SURVEYING DEFINITION AND PRIMARY DIVISIONS (PLANE AND GEODETIC SURVEYING)

WHAT IS SURVEYING ?


Surveying is the art and science of determining the relative position of various points or stations on, above or beneath the surface of the earth by measuring the horizontal and vertical distances, angles and using the details of these points to prepare a map or plan to any suitable scale.

The knowledge of surveying is advantageous in many phases of engineering. 

The earliest surveys were made in connection with land surveying. 

Practically, every engineering project such as water supply and irrigation schemes, railroads and transmission lines, mines, bridges and buildings etc. require surveys. 

Before plans and estimates are prepared, boundaries should be determined and the topography of the site should be ascertained. 

After the plans are made, the structures must be staked out on the ground. 

As the work progresses, lines and grades must be given.

In surveying, all measurements of lengths are horizontal, or else are subsequently reduced to horizontal distances. 

The object of a survey is to prepare plan or map so that it may represent the area on a horizontal plane. 

A plan or map is the horizontal projection of an area and shows only horizontal distances of the points. 

Vertical distances between the points are, however, shown by contour lines, hachures or some other methods. 

Vertical distances are usually represented by means of vertical sections drawn separately.


PRIMARY DIVISIONS OF SURVEY

Primarily surveying can be divided into two classes : 

(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 as plane triangles. 

The level line is considered as straight and all plumb lines are considered parallel. 

In everyday life we are concerned with small portions of earth's surface and the above assumptions seem to be reasonable in light of the fact that the length of an arc 12 kilometres long lying in the earth's surface is only 1 cm 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.

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