Surveying principal and method of Civil Engineering

 Surveying is the process of determining the relative positions of different objects on the surface of the earth by measuring horizontal distances between them and preparing a map to any suitable scale.

Measurements are taken in the horizontal plane only. Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurements of distance, direction and elevation.

Levelling is a branch of surveying the object of which is to find the elevations of points with respect to a given or assumed datum and to establish points at a given elevation or at different elevations with respect to a given or assumed datum.

The survey is performed to prepare a map of relative positions on the surface of the earth. It shows the natural feature of a country such as towns, villages, roads, railways, etc. They may also include details of different engineering works such as bridge, canal, dam etc.

Before commencement of any major engineering project, the survey is carried out to determine the location of the site and its surrounding. To understand the topography of the site and natural resource availability. A survey is carried out to prepare the cadastral map showing property boundaries.

Principles of Surveying

The fundamental principle upon which the various methods of plane surveying are based can be stated under the following two aspects.

Location of a point by measurement from two points of reference

According to this principle, the relative position of a point to be surveyed should be located by measurement from at least two points of reference, the positions of which have already been fixed.

If P and Q are the two reference points on the ground, any other point, such as R, can be located by any of the direct methods shown in the above figures. But, although a single method is sufficient to locate the relative position of ‘R’ with respect to reference points P and Q, it is necessary to adopt at least any two methods to fix the position of point ‘R’.

While the measurements made in either of the first method or second method will be helpful in locating the point ‘R’, the measurements made in the other method will act as a check.

Working from whole to part

According to this principle, it is always desirable to carry out survey work from whole to part. This means, when an area is to be surveyed, first a system of control points is to be established covering the whole area with very high precision. Then minor details are located by less precise methods.

The idea of working this way is to prevent the accumulation of errors and to control and localize minor errors which, otherwise, would expand to greater magnitudes if the reverse process is followed, thus making the work uncontrolled at the end.

Method of Surveying in Civil Engineering

Primary types of Surveying are:

• Plane surveying
• Geodetic surveying

1. Plane surveying

Plane surveying is conducted by state agencies as well as private agencies. As we know earth is spherical in shape but its diameter is big enough to consider a plane in small dimensions. It is that type of surveying in which the mean surface of the earth is considered as a plane and the spheroidal shape is neglected. All triangles formed by survey lines are considered plane triangles. The level line is considered straight and the plumb lines are considered parallel. Plane surveying is done of the area of survey is less than 250 km2.

2. Geodetic surveying

The geodetic survey is conducted by the survey department of the country. It is that type of surveying in which the curved shape of the earth is taken into account. The object of geodetic survey is to determine the precise position on the surface of the earth, of a system of widely distant points which form control stations in which surveys of less precision may be referred. . Line joining two points is considered as curved line and angles are assumed as spherical angles. It is carried out if the area exceeds over 250 km2.

Secondary classification of Surveying

Surveys may be classified based on the nature of the field of survey, the object of survey and instruments used.

1) Surveying based on Nature of Survey

a) Topographical Surveys

They are carried out to determine the position of natural features of a region such as rivers, streams, hills etc. and artificial features such as roads and canals. The purpose of such surveys is to prepare maps and such maps are called topo-sheets.

b) Hydrographic Survey

The hydrographic survey is carried out to determine M.S.L. (Mean Sea Level), water spread area, depth of water bodies, the velocity of flow in streams, the cross-section area of flow etc.

c) Astronomical Survey

The Astronomical Survey is carried out to determine the absolute location of any point on the surface of the earth. The survey consists of making observations of heavenly bodies such as stars.

d) Engineering Survey

This type of survey is undertaken whenever sufficient data is to be collected for the purpose of planning and designing engineering works such as roads, bridges and reservoirs.

e) Archeological Survey

This type of survey is carried out to gather information about sites that are important from archaeological considerations and for unearthing relics of antiquity.

f) Photographic Survey

In this type of survey, information is collected by taking photographs from selected points using a camera.

g) Aerial Survey

In this type of survey data about large tracks of land is collected by taking photographs from an aero-plane.

h) Reconnaissance Survey

In this type of survey, data is collected by marking physical observation and some measurements using simple survey instruments.

