SURVEYING

LECTURE-1

Introduction

• Surveying is defined as the science of making measurements of the earth specifically the surface of the earth.

• This is being carried out by finding the spatial location (relative / absolute) of points on or near the surface of the earth.

The primary aims of field surveying are :• To measure the Horizontal Distance between

points.• To measure the Vertical elevation between

points.• To find out the Relative direction of lines by

measuring horizontal angles with reference to any arbitrary direction and

• To find out Absolute direction by measuring horizontal angles with reference to a fixed direction.

Objectives of Surveying

• To collect field data;• To prepare plan or map of the area surveyed;• To analyse and to calculate the field

parameters for setting out operation of actual engineering works.

• To set out field parameters at the site for further engineering works.

Classification of Surveying

• Based on Accuracy Desired:-1.Plane Survey-2.Geodetic Survey-

Based on Purpose of survey

1. Engineering survey2. Defence survey3. Geological survey4. Geographical survey5. Mine survey6. Route survey

Based on place of survey

1.Land survey2.Hydrographical Survey :- Determine shape of

area under water surface like Lakes, streams.3.Underground Survey4.Aerial Survey-used for large scale map , used

where ground survey will be slow or difficult.

Based on Instrument used

1. Chain Survey – linear measurement2. Traverse Survey – Direction or angle measure3. Tacheometry – using transit equipped with

special telescope having stadia wires and analytical lens.

4. Levelling5. Plane Table Surveying6. Triangulation

•  Geographical survey : To prepare a plan/ map of a region which includes natural as well as and man-made features including elevation.

•  Route survey : To plan, design, and laying out of route such as highways, railways, canals, pipelines, and other linear projects.

• Construction surveys : Surveys which are required for establishment of points, lines, grades, and for staking out engineering works (after the plans have been prepared and the structural design has been done).

• Mine surveys : To carry out surveying specific for opencast and under ground mining purposes.

•  Route survey : To collect data for planning, design and execution of engineering projects. Three broad steps are1)  Reconnaissance survey : To explore site

conditions and availability of infrastructures.2) Preliminary survey : To collect adequate data

to prepare plan / map of area to be used for planning and design.3)  Location survey : To set out work on the

ground for actual construction / execution of the project.

SURVEYING

• GENERAL PRINCIPLE :1. Working from “ WHOLE TO THE PART “ To localise the errors and To control the accumulation of errors.

2. To locate a New Point by at least two reference known measurements.

Divisions of Surveying

Based upon the consideration of the Accuracy and shape of the earth, surveying is broadly divided into two types.

• Geodetic Surveying• Plane Surveying

Geodetic Surveying

• In this branch of surveying, the true shape of the earth is taken into consideration.

• This type of surveying is being carried out for highly precise work and is adopted for surveying of large area.

Plane Surveying

• In this method of surveying, the mean surface of the earth is considered to be a plane surface. This type of survey is applicable for small area (less than 200 square kilometer).

• Thus for most of the Civil Engineering projects, methods of plane surveying are valid.

Accessories for linear measurement

PegsArrowsRanging RodsOffset RodsPlumb BobChainsTapes

Chains• Metric chain- To enable the reading of a chain,

tallies are fixed at every five meter length and small brass rings at every meter length.

• Steel band chain-It is 20 or 30long and 16 mm wide.

• Gunter’s Chain-It is 66 fit long and is divided into 100 links.

• Revenue Chain-used for measuring fields in cadastral survey

• Engineers chain-It is 100 ft long divided into 100 links.

Tapes

• Cloth or linen Tape• Metric Woven Metallic Tape• Metric steel Tape• Invar tape- used for highest precision work,

made of an alloy of steel and nickel. It is 6 mm wide and may be length of 30m and 100m.

• Synthetic Tape- made of glass fibre.

Linear measurement on sloping ground

• Two methods for getting the horizontal distance b/w two points:-

1.Direct Method:- used for Irregular slope.2.Indirect Method:-used for regular or even

slope. a) Angle measured b) Difference in level measured c) Hypotenusal allowance

SCALES

• Scale fixed ratio that every distance on the map bears with corresponding distance on the ground.

1.One cm on the plan represents some whole number of meters on the ground , Called engineering scale.

2.Representative fraction:- ratio of map distance to ground distance ie units less measurement.

