SURVEYING

General purpose of Surveying –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. To locate correct position of any point in horizontal and vertical plane. 

Definition of Survey : 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.

SURVEYING - Normally determining position in Horizontal plane is called surveying. Art of making measurement and determining the position of point in horizontal plane. Surveying is done by various instruments such as surveyor’s chain, compass, measuring tape, ranging rods, pegs etc.

LEVELLING - The art of determining relative altitudes of points on the surface of the earth or beneath the surface of earth is called LEVELLING. Determining the position of point in vertical plane. Determining relative heights or depth 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).

d)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.
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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.

g) 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 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.

ii) 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.

iii) 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.

iv) Digital Auto level

Advantage of automatic level

i) Much simpler to use

ii) 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  100 120

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 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 erect 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 sight

2. Level Tube – to make line of sight horizontal

3. 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 telescope

ii) to attach the level to the tripod

iii) to provide a means for level (foot screws)

Adjustment of level :

i) Temporarily Adjustments – adjustments which are made for every setting of alevel. 

ii) Permanent adjustments- required if some error is there in instrument.

i) Temporary Adjustments : includes

a) 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.

1. Level the instrument correctly.

2. Direct the telescope towards the staff held

3. 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 point

5. Direct the telescope towards C and focus it again

6. 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 apart

ii) 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. Σ BS - Σ

FS

Booking and reduction of the levels may be done by following 2 methods.

i) 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 ofeach 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.

ii) 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. – FS     
Or = 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.             

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 at-least two independent processes – By
linear and Angular

Classification of Surveys : Surveying is divided into two main categoriesi)
i)Geodetic Survey
ii) Plane survey

i) 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).

A) Classification based upon equipment used:

i) Chain survey
ii) Compass survey
iii) Theodolite survey
iv) Plane Table survey
v) Tachometric survey
vi) Aerial Photographic Survey
vi) Remote sensing.
vii)Hydrographic survey.

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.

CHAIN SURVEY – sides of various triangles are measured directly on fairly plane ground. No angular measurement is taken. Longest chain line of the survey is called Base line. Base line should be straight and should be at the centre of area. Line joining the apex of a triangle to some fixed points is called proof line or check line. Line joining some fixed point on main survey line is called tie line.
There are two types of offsets, namely – perpendicular offsets and oblique offsets.

COMPASS SURVEY - Magnetic compass and Prismatic compass are used for  measuring bearings of lines on horizontal plane. Theodolite and Total Stations are  used for measuring vertical as well as horizontal angles.

Whole circle bearing (WCB) – bearing is measured clockwise from the north direction towards the line right round the circle. The value varies between 00 to 3600 . The bearings observed with a Prismatic compass or a Theodolite are the WCB.

Reduced bearing (RB) - the bearing is measured clockwise or anticlockwise from
north or south point whichever is nearer the line towards the east or west. Therefore the plane around the station is divided into four quadrants by the lines at right angles to each other. When the WCB of a line exceeds 900 it must be reduced to corresponding angle less than 900 (NE, SE, NW and SW).

Fore bearing and Back bearing – the bearing of a line in the direction of the progress of the survey is called Fore bearing while in opposite direction is known as Back bearing. Fore and Back bearing of any line always differ by 1800 .

Back bearing = Fore bearing + 180

Plane Table Surveying – it is a graphical method of surveying in which the field work and plotting are done simultaneously. It is most suitable for filling in details
between the stations previously fixed by triangulation where much accuracy is not required.

Theodolite – It consists of a telescope by means of which distant objects are sighted. It is one of the most accurate instrument for measurements of horizontal and vertical angles.

The telescope has two distinct motions, one in horizontal plane and other in vertical plane. The horizontal angle is measured on a graduated horizontal circle by means of a set of verniers and vertical angles on a vertical circle by two verniers. It can be used for locating points on line, prolonging survey lines, establishing grades, difference in elevations etc.

Total Stations – Why it is called total station ?

Gives total solution.

Basic Principle

A total station integrates the functions of a theodolite for measuring angles, an EDM for measuring distances, digital data and a data recorder. Examples of total
stations are the Sokkia Set4C and the Geodimeter 400 series. All total stations have similar constructional features regardless of their age or level of technology, and all perform basically the same functions.

Total solution for surveying work,
Most accurate and user friendly,
Gives position of a point (x, y and z) w. r. t. known point (base point),
EDM is fitted inside the telescope, Digital display,
On board memory to store data,
Compatibility with computers,
Measures distance and angles and displays coordinates,
Auto level compensator is available,
Can work in lesser visibility also,
Can measure distances even without prismatic target for lesser distances,
Is water proof,
On board software are available,
Can be used for curve layout after feeding data.
New total stations have atmospheric correction, and auto-focus. In addition, these series incorporates a quick distance measuring mode and a high data storage capacity for increased productivity.

The new Total station gives the unique opportunity for long range distance monitoring of up to 9000m to a single prism. Using the scan functionality of software allows fully automated monitoring of the prism in direction of the line of sight.

Total Stations can be used for:

General purpose angle measurement
• General purpose distance measurement
• Provision of control surveys
• Contour and detail mapping
• Setting out and construction work

Factors influencing the use of Total Stations:

• A clear line of sight between the instrument and the measured points is essential.
• The precision of the instrument is dependent on the raw repeatabilities of the direction and distance measurements.
• A well defined measurement point or target/prism is required to obtain optimal
precision and accuracy.
• The accuracy of direction and distance measurement is subject to a number of instrumental errors and the correct field procedures.

Auxiliary Equipment Required

• Targets or Prisms to accurately define the target point of a direction
measurement.
•  A data recorder if one is not integrated into the total station.
•  A download cable and software on a PC to capture and process the captured
digital data to produce contour and detail maps.

ROBOTIC TS

• Display at target also,

• No need of operator on station,

• Moves automatically to predetermined direction and focuses automatically at target at specified distance,

• Can be integrated with GPS also. 

it is a recent and most accurate type of electronic instrument comprising mainly an EDM, an electronic digital Theodolite and computer. An electronic distance meter (EDM) is mounted co-axially with a digital Theodolite as a single unit. Thus it is capable of measuring angles and distances electronically. Total Stations are operated using a multi-function keyboard which is connected to a microprocessor built in the instrument. Because of this arrangement, it is possible to carry out all observations required at a particular work station such as co-ordinate points as well as inaccessible points viz. chimney, towers, building tops etc.