Lesson 3. On Chain Surveying Introduction

3.1 PRINCIPLE OF CHAIN SURVEYING

The principle of chain surveying is triangulation. This means that the area to be surveyed is divided into a number of small triangles which should be well conditioned. In chain surveying the sides of the triangles which should be well conditioned. In chain surveying the sides of the triangles are measured directly on the field by chain or tape, and no angular measurements are taken. Here, the tie lines and check lines control the accuracy of work.

It should be noted that plotting triangles requires no angular measurements to be made, if the three sides are known.

Chain surveying is recommended when:

1.     The ground surface is more or less level

2.     A small area is to be surveyed

3.     A small-scale map is to be prepared and

4.     The formation of well-conditioned triangles is easy

Chain surveying is unsuitable when:

1.     The area is crowded with many details

2.     The area consists of too many undulations

3.     The area is very large and

4.     The formation of well-conditioned triangles becomes difficult due to obstacles

A. Large-Scale and Small-Scale Maps

When 1 cm of a map represents a small distance, it is said to be a large-scale map.

For example,

         

When 1 cm of the map represents a large distance, it is called a small-scale map.

For example,

A map having an RF of less than 1/500 is considered to be large-scale. A map of RF more than 1/500 is said to be small-scale.

3.2 WELL-CONDITIONED AND ILL-CONDITIONED TRIANGLES

A triangle is said to be well-conditioned when no angle in it is less than 30⁰ or greater than 120⁰ . An equilateral triangle is considered to be the best-condition or ideal triangle

Well-conditioned triangles are preferred because their apex points are very sharp and can be located by a single ‘dot’. In such a case, there is no possibility of relative displacement of the plotted point.

A triangle in which an angle is less than 30⁰ or more than 120⁰ is said to be ill-conditioned

Well - conditioned triangles are not used in chain surveying. This is because their apex points are not sharp and well defined, which is why a slight displacement of these points may cause considerable error in plotting.

3.3 RECONNAISSANCE SURVEY AND INDEX SKETCH

Before the commencement of any survey work, the area to be surveyed is thoroughly examined by the surveyor, who then thinks about the possible arrangement of the framework of survey. This primary investigations of the area is termed as reconnaissance survey or reconnoitre.

During reconnaissance survey, the surveyor should walk over the area and note the various obstacles and whether or not the selected stations are intervisible. The main stations should be so selected that they enclose the whole area. The surveyor should also take care that

The neat hand sketch of the area which is prepared during reconnaissance survey is known as the ‘index sketch’ or ‘key plan’. The index sketch shows the skeleton of the survey work. It indicates the main survey stations, sub-stations, tie stations, base line, arrangement for framework of triangles and the approximate positions of different objects. This sketch is an important document  for the surveyor and for the person who will plot the map. It should be attached to the starting page of the field book

3.4 DEFINITIONS AND ILLUSTRATIONS

A. Survey Stations

Survey stations are the points at the beginning and the end of a chain line. They may also occur at any convenient points on the chain line. Such stations may be:

1.     Main stations

2.     Subsidiary stations and

3.     Tie stations

2.     Main stations Stations taken along the boundary of an area as controlling points are known as  ‘main survey lines’. The main survey lines should cover the whole area to be surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc. The chain lines are denoted by “__ … __ ... __...__...__...__”.

3.     Subsidiary stations Stations which are on the main survey lines or any other survey lines are known as “Subsidiary stations”. These stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details. These stations are denoted by ‘’ with letters S₁,S₂,S₃, etc.

4.     Tie stations  These are also subsidiary stations taken on the main survey lines. Lines joining the tie stations are known as tie lines. Tie lines are mainly taken to fix the directions of adjacent sides of the chain survey map. These are also taken to form ‘chain angles’ in chain traversing, when triangulation is not possible. Sometimes tie lines are taken to locate interior details. Tie stations are denoted by ‘’ with letters T₁, T₂, T₃. Etc.

B.     Base Line

The line on which the framework of the survey is built is known as the ‘base line’. It is the most important line of the survey. Generally, the longest of the main survey lines is considered the base line. This line should be taken through fairly level ground, and should be measured very carefully and accurately. The magnetic bearings of the base line are taken to fix the north line of the map.

