# 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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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