After reading this essay you will learn about:- 1. Definition of Surveying 2. Object of Surveying 3. Main Division 4. Measurements 5. Classification 6. Principles 7. Precision.
Contents:
- Essay on the Definition of Surveying
- Essay on the Object of Surveying
- Essay on the Main Division of Surveying
- Essay on the Measurements in Surveying
- Essay on the Classification of Surveying
- Essay on the Principles of Surveying
- Essay on the Precision in Surveying
1. Definition of Surveying:
Surveying is the art of determining relative positions of objects on the surface of the earth by taking measurements in the horizontal and vertical plane. The measurements are drawn to convenient reduced size on papers known as maps or plans.
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The part of surveying which deals with the measurements in the vertical plane is known as levelling and the papers on which these measurements are drawn are called sections and elevations. Surveying and levelling are considered to be two different operations but in the wider sense, the term surveying includes levelling.
2. Object of Surveying:
The main object of surveying is to obtain a map or a plan of the area surveyed. The art of surveying and map making has been practised from the very ancient times. The earliest surveys known’ were performed only for the purpose of establishing the boundaries of land, but their application has become increasingly important as the time has progressed.
Now-a- days for the purpose of designing any engineering project such as a road, railway, canal, water supply or sanitary scheme etc., an engineer would require to study, as the very first item, the features of the earth’s surface in which the project is to be located, and prepare a map of that area.
The success of any engineering project is based upon the accurate and complete survey work. An engineer therefore, must be thoroughly familiar with the principles and practice of surveying.
3. Main Division of Surveying:
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Surveying may be divided into the following two main classes:
i. Plane Surveying,
ii. Geodetic Surveying.
i. Plane Surveying:
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The earth is somewhat elliptical in nature but is generally assumed to be a spheroid. Therefore, a line joining any two points on the surface of the earth is an arc of a great circle and not a straight line. But a survey plan is a projection upon a horizontal plane, and in its construction all linear and angular measurements drawn must be horizontal.
When the surveys extend over limited areas, the difference in length between the arc and the subtended chord for any two points on the earth is very small and can be neglected. The surveys, where the effect of curvature of the earth is neglected assuming the earth’s surface to be plane, are called plane surveys.
The degree of accuracy obtained in this type of surveying is comparatively low. Generally areas less than 260 sq. km (100 sq. miles) are treated as plane. However, the controlling factor should be the degree of accuracy required rather than the area of the survey. The scope and use of plane surveying is very wide as it is employed in majority of surveys conducted for the purpose of engineering works.
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ii. Geodetic Surveying:
When the surveys extend over large areas or the degree of accuracy required is great, the curvature of the earth cannot be ignored. Those surveys, where the curvature of the earth is taken into account, are called Geodetic Surveys. They are also known as Trigonometrical surveys as they involve the knowledge of Trigonometry during different operations.
Geodetic Surveying is generally adopted to locate the widely distant control points for different surveys to be conducted in between these points. It requires the use of very refined instruments as well as methods of observation and adjustment. It is conducted only through Government agencies such as in India, it is conducted by the Great Trigonometrical Survey (G.T.S.) department of India.
4. Measurements in Surveying (With Units):
In surveying, a surveyor has to generally deal with linear and angular measurements both in the horizontal and vertical planes. Linear measurements taken in the horizontal plane are known as horizontal distances whereas those taken in the vertical plane are called vertical distances. Similarly the angular measurements are horizontal angles and vertical angles when taken in the horizontal and vertical planes respectively.
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(a) Linear Measurements:
F.P.S. system:
Previously India followed F.P.S. system of measurements but from 1956, according to the Weights and Measures Act, the Metric system has become the only recognised system in India. Therefore a metre is the basic unit of length now-a-days and the units of area and volume are derived from it.
The units of length, area and volume in Metric system and their conversion to British system are given in the following tables:
(b) Angular Measurements:
A radian is the unit of plane angle and is equal to the angle subtended at the centre of a circle by an arc equal in length to its radius.
radians = 2 right angles
1 right angle = 100 grades or 90 degrees.
A degree is the basic unit of angle used in India.
1 degree (°) = 60 minutes (‘)
1 minute (‘) = 60 seconds (“)
Instruments used for taking Measurements:
The instruments commonly used for taking surveying measurements may be divided into the following main classes:
i. Instruments used for ranging and laying out survey lines such as ranging rods, line ranger, cross-staff, optical square etc.
ii. Instruments used for direct measurements of lengths such as chain, tape, steel band etc.
iii. Instruments used for measurement of length indirectly such as tacheometer, subtence bar, Electronic Distance Measuring Instruments etc.
iv. Instruments used for determining direction of lines such as prismatic compass, box-sextant, theodolite etc.
v. Instruments used for measuring slopes and heights such as clinometer, level etc.
vi Instruments used for surveying and plotting simultaneously such as plane table.
vii. Instruments used for surveying by photographic means such as photo theodolite.
