the surveyor to obtain elevation. B.S. enables the surveyor to obtain HI +sight i.e. Height of Instrument or line of HI +sight i.e. Height of Instrument or line of sight.sight.jj) Fore Sight : (F.S.) –) Fore Sight : (F.S.) – It is the last staff reading It is the last staff reading taken from a setting of the level. It is also taken from a setting of the level. It is also termed as minus sight.termed as minus sight.Fore sight is the sight taken on a levelling Fore sight is the sight taken on a levelling staff held at a point of unknown elevation to staff held at a point of unknown elevation to ascertain the amount by which the point is ascertain the amount by which the point is above or below the line of sight. This is also above or below the line of sight. This is also called minus sight as the foresight reading is called minus sight as the foresight reading is always subtracted from height of Instrument.always subtracted from height of Instrument.

k) Change Point (CP) :k) Change Point (CP) : The point on The point on which both the foresight and back which both the foresight and back sight are taken during the operation sight are taken during the operation of levelling is called change point.of levelling is called change point.

l) Intermediate Sight (IS) :l) Intermediate Sight (IS) :The foresight taken on a levelling The foresight taken on a levelling staff held at a point between two staff held at a point between two turning points, to determine the turning points, to determine the elevation of that point, is known as elevation of that point, is known as intermediate sight.intermediate sight.

TYPES OF LEVELS:TYPES OF LEVELS:

i) i) Dumpy levelDumpy level

ii) Tilting levelii) Tilting level

iii) Automatic leveliii) Automatic level

iv) Digital Auto leveliv) Digital Auto level

DUMPY LEVELDUMPY LEVEL

This are the basic levels used in construction work.

The telescope is attached to a single bubble and the assembly is adjusted either by means of a screwed ball-joint or by footscrews which are adjusted first in one direction then at 90 degrees.

DUMPY LEVELDUMPY LEVEL

ADVANTAGES:ADVANTAGES:

It is 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

TILTINGTILTING LEVELLEVEL

It consists of a telescope attached It consists of a telescope attached with a level tube which can be tilted with a level tube which can be tilted within few degrees in vertical plane within few degrees in vertical plane by a tilting screw.by a tilting screw.

The main peculiarity of this level is The main peculiarity of this level is that the vertical axis need not be that the vertical axis need not be truly vertical, since the line of truly vertical, since the line of collimation is not perpendicular to it. collimation is not perpendicular to it. The line of collimation, is, however, The line of collimation, is, however, made horizontal for each pointing of made horizontal for each pointing of telescope by means of tilting screw. telescope by means of tilting screw. It is mainly designed for precise It is mainly designed for precise levelling work.levelling work.

ADVANTAGES :ADVANTAGES :Helpful in quick levelling.Ball and socket arrangement

permits the head to be tilted and quickly locked nearly level.

AUTO LEVELAUTO LEVEL

The Automatic level : The Automatic level : Also termed as Also termed as self aligning level. It has an self aligning level. It has an compensator which consists of an compensator which consists of an arrangement of three prisms.The two arrangement of three prisms.The two outer ones are attached to the barrel outer ones are attached to the barrel of the telescope.The middle prism is of the telescope.The middle prism is suspended by fine wiring and reacts to suspended by fine wiring and reacts to gravity .The instrument is first levelled gravity .The instrument is first levelled appproximately by the circular appproximately by the circular bubble ,the compensator then deviate bubble ,the compensator then deviate the line of sight by the amount that the line of sight by the amount that the telescope is out of sight.the telescope is out of sight.

ADVANTAGES:ADVANTAGES:i)i) Much simpler to use Much simpler to useii) ii) High precisionHigh precision – Mean elevation error – Mean elevation error on staff graduated to 5mm division on staff graduated to 5mm division varies between +0.5 to 0.8 mm per km varies between +0.5 to 0.8 mm per km of forward and backward levelling.of forward and backward levelling.iii) iii) High speedHigh speed : The speed of Dumpy : The speed of Dumpy level is about 25% lower than tilting level is about 25% lower than tilting level.level. iv) iv) Freedom from errorsFreedom from errors – Accuracy – Accuracy is increased by an errect telescope is increased by an errect telescope image. image. v) Range of applicationv) Range of application – level can be – level can be used on medium and large sized used on medium and large sized projects and setting bench marks. projects and setting bench marks.

DIGITAL LEVEL:DIGITAL LEVEL:

It uses digital-electronic image processor using a charged-coupled device(CCD) for determining heights and distances.

Its accuracy in rod reading is .5mm and the maximum range is 100m.

ADVANTAGES :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.

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