2) Surveying based on Type of Instruments

a) Chain Surveying

Chain surveying is the basic and oldest type of surveying. The principle involved in chain survey is triangulation. The area to be surveyed is divided into a number of small triangles. Angles of triangles must not be less than 30 degrees and greater than 120 degrees. Equilateral triangles are considered to be ideal triangles. No angular measurements are taken, tie line and check lines control accuracy of the work.

This method is suitable on level ground with little undulations and the area to be survey is small.

b) Compass Surveying

Compass survey uses the principle of traversing. This method does not require the need to create triangles. It uses a prismatic compass for measuring the magnetic bearing of the line and the distance is measured by the chain. A series of connecting lines is prepared using a compass and measuring distances using a chain. Interior details are located using offset from main survey lines.

They suitable for large area surveying crowded with many details. It can be used to survey a river course.

c) Plane Table Surveying

The principle of the plane table survey is parallelism. They are plotted directly on paper with their relative position. The rays are drawn from the station to the object on the ground. The table is placed at each of the successive station parallels to the position of the last station.

They are basically suitable for filling interior detailing and is recommended when great accuracy is not required.

d) Theodolite Surveying

The theodolite is an instrument used mainly for accurate measurement of the horizontal and vertical angles. They are accurate to measure up to 10″ or 20″ angles.

Theodolite can be used to measure:

• Horizontal angles
• Vertical angles
• Deflection angle
• Magnetic bearing
• The horizontal distance between two points
• Vertical height between two points
• Difference in elevation

Nowadays theodolite is shadowed and replaced by the use of Total Station which can perform the same task with greater ease and accurate results

e) Tacheometric Surveying

Tachometer is a branch of surveying in which horizontal and vertical distances are determined by taking angular observations with an instrument known as a tacheometer. Tacheometer is nothing but a transit theodolite fitted with a stadia diaphragm and an anallatic lens. There is no need for chaining in such a survey. The principle of the Tacheometer is based on the property of the isosceles triangle, where the ratio of the distance of the base from the apex and the length of the base is always constant.

A different form of stadia diaphragm commonly used:

f) Photographic Surveying

The photographic survey is based on the technique of taking photographs from a different angle to prepare topographic details with relatively high speed.

There are two types of photographic surveying

i). Terrestrial or ground photogrammetry

In terrestrial photogrammetry, maps are prepared from ground photographs from different points on the earth surface for measurement purpose.

ii). Aerial photogrammetry

In aerial photogrammetry, maps are produced from the air from an aeroplane or helicopter.

Photogrammetry encompasses two major areas of specialization.

• Metrical photogrammetry
• Interpretive photogrammetry

Metrical photogrammetry is of principal interest to surveyors since it is applied to determine distances, elevations, areas, volume, etc. to compile topographic maps made from measurements on photographs.

Intuitive photogrammetry involves objects from their photographic image and their significance. Critical factors considered in identifying an object of shape, sizes, patterns, shadow.

Advantage and disadvantage of plane table survey

 Advantage

•       It is most suitable for preparing for small scale maps.

•       It is the most rapid method

•       Field book is not necessary as plotting is done in the field concurrently with the fieldwork and hence the mistakes in booking the field notes are avoided.

•       The surveyor can compare the plotted work with the actual features of the area surveyed and thus can ascertain if it represents them properly and cannot overlook any essential features.

•       There is no possibility of omitting the necessary measurements as the map are plotted in the field.

•       Errors of measurements and the plotting may be readily detected by check lines.

•        Counters and irregular objects maybe represented accurately since the tract is in view.

•       It is particularly advantageous in a magnetic area where compass survey is not reliable.

