Surveying and Principle

What is Surveying?

Surveying is the technique of determining the relative position of different features on, above or beneath the surface of the earth by means of direct or indirect measurements and finally representing them on a sheet of paper known as plan or map.

Surveying is primarily classified as under:

1. Plane surveying
2. Geodetic Surveying

Plane Surveying 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 all plumb lines are considered parallel. In everyday life were are concerned with small portion of earth’s surface and the above assumptions seems to be reasonable in light of the fact that the length of an arc 12 kilometers long lying in the earth’s surface is only 1cm greater than the subtended chord and further that the difference between the sum of the angles in a plane triangle and the sum of those in a spherical triangle is only one second for a triangle at the earth’s surface having an area of 195 sq. km.

Geodetic Surveying is that type of surveying in which the shape of the earth is taken into account. All lines lying in the surface are curved lines and the triangles are spherical triangles. It therefore, involves spherical trigonometry. All Geodetic surveys include work of larger magnitude and high degree of precision. 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 to which surveys of less precision may be referred.

Classification of surveying

Surveys may be secondarily classified under no. of headings which define the uses or purpose of resulting maps.

Classification based upon nature of field:

There are three types of surveying based upon the nature of field which are as follows:

1. Land Surveying: It can be further classified as i) Topographical survey ii) Cadastrial Survey and iii) City Survey. It generally deals with natural or artificial features on land such as rivers, streams, lakes, wood, hills, roads, railways, canals, towns, water supply systems, buildings & properties etc.
2. Marine Surveying: Marine or hydrographic survey deals with bodies of water for purpose of navigation, water supply, harbour works or for determination of mean sea level. The work consists in measurement of discharge of streams, making topographic survey of shores and banks, taking and locating soundings to determine the depth of water and observing the fluctutations of the ocean tide.
3. Astronomical Surveying: The astronomical survey offers the surveyor means of determining the absolute location of any point or the absolute location of and direction of any line on the surface of the earth. This consists in observations to the heavenly bodies such as the sun or any fixed star.

Classification based on object:

Based on object, there are four types of surveying which are as follows:

1. Geological Surveying
2. Mine Surveying
3. Archaeological surveying
4. Military surveying

Classification based on instruments used:

Based on various types of instruments used, surveying can be classified into six types.

1. Chain surveying
2. Compass surveying
3. Plane table surveying
4. Theodolite surveying
5. Tacheometric surveying
6. Photographic surveying

Classification based on methods used:

Based on methods adopted, surveying can be categorized into:

1. Triangulation surveying
2. Traverse surveying

Principles of Surveying

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

1)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 the 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.

2)Working from whole to part

According to this principle, it is always desirable to carryout 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