C.    Check Line

The line joining the apex point of a triangle to some fixed point on its base is known as the ‘check line’. It is taken to check the accuracy of the triangle. Sometimes this line helps to locate interior details.

D.    Offset

The lateral measurement taken from an object to the chain line is known as ‘offset’. Offsets are taken to locate objects with reference to the chain line. They may be of two kinds  - perpendicular and oblique.

1. Perpendicular offsets When the lateral measurements are taken perpendicular to the chain line, they are known as perpendicular offsets

Perpendicular offsets may be taken in the following ways:

(a) By setting a perpendicular by swinging a tape from the object to the chain line. The point of minimum reading on the tape will be the base of the perpendicular

(b) By setting a right angle in the ratio 3 : 4 : 5

(c) By setting a right angle with the help of builder’s square or tri-square

(d) By setting a right angle by cross-staff or optical square.

2. Oblique offsets Any offset not perpendicular to the chain line is said to be oblique. Oblique offsets are taken when the objects are at a long distance from the chain line or when it is not possible to set up a right angle due to some difficulties. Such offsets are taken in the following manner.

Suppose AB is a chain line and p is the corner of a building. Two points ‘a’ and ‘b’ are taken on the chain line. The chainages of ‘a’ and ‘b’ are noted. The distances ‘ap’ and ‘bp’ are measured and noted in the field book. Then ‘ap’ and ‘bp’ are the oblique offsets. When the triangle abp is plotted, the apex point p will represent the position of the corner of the the building.

Perpendicular offsets are preferred for the following reasons:

(a) They can be taken very quickly

(b) The progress of survey is not  hampered

(c) The entry in the field book becomes easy

(d) The plotting of the offsets also becomes easy

3. Number of offsets The offsets should be taken according to the nature of the object. So, there is no hard and fast rule regarding the number of offsets. It should  be remembered that the objects are to be correctly represented and hence the number of offsets should be decided on the field. Some guidelines are given below:

(a) When the boundary of the object is approximately parallel to the chain line, perpendicular offsets are taken at regular intervals

(b) When the boundary is straight, perpendicular offsets are taken at both ends of it

(c) When the boundary line is zigzag, perpendicular offsets are taken at every point of bend to represent the shape of the boundary accurately. In such a case, the interval of the offsets may be irregular

(d) When a road crosses the chain line perpendicularly, the chainage of the intersection point  is to be noted

(e) When a road crosses a chain line obliquely, the chainages of intersection points ‘a’ and ‘b’ are noted. Then at least one offset is taken on both sides of the inter-section points. More offsets may be taken depending on the nature of the road. Here, perpendicular offsets are taken at ‘c’ and ‘d’

(f) When the building is small, its corners are fixed by perpendicular or oblique offsets and the other dimensions are taken directly on the field and noted in the field book.

(g) When the building is large, zigzag in shape and oblique to the chain line, then the corners are fixed by perpendicular or oblique offsets. Then the full plan of the building is drawn on a separate page along with all the dimensions. This page should be attached with the field book at the proper place.

(h) When the object is circular, perpendicular offsets are taken at short and regular intervals

4. Limiting length of offset The maximum length of the offset should not be more than the length of the tape used in the survey. Generally, the maximum length of offset is limited to 15m. However, this length also depends upon the following factors:

(a) The desired accuracy of the map

(b) The scale of the map

(c) The maximum allowable deflection of the offset from its true direction and

(d) The nature of the ground

Problems on limiting length of offset

Problem 1  An offset was laid out 5⁰ from its true direction and the scale of the map was 20 m to 1 cm. Find the maximum length of offset for the displacement of a point on the paper not to exceed 0.03 cm.

Solution Let AB be the actual length of offset which was laid  out 5⁰ from its true direction. So, BC is the displacement of the point.

            Let the maximum length of offset,     AB = L m

                         

or         BC = AB sin 5⁰ = L sin 5⁰ m  (displacement of the ground)

Since the scale is 1 cm to 20 m, 20 m on the ground represents 1 cm on the paper.

             

     = 6.884 m

            Therefore, the maximum length of offset should be 6.884 m.