•       It is less costly than a theodolite survey.

•       No great skill is required to prepared maps.

Disadvantage

•       Plane table is essentially a tropical instrument.

•       It is not suitable for work in a wet climate.

•       It is heavy and awkward to carry.

•       There are several accessories to be carried they are like to be lost.

•       It is not intended for accurate work.

•       If the survey is not to be re-plotted to a different scale or quantities are not to be computed.

•       It is a great inconvenience in absence of the field notes.

Obstacle in Chaining

 

Sometimes happens that a survey line passes through some object such as a pond, the building, a river, a hedge etc. which prevents the direct measurement of that part of the line which the object intersects. The interfering object in such a case is called on the obstacle.

The various obstacle may be classed as:

1. When Chaining is free, but Vision is obstructed.

2. When Chaining is Obstructed, but Vision is Free.

3. When Chaining and Vision Both are obstructed.

It is necessary to overcome obstacles so that chaining may be continued in a straight line. Special methods are, therefore, employed in measuring distances across the obstacles.

1. When Chaining is free, but Vision is obstructed.

Such a the problem arise when rising ground or hill intervening, the jungle is interrupted etc. Hence the end stations are not inter-visible. There are two cases

Case-I

In this method difficulty got over by reciprocal ranging. The reciprocal ranging may also, be used in ranging a line across a hollow.

Case-II

The end stations are not visible from intermediate points when a jungle area comes across the chain line. In this case, the obstacle may be crossed over using a random line.

Procedure

1.     Let the AB be the line whose length is required.  From A run a line ‘AB1’ called the random line, in any conventional direction but as nearly towards ‘B’  as can be the jungle and continue it until the point ‘B’ is visible from ‘B1’. Chain the line to ‘B1’ where ‘ BB1’ is perpendicular to ‘AB1’ and measure ‘AB1’ 

                                    AB = √(AB1² + BB1²) 

1.       If any length ‘AC1’ is measured along ‘AB1’ at point ‘C’ is located on the line ‘AB’ by measuring a perpendicular distance ‘CC1’.

                                        (CC1 / BB1) = (AC1 /AB1)

                             CC1  = (AC1 /AB1) x BB1

1.      In this corner a sufficient number of points can be located. The line is cleared and the distance measured.

2. When Chaining is Obstructed, but Vision is Free.

Such a the problem arise when a pound or a river tank, plantation comes across the chain line. The typical obstacle of this type is a sheet of water, the width of which in the direction of measurement exceeds the length of the chain or tape. The problem consists of finding the distance between convenient points on the chain line on either side of the obstacle.

Case-I

In which it is possible to chain around the obstacle e.g. a pound. A bend in the river etc.

Part-1

 Suppose AB is a chain line. Two-point ‘C’ and ‘D’ are selected on it on the opposite bank of the pound.    Equal perpendicular ‘CE’ and ‘DF’  erected at ‘C’  and ‘D’.  Distance ‘EF’ is measured (i.e. CD= EF)

Part-2 The the pond may also be crossed by forming a triangle as shown. Then

                            CD = √(ED² + CE²) 

Part-3 Find by optical square or a cross-staff a point C such that ∠PRQ is right angle (Fig. 4) Measure PR and RQ. Then

                                   CD = √(PR² + RQ²) 

Case-II

Sometimes it is not possible to go around the obstruction {i.e. obstacle is a river}

Part-1 Imagine a small river comes across the chain line 

 Suppose ‘AB’ is the chain line. Two points ‘C’ and ‘D’ selected on this line on the opposite bank of the river.At ‘C’ a perpendicular ‘CE’ is erected and bisected at ‘F’.A perpendicular is set out at ‘E’ and a point ‘G’ are in the same straight line. From triangle ‘DCE’ and ‘GEF’ the distance ‘GE’ is measured and ‘CD’ is obtained indirectly 

Part-2 Imagine a large river comes across the chain line.