3. Graphical scale:-Typically referred to as bar scales have the advantage of accurately representing scale, enlargements and reductions do not affect the accuracy of the scales

TYPES OF SCALES

1. Plain Scale:-

2. Diagonal Scale

• 1-1 represent 1/10 PQ• 2-2 represent 2/10 PQ• …………………………………• …………………………………• 9-9 represent 9/10 PQ

3. THE VERNIER SCALE

• The vernier scale is a clever way of improving the precision of a reading on an instrument scale.

• The scale has been especially useful for the so very fine angle scales of a theodolite.

• Least count of the vernier = the difference between smallest division on the main division and smallest division on the vernier scale.

• Direct virnier:- least count = s/nS = value of one smallest division on main scaleV= value of one smallest division on the vernierN= number of divisions on the vernier.V= ((n-1)/n)*s

SHRUNK SCALE

“Shrunk scale = shrinkage factor * original scale” shrinkage factor is equal to ratio of shrunk

length to the actual length.

Correction for measured length

• Cumulative errors :- These errors always accumulate in one direction and are serious in nature. They affect the survey work considerably.

• Compensative errors• These errors occur in either direction and are

likely to compensate.• They are classified as follows:• Positive error• Negative error

ERRORS

• Positive error• These errors make the measured length more

than the actual length which results into wrong calculations by the Surveyor.

• Negative errors• These errors make the measured length less

than the actual length.

Errors in chains

1. Erroneous length of chain or tape (Cumulative +,-)2. Bad Ranging (Cumulative , +)3. Care less holding and marking(Cumulative, +,-)4. Bad Straightening (Cumulative +)5. Non-horizontality (Cumulative +)6. Sag in chain(Cumulative +)7. Variation in temperature (Cumulative + , -)8. Variation in pull (Compensating & cumulative +,-)9. Personal mistake.

• NOTE:- Cumulative errors are more important than compensating errors.

CORRECTIONS IN TAPE OR CHAIN

Correction for absolute length Correction for pull Correction for sag Correction for temperature Correction for Slope

Correction for absolute length

• Absolute length of tape is its actual length under specified condition

• Ca = L.C/l• Where, Ca = correction for absolute length ‘m’• L = measured length in ‘m’• l = Designed or nominal length of tape in ‘m’• C = correction per tape length.

• Correction for TemperatureLet T0 = Temperature at which tape is standardizedTm = Mean temperature during measurementα = Coefficient of thermal expansion of the material of the tape andL = Measured length,Then the temperature correction Ct is given by,Ct = α.(Tm – T0).L

Corrections for Pull

• If pull applied while standardizing the length of tape and pull applied in the field are different, then correction is required.Let, P0 = Standard pullP = Pull applied in the field (N)A = Cross-sectional area of the tape(cm2)L = Measured length of line(m)E = Young’s modulus of the material of tape (N/cm2), then

• Cp = [(P-P0)L] / AE

• Correction for Sag• When the tape is suspended from two ends in

air, A tape not supported along its length will sag and form a catenary between supports.

• Sag correction, Cs1 = L1(W1)2/24P2• Where, L1 = horizontal distance between

supports in meters• W1 = weight of tape in N = w.L1

• P = Pull applied in N• w = wt. of tape per unit length (N/m)

• Correction for SlopeIf the length measured is ‘L’ and the difference in the levels of first and the last point is ‘h’, then slope correction

OBSTACLES IN CHAINING

1. OBSTACLES IN RANGING 2. OBSTACLES IN CHAINING3. OBSTACLES IN BOTH CHAINING AND

RANGING

OBSTACLES IN RANGING

• Two cases:-1.Both ends of line may be visible from

intermediate points2.Both ends point may not be visible from

intermediate point

OBSTACLES IN CHAINING

• Two cases:-1.Possible to chain round the obstacle like pond.2.Not possible to chain round the obstacle like

river.

OBSTACLES IN BOTH CHAINING AND RANGING

• This problem lies in prolonging line beyond the obstacle and determine the distance across it.

Cross Staff 

• This consists of two pairs of vanes set at right angle to each other with a wide and narrow slit in each vane.

• The instrument is mounted upon a pole, so that when it is set up it is at normal eye level.

• It is also used for setting out lines at right angle to the main chain line.