Problem 2  The length of the offset is 15 m and the scale of the plan 10 m to 1 cm. If the offset is laid out 3⁰ from its true direction, find the displacement of the plotted point on the paper

(i) perpendicular to the chain line, and

(ii) parallel to the chain line.

Solution Let AB be the actual length of offset, which is 15 m long and deflected by 3⁰ from its true direction.

Here,

            BC = Displacement parallel to chain line

                        CD = displacement perpendicular to chain line

(i)  CD = AD – AC = AB - AC

             = 15 – 15 cos 3⁰

             = 15 (1 – cos 3⁰) m (displacement on the ground)

            Since the scale is 1 cm to 10 m,

            10 m on the ground = 1 cm on the map

            

                                                          =  0.002 cm on the map

            Required displacement perpendicular to chain line

          = 0.002 cm (on paper)

(ii) BC = AB sin 3⁰ = 15 sin 3⁰ = 0.7850 m (displacement on ground)

     

E. Degree of Accuracy

Degree of accuracy is determined before the starting of any survey work. It is worked out according the following factors:

(a) Scale of plotting

(b) Permissible error in plotting

During reconnaissance survey, the length of the main survey lines are approximately determined by the pacing method. One pace or walking step of a man is considered to equal 80 cm. When the length of the survey lines or the extent of area to be surveyed is approximately known, the scale of the map may be assumed. Again, the permissible error in plotting may be obtained from the concerned department. Then the degree of accuracy in measurement is ascertained.

Let us now consider an example.

Suppose the scale of plotting is 5 m to 1 cm and the allowable error is 0.02 cm.

Then,  1 cm on the map = 500 cm on the ground

        0.02 cm on the map = 500 x 0.02 = 10 cm on the ground

So, the measurement should be taken nearest to 10 cm.

3.5 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.

LESSON NOTE 3A ON CHAIN SURVEY FIELD BOOK

 EQUIPMENTS FOR CHAIN SURVEY

The following equipments are 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                                           =          10 nos.
13. Good pencil                                        =          1 no
14. Pen knife                                             =          1 no.
15. Eraser (rubber)                                    =          1 no.

3.7 THE FIELD BOOK

The notebook in which field measurements are noted is known as the ‘field book’. The size of the field book is 20 cm x 12 cm and it opens lengthwise. Field books may be of two types:

1. Single –line , and
2. Double-line.

1. Single-line field book In this type of field book, a single red line is drawn through the middle of each page. This line represents the chain line, and the chainages are written on it. The offsets are recorded, with sketches, to the left or right of the chain line. The recording of the field book is started from the last page and continued towards the first page. The main stations are marked by ‘’ and subsidiary stations or tie stations are by ‘’

2. Double-line field book In this type of field book, two red lines, 1.5 cm apart, are drawn through the middle of each page. This column represents the chain line, and the chainages are written in it. The offsets are recorded, with sketches, to the left or right of this column. The recording is begun from the last page and continued towards the first. The main stations are marked by ‘’ and subsidiary or tie stations by ‘’ This type of field book is commonly used.

A. Problems on Entering Records in Field Book

Problem 1 While measuring a chain line AB, the following offsets are taken. How would you enter the field book ?

(a) A telegraph post is 10 m perpendicularly from chainage 2.5 m to the right of the chain line.

(b) A road crosses obliquely from left to right at chainage 10 m and 14 m. Perpendicular offsets are 2m and 3m to the side of the road from chainage 5m and 20 m respectively.

(c) A tube-well is 5m perpendicularly from chainage 30 m to the left of the chain line.

(d) Total chainage of AB is 45 m.

Problem 2 The base line AC of a chain survey is measured and the following records are noted. Make the necessary entries in a field book.

(a) The corners of a building are 9 and 9,5m from chainage 7.5 and 18 m to the left of the chain line. The building is 7m wide.

(b) A 4 m wide road runs about parallel to the right of the chain line. Offsets are 2,2.1,2.2, and 2.15m at chainages 0,20,40, and 55.5m respectively.

(c) A check line is taken from the sub-station at chainage 25 m to the left.

(d) The total chainage of the base line is 55.5m.

(e) The fore bearing and back bearing of the base line are 30⁰30’ and 210⁰3C’ respectively.