1    Let ‘AB’ be the chain line. Point ‘C’, ‘D’ and ‘E’ are selected on this line such that ‘D’ and ‘E’ are on the opposite banks of the river. The perpendicular ‘DE’ and ‘CG’ are erected on the chain line in such a way that ‘E’, ‘F’ and ‘G’ are on the same straight line. Line ‘FH’ take parallel to “CD’. From triangle ‘DEF’ and ‘HEG’.

(ED / DF) = (FH / HG)

ED  = (FH / HG) x  DF

ED  = (FH / (CG - DF)) x  DF

3. When Chaining and Vision Both are obstructed.

 Such a problem arise when a building comes across the chain line. The problem, in this case, consists both in prolonging the line beyond the obstacle and finding the distance across it.

 Suppose ‘AB is the chain line. Two-point ‘C’ and ‘D’ are selected on it at on the side of the building.

1.      Equal perpendicular ‘CC1’ and ‘DD1’ are erected, the line ‘C1D1’ is extended until the building is crossed. On the extended line, two points ‘E1’ and ‘F1’ are selected. Then perpendiculars ‘EE1’ and ‘FF1’ are so erected that.

                                        EE1 = FF1 = =DD1 = CC1

Thus the points ‘C’, ‘D’, ‘E’ AND ‘F’ will lie on the same straight line ‘AB’

    DE =   D1E1

             

Conventional symbol

CONVENTIONAL SYMBOLS

In a map, the objects are shown by symbols and not by names. So the surveyor should know the following standard conventional symbols for some common objects.

SELECTION OF SURVEY STATIONS

SELECTION OF SURVEY STATIONS

The following points should be remembered during the selection of survey stations:

1. The stations should be so selected that the general principle of surveying may be strictly followed.

2. The stations should be intervisible.

3. The stations should be selected in such a way that well-conditioned triangles may be formed.

4. The base line should be the longest of the main survey lines.

5. The survey lines should be taken through fairly level ground, as far as practicable.

6. The main survey lines should pass close to the boundary line of the area to be surveyed.

7. The survey lines should be taken close to the objects so that they can be located by short offsets.

8. The tie stations should be suitably selected to fix the directions of adjacent sides.

9. The subsidiary stations should be suitably selected for taking check lines.

10. Stations should be so selected that obstacles to chaining are avoided as far as possible.

11. The survey lines should not be very close to main roads, as survey work may then be interrupted by traffic.

TYPES OF EQUIPMENTS FOR CHAIN SURVEY

The following equipment is required for conducting chain survey:

1. Metric chain (20 m)                             =          1 no.
2. Arrows                                                 =          10 nos. 
3. Metallic tape (15 m)                            =          1 no. 
4. Ranging rods                                       =          3 nos. 
5. Offset rod                                            =          1 no. 
6. Clinometer                                          =          1 no.
7. Plumb bob with thread                        =          1 no.
8. Cross staff or optical square               =          1 no.
9. Prismatic compass with stand            =          1 no. 
10. Wooden pegs                                      =          10 nos. 
11.  Mallet                                                  =          1 no.
12. Field book                                           =          1 no.
13.  Good pencil                                        =          1 no.
14. Pen knife                                             =          1 no.
15. Eraser (rubber)                                    =          1 no.

CROSS-STAFF AND OPTICAL SQUARE

CROSS-STAFF AND OPTICAL SQUARE

A. Cross-staff

The cross-staff is a simple instrument for setting out right angles. There are three types of cross-staves.

1. Open
2. French
3. Adjustable

The open cross-staff is commonly used.

1) Open cross-staff

The open cross-staff consists of four metal arms with vertical slits. The two pairs of arms (AB and BC) are at right angles to each other. The vertical slits are meant for sighting the object and the ranging rods. The cross-staff is mounted on a wooden pole of length 1.5m and diameter 2.5 cm. The pole is fitted with an iron shoe.

