PRIMARY DIVISIONS IN SURVEYING

The earth is an oblate spheroid, polar axis being 12713.80 km and length of equatorial axis being 12756.75 km. Since on the earth the difference between these two axes and irregularities 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, Given figure shows a circular plane passing through a point A on the earth surface. The gravitational force is directed towards the centre of the earth always.

Therefore, the plumb-line shown in given figure is a vertical line. Line which is perpendicular to vertical line (tangential to earth surface) is called horizontal line. all measurement at any point are in the direction of these two lines in surveying.

                  Vertical and horizontal lines                                    Plane and spherical triangles

Clearly, 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 which lying on the earth's surface are curved lines and all triangles are spherical triangles as shown in above figure. Therefore, surveying involves spherical trigonometry.

The curvature of the earth may be neglected if the area to be surveyed is small, and all plumb lines treated as the same vertical. Therefore, the lines normal to plumb line at any point in the area are treated as the same horizontal. In the area all triangles may be treated as plane triangles. Survey in which earth curvature is neglected is known Plane Surveying and the survey in which earth's curvature is considered is called Geodetic Surveying.

No definite value may be assigned to the area up to which a survey may be treated as plane, as the degree of accuracy required forms the controlling factor. Conversely, the points should be noted are following:

1.      On earth's mean surface Length of an arc of 1.2 km is only 1 mm more than the straight line connecting those two points.

2.      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 approximate 200 square kilometres of area.

Therefore, in most of engineering projects plane surveying is used. The geodetic surveying is used, on the surface of the earth to find out the precise positions of control stations to which plane survey details are connected in works of larger magnitude like preparing maps of countries. So, in surveying there are two primary divisions For example: Geodetic Surveying and Plane Surveying.