5. Classification of Surveys:
Surveys may be classified in different ways:
I. According to the instrument used, such as:
i. Chain Survey
ii. Compass Survey
iv. Theodolite Survey, etc.
II. According to the purpose of Survey, such as:
i. Mine Survey to explore the mineral wealth such as gold, copper, coal etc. within the earth’s crust.
ii. Geological Survey to determine different strata in the earth’s crust.
iii. Archaeological Survey to trace customs or relics of the past.
iv. Military Survey to determine points of strategic importance.
III. According to the method employed, such as:
i. Triangulation Survey.
ii. Traverse Survey.
IV. According to the place of work, such as:
i. Land Survey for objects on earth’s surface.
ii. Hydrographical or Marine Survey for objects under water.
iii. Aerial Survey by Aeroplane in air.
Land Survey can be further sub-divided into the following classes:
(a) Topographical Survey:
This is used for determining the natural features of the country such as lakes, rivers, hills, woods and nallahs etc., and also the artificial objects such as canals, railways, roads, towns and villages etc.
(b) Cadastral Survey:
This is usually plotted to a larger scale than topographical survey. Additional details such as boundaries of fields, houses and other property are determined.
(c) City Survey:
This is performed in connection with town planning schemes such as drainage, water supply etc. and for laying out plots, roads, streets etc.
(d) Engineering Survey:
This is carried out for determining the feasibility of any engineering project and collection of field-data as required for the design etc.
6. Principles of Surveying:
The following two basic principles of surveying should be kept in view while determining the relative positions of points on the surface of the earth:
i. Determining Suitable Method for Locating a Point:
It is always practicable to select two points in the field and to measure the distance between them. These can be represented on paper by two points placed in a convenient position. From these initial points also known as reference points others can be located by two suitable measurements in the field and drawn in their relative positions on the sheet. Points so obtained serve in turn to fix the positions of others.
The common methods of locating a point such as C with respect to two reference points such as A and B are illustrated in fig. 1.2.
(a) Distances AC and BC are measured, and C is plotted as the intersection point of two arcs with centers A and B radii the measured distances. The method is much used in chain surveying.
(b) Perpendicular CD and distance AD or BD measured and C is plotted by the use of a set square. This method is termed as offsetting and is used for locating details.
(c) Distance AC and the angle BAC are measured, and C is plotted by means of a protractor or trigonometrically. This method is used in traversing.
(d) Angles ABC and BAC are measured, and C is plotted by a protractor or by solution of triangle ABC. This method is much used in triangulation for very extensive work.
(e) Angle BAC and distance BC are measured, and C is plotted by protractor or trigonometrically. This method is of minor utility and is used in exceptional cases.
One or more than one of the above methods may be employed in the same survey according to the instruments available for the angular and linear measurements. A survey may therefore be conducted in several ways by different combinations of methods and instruments depending upon the purpose of survey, the degree of accuracy required, the nature of the country, the extent of the survey and the time available.
ii. Working from Whole to the Part:
In surveying an area, it is essential to establish first of all a system of control points with great precision. Minor control points can then be established by less precise methods and the details can be located afterwards by the method of triangulation or traversing between the control points.
This way the minor errors are automatically controlled and localised and do not accumulate. On the other hand, if we work from part to the whole, the small errors are magnified and become uncontrollable at the end.
7. Precision in Surveying:
The degree of accuracy required in survey work mainly depends upon the purpose of survey and the scale of plotting. Since the land in cities and towns is much more valuable than that in rural areas, so greater accuracy is required in the survey of urban areas and measurements must be made up to the nearest centimetre.
But in the survey of fields, measuring upto smaller fractions is sheer wastage of time. They may be taken to the nearest decimetre. At the same time, it may be pointed out that very accurate surveys take much time and are more expensive.
Summing up the various factors, the accuracy required in survey-work depends upon the following:
i. The purpose of the survey,
ii. The nature and size of the country to be surveyed,
iii. The instruments and methods of observation employed,
iv. The sources of error,
v. The funds available,
vi. The time available and,
vii. The scale used for plotting.
Keeping in view the above factors, the surveyor may decide upon the method and instruments most suitable for particular case. By properly planning and methodically conducting the survey-work, he can attain the required degree of accuracy at a minimum cost and in a minimum time.