Problem 3 Enter the field book according to the following field notes:

(a) Chainage of line AB is 95.5m

(b) The offsets to the pond at the left of chain line are as follows:

Chainage – 10,15,20,25,30 m

Offset – 16,12,10,14,20 m

(c)    The offsets to the river at the right of the chain line are :

Chainage – 5,25,40,80 m

Offset -13,17,19,19.5m

B. Precautions to be Taken While Entering the Field book

1. All measurements should be noted as soon as they are taken.

2. Each chain line should be recorded on a separate page. Normally it should start from the bottom of one page and end on the top of another. No line should be started from any intermediate position.

3. Over –writing should be avoided.

4. Figures and hand-writing should be neat and legible.

5. Index-sketch, object-sketch and notes should be clear.

6. Reference sketches should be given in the field book, so that the station can be located when required.

7. The field book should be entered in pencil and not in ink.

8. If an entry is incorrect or a page damaged, cancel the page and start the entry from a new one.

9. Erasing a sketch, measurement or note should be avoided.

10. The surveyor should face the direction of chaining so that the left-hand and right-hand objects can be recorded without any confusion.

11. The field-book should be carefully preserved.

12. The field-book should contain the following: 
            (i) name,

(ii) location, and

(iii) date, of survey,

(iv) name of party members, and

(v) page index or chain line.

Lesson Note2 On Classification and basic principles – Linear Measurement

Lesson 2. Classification and basic principles – Linear Measurement

Surveying

The practice of measuring angles and distances on the ground so that they can be accurately plotted on a map

GENERAL PRINCIPLE OF SURVEYING

The general principles of surveying are:

1. To work from the whole to the part, and

2. To locate a new station by at least two measurements (linear or angular) from fixed reference points.

According to the first principle, the whole area is first enclosed by main stations (i.e. controlling stations) and main survey lines (i.e. controlling lines). The area is then divided into a number of parts by forming well conditioned triangles. A nearly equilateral triangle is considered to be the best well-conditioned triangle. The main survey lines are measured very accurately with a standard chain. Then the sides of the triangles are measured. The purpose of this process of working is to prevent accumulation of error. During this procedure, if there is any error in the measurement of any side of a triangle, then it will not affect the whole work. The error can always be detected and eliminated.

·         But, if the reverse process (i.e. from the part to the whole) is followed, then the minor errors in measurement will be magnified in the process of expansion and stage will come when these errors will become absolutely uncontrollable.

According to the second principle, the new stations should always be fixed by at least two measurements (linear or angular) from fixed reference points. Linear measurements refer to horizontal distances measured by chain or tape. Angular measurements refer to the magnetic bearing or horizontal angle taken by a prismatic compass or theodolite.

·         In chain surveying, the positions of main stations and directions of main survey lines and check lines.

CLASSIFICATION OF SURVEYING

Generally, surveying is divided into two major categories: plane and geodetic surveying.

PLANE SURVEYING is a process of surveying in which the portion of the earth being surveyed is considered a plane. The term is used to designate survey work in which the distances or areas involved are small enough that the curvature of the earth can be disregarded without significant error. In general, the term of limited extent. For small areas, precise results may be obtained with plane surveying methods, but the accuracy and precision of such results will decrease as the area surveyed increases in size. To make computations in plane surveying, you will use formulas of plane trigonometry, algebra, and analytical geometry.

A great number of surveys are of the plane surveying type. Surveys for the location and construction of highways and roads, canals, landing fields, and railroads are classified under

plane surveying. When it is realized that an arc of 10 mi is only 0.04 greater that its subtended chord; that a plane surface tangent to the spherical arc has departed only about 8 in. at 1 mi from the point of tangency; and that the sum of the angles of a spherical triangle is only 1 sec greater than the sum of the angles of a plane triangle for a triangle having an area of approximately 75 sq mi on the earth’s surface, it is just reasonable that the errors caused by the earth’s curvature be considered only in precise surveys of large areas.

In this training manual, we will discuss primarily the methods used in plane surveying rather than those used in geodetic surveying.

