Surveying is the art of making measurements of objects on, above or beneath the ground to show their relative positions on paper. The relative position required is either horizontal, or vertical, or both. Less precisely the term Surveying is used to the measurement of objects in their horizontal positions. Measurements to determine their relative vertical positions is known as levelling.
Object and Uses of Surveying
As stated in the definition, object of surveying is to show relative positions of various objects of an area on paper and produce plan or map of that area. Various uses of surveying are listed below:
- Plans prepared to record property lines of private, public and government lands help in avoiding unnecessary controversies.
- Maps prepared for marking boundaries of countries, states, districts etc., avoid disputes.
- Locality plans help in identifying location of houses and offices in the area.
- Road maps help travelers and tourist.
- Topographic maps showing natural features like rivers, streams, hills, forests help in planning irrigation projects and flood control measures.
- For planning and estimating project works like roads, bridges, railways, airports, water supply and waste water disposal surveying is required.
- Marine and hydrographic survey helps in planning navigation routes and harbours.
- Military survey is required for strategic planning.
- Mine surveys are required for exploring mineral wealth.
- Geological surveys are necessary for determining different strata in the earth crust so that proper location is found for reservoirs.
- Archeological surveys are useful for unearthing relics of antiquity.
- Astronomical survey helps in the study of movements of planets and for calculating local and standard times.
Primary Divisions in Surveying
The earth is an oblate spheroid, length of equatorial axis being 12756.75 km and polar axis being 12713.80 km. Since the difference between these two axes and irregularities on the earth surface are very small (Note. Height of Mount Everest is 8.79 km) compared to these two axes, the earth may be treated as a sphere,
Figure 1 shows a circular plane passing through a point A on the earth surface. The gravitational force is always directed towards the centre of the earth. Hence, the plumb-line shown in Fig. 1 is a vertical line.
Line perpendicular to vertical line (tangential to earth surface) is known as horizontal line. In surveying all measurement at any point are in the direction of these two lines.
Obviously, the vertical and horizontal lines at another point B are not parallel to the respective lines at A. It should be noted that all lines lying on the earth’s surface are curved lines and all triangles are spherical triangles as shown in Fig. 2.
Hence, surveying involves spherical trigonometry. If the area to be surveyed is small, the curvature of the earth may be neglected and all plumb lines treated as the same vertical. Hence, the lines normal to plumb line at any point in the area are treated as the same horizontal. All triangles in the area may be treated as plane triangles.
The survey in which earth curvature is neglected is called Plane Surveying and the survey in which earth’s curvature is considered is known as Geodetic Surveying. No definite value can be assigned to the area up to which a survey may be treated as plane, since the degree of accuracy required forms the controlling factor. However, the following points should be noted:
- The length of an arc of 1.2 km on earth’s mean surface is only 1 mm more than the straight line connecting those two points.
- The sum of the interior angles of a geometrical figure laid on the surface of the earth differs from that of the corresponding figure only to the extent of one second for about 200 square kilometres of area.
Hence, in most of engineering projects plane surveying is used. The geodetic surveying is used to determine the precise positions of control stations on the surface of the earth to which plane survey details are connected in works of larger magnitude like preparing maps of countries. Thus, in surveying there are two primary divisions viz. Geodetic Surveying and Plane Surveying.
Fundamental Principles of Surveying
To get accurate results in surveying one should follow the following fundamental principles:
- Work from whole to part
- Take extra care in fixing new control points.
Work from Whole to Part
In surveying large areas, a system of control points is identified and they are located with high precision. Then secondary control points are located using lesser precise methods. The details of the localised areas are measured and plotted with respect to the secondary control points. This is called working from whole to part.
This principle in surveying helps in localising the errors. If the surveying is carried out by adding localised areas errors accumulated and may become unacceptable when large area is covered.
Extra Care in Fixing New Control Points
Figure 3 shows the various methods of fixing point C with respect to already fixed points A and B by measuring sides, angles or setting perpendiculars. For fixing new control points (stations) with respect to already fixed points at least two independent processes should be followed.
If A and B are already located control points and with respect to them new control point C is to be located, apart from the minimum measurements required as shown in Fig. 3, one more measurement should be taken. Measuring the lengths of check lines and tie lines will also serve this purpose (Ref. Fig. 4).
Classification of Surveying
Surveying may be classified on the following basis:
- Nature of the survey field
- Object of survey
- Instruments used and
- The methods employed.
Classification Based on Nature of Survey
Field On this basis survey may be classified as land survey, marine or hydraulic survey and astronomical survey.
Land Survey: It involves measurement of various objects on land. This type of survey may be further classified as given below:
- Topographic Survey: It is meant for plotting natural features like rivers, lakes, forests and hills as well as manmade features like roads, railways, towns, villages and canals.
- Cadestal Survey: It is for marking the boundaries of municipalities, villages, talukas, districts, states etc. The survey made to mark properties of individuals also come under this category.
- City Survey: The survey made in connection with the construction of streets, water supply and sewage lines fall under this category.
Marine or Hydrographic Survey: Survey conducted to find depth of water at various points in bodies of water like sea, river and lakes fall under this category. Finding depth of water at specified points is known as sounding.
Astronomical Survey: Observations made to heavenly bodies like sun, stars etc., to locate absolute positions of points on the earth and for the purpose of calculating local time is known as astronomical survey.
Classification Based on Object of Survey
On the basis of object of survey the classification can be as engineering survey, military survey, mines survey, geological survey and archeological survey.
(a) Engineering Survey: The objective of this type of survey is to collect data for designing civil engineering projects like roads, railways, irrigation, water supply and sewage disposals. These surveys are further sub-divided into:
- Reconnaissance Survey for determining feasibility and estimation of the scheme.
- Preliminary Survey for collecting more information to estimate the cost of the project, and Location Survey to set the work on the ground.
(b) Military Survey: This survey is meant for working out plans of strategic importance.
(c) Mines Survey: This is used for exploring mineral wealth.
(d) Geological Survey: This survey is for finding different strata in the earth’s crust.
(e) Archeological Survey: This survey is for unearthing relics of antiquity.
Classification Based on Instruments Used
Based on the instruments used, surveying may be classified as:
- Chain survey
- Compass survey
- Plane table survey
- The odolite survey
- Tacheometric survey
- Modern survey using electronic distance meters and total station
- Photographic and Aerial survey – The survey is taught to students mainly based on this classification.
Classification Based on Methods Employed
On this basis surveying is classified as triangulation and traversing.
(i) Triangulation: In this method control points are established through a network of triangles.
(ii) Traversing: In this scheme of establishing control points consists of a series of connected points established through linear and angular measurements. If the last line meets the starting point it is called as closed traverse. If it does not meet, it is known as open traverse.