For setting out perpendicular on a chain line, the cross-staff is held vertically at the approximate position. Suppose slits A and B are directed to the ranging rods (R, R₁) fixed at the end stations. Slits C and D are directed to the object (O). Looking through slits A and B, the ranging rods are bisected. At the same time, looking through slits C and D, the object O is also bisected. To bisect the object and the ranging rods simultaneously, the cross-staff may be moved forward or backwards along the chain line

2) French cross-staff:

It consists of an octagonal brass tube with slits on all the eight sides. It has an alternate vertical sighting slit and an opposite vertical window with a vertical fine wire or horsehair on each of the four sides. These are used for setting out right angles. On the other sides are vertical slits, which are at 45° to those mentioned above for setting out angles of 45°. The sights are about 8 cm apart.

It carries a socket at the base so that it may be mounted on the pointed staff when in use. The sights being too close, it is not as accurate as of the open cross-staff.

French Cross staff

 

For setting out a right angle at a given point on a chain line, it is held vertically over the given point and turned until one of them saw cuts or (the pair of sights) is ranged along the chain line. Then the line of sights through the other saw cut or the pair of sights) is at right angles to the chain line and a ranging rod can be fixed in this direction at a convenient position.

To find the foot of the perpendicular from an object on a chain line, the cross-staff is held vertically at an approximate position of the required object and turned until one of the saw cuts or (the pair of sights) is ranged along the chain line.

If on looking through the another saw cut or (the pair of sights) is seen the object from which the foot of the perpendicular is to be drawn, then the point where the cross-staff is held is the correct position of the foot of the perpendicular from the given object. If not, the cross-staff is moved forward or backwards along the chain line until the line of sight through the saw cut or (the pair of sights) at right angles to the chain line bisect the given object.

3) Adjustable cross-staff:

It consists of a brass metal cylindrical tube about 8 cm in diameter and 10 cm in deep (divided in the centre). The upper cylinder can be rotated relatively t the lower one by a circular rack and pinion arrangement actuated by mill headed screw. Both are provided sighting slits.

The lower part is graduated to degree and subdivision, which the upper one carries a vernier.it has a magnetic compass at the top which may serve to take the bearing of the line.

It is used for setting out angles of any magnitude.

B. Optical Square

An optical square is also used for setting out right angles. It consists of a small circular metal box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides around the box to cover the slits. The following are the internal arrangements of the optical square.

1. A horizon glass H is fixed at the bottom of the metal box. The lower half of the glass is unsilvered and the upper half is silvered.

2. An index glass I is also fixed at the bottom of the box which is completely silvered.

3. The angle between the index glass and horizon glass is maintained at 45⁰.

4. The opening ‘e’ is a pinhole for eye E, ‘b’ is a small rectangular hole for ranging rod B, ‘P’ is a large rectangular hole for object P.

5. The line EB is known as horizon sight and IP as index sight.

6. The horizon glass is placed at an angle of 120⁰ with the horizon sight. The index glass is placed at an angle of 105⁰ with the index sight.

7. The ray of light from P is first reflected from I, then it is further reflected from H, after which it ultimately reaches the eye E

Principle

According to the principle of reflecting surfaces, the angle between the first incident ray and the last reflected ray is twice the angle between the mirrors. In this case, the angle between the mirrors is fixed at 45⁰. So, the angle between the horizon sight and index sight will be 90⁰.

Setting up the perpendicular by optical square

1.      The observer should stand on the chain line and approximately at the position where the perpendicular is to be set up.

2.      The optical square is held by the arm at the eye level. The ranging rod at the forward station, B is observed through the unsilver portion on the lower part of the horizon glass.

3.      Then the observer looks through the upper silvered portion of the horizon glass to see the image of the object P.

4.      Suppose the observer finds that the ranging rod B and the image of object P do not coincide. Then he should move forward or backwards along the chain line until the ranging rod B and the image of P exactly coincide

5.      At this position, the observer marks a point on the ground to locate the foot of the perpendicular.