GEODETIC SURVEYING is a process of surveying in which the shape and size of the earth are considered. This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods are required for this type of surveying than for plane surveying. The shape of the earth is thought of as a spheroid , although in a technical sense, it is not really a spheroid. In 1924, the convention of the International Geodetic and Geophysical Union adopted 41,852,960 ft as the diameter of the earth at the equator and 41,711,940 ft as the diameter at its polar axis. The equatorial diameter was computed on the assumption that the flattening of the earth caused by gravitational at traction is exactly 1/297. Therefore, distances measured on or near the surface of the earth are not along straight lines or planes, but on a curved surface. Hence, in the computation of distances in geodetic surveys, allowances are made for the earth’s minor and major diameters from which a spheroid of reference is developed. The position of each geodetic station is related to this spheroid. The positions are expressed as latitudes (angles north or south of the Equator) and longitudes (angles east or west of a prime meridian) or as northings and castings on a rectangular grid.

Classifications of Surveying

Based on the purpose (for which surveying is being conducted), Surveying has been classified into:

• Control surveying :

To establish horizontal and vertical positions of control points.

• Land surveying :

To determine the boundaries and areas of parcels of land, also known as property survey, boundary survey or cadastral survey.

• Topographic survey :

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

• Engineering survey :

To collect requisite data for planning, design and execution of engineering projects. Three broad steps are

1) 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.

•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).

•Astronomic surveys :

To determine the latitude, longitude (of the observation station) and azimuth (of a line through observation station) from astronomical observation.

•Mine surveys :

To carry out surveying specific for opencast and underground mining purposes

SPECIAL SURVEYS

As mentioned earlier in this chapter, SPECIAL SURVEYS are conducted for a specific purpose and with a special type of surveying equipment and methods. A brief discussion of some of the special surveys familiar to you follows.

LAND SURVEYS (sometimes called cadastral or property surveys) are conducted to

establish the exact location, boundaries, or subdivision of a tract of land in any specified area.

This type of survey requires professional registration in all states. Presently, land surveys generally consist of the following chores:

1. Establishing markers or monuments to define and thereby preserve the boundaries of land belonging to a private concern, a corporation, or the government.

2. Relocating markers or monuments legally established by original surveys. This requires examining previous survey records and retracing what was done. When some markers or monuments are missing, they are re-established following recognized procedures, using whatever information is available.

3. Rerunning old land survey lines to determine their lengths and directions. As a result of the high cost of land, old lines are re-measured to get more precise measurements.

4. Subdividing landed estates into parcels of predetermined sizes and shapes.

5. Calculating areas, distances, and directions and preparing the land map to portray the survey data so that it can be used as a permanent record.

6. Writing a technical description for deeds.

CONTROL SURVEYS provide "basic control" or horizontal and vertical positions of points to which supplementary surveys are adjusted. These types of surveys (sometimes termed and traverse stations and the elevations of bench marks. These control points are further used as References for hydrographic surveys of the coastal waters; for topographic control; and for the control of many state, city, and private surveys.

Lesson Note On Surveying – Introduction

Lesson Note On Surveying – Introduction

Introduction to Surveying

Surveying is the art of determining the relative positions of different objects on the surface and below the surface of the earth by measuring the horizontal and vertical distances between them and by preparing a map to any suitable scale. Thus in discipline, the measurements are taken in the horizontal plane alone.

Levelling is the art of determining the relative vertical distances of different points on the surface of the earth. Therefore, in levelling, the measurements are taken only in the vertical plane.

Objective of surveying

The aim of surveying is to prepare a plan or map to show the relative positions of the objects on the surface of the earth. The map is drawn to some suitable scale .It shows the natural features of a country such as towns, villages, roads, railways, rivers, etc. Maps may also include details of different engineering works, such as roads, railways, irrigation, canals, etc.

Uses of surveying

Surveying may be used for the following various applications.

To prepare a topographical map which shows the hills, valleys, rivers, villages, towns, forests ,etc. of a country.

To prepare a cadastral map  showing  the boundaries of fields, houses, and other properties.

To prepare an engineering map showing details of engineering works such as roads, railways, reservoirs, irrigation canals, etc.

To prepare a military map showing the road and railway communications with different parts of a country. Such a map also shows the different strategic points important for the defence of a country.

To prepare a contour map to determine the capacity of reservoir and to find the best possible route for roads, railways, etc.

To prepare a geological map showing areas including underground resources exist.

To prepare an archeological map including places where ancient relics exist.