Primary divisions of surveying

2. PRIMARY DIVISIONS OF SURVEYING

Earth is a spheroid. The line connecting any two points on Earth's surface is not straight, but curved. The angles subtended by two lines are not plane angles, but spherical angles. See Fig. 1. Considering the curvature effects of Earth, extent of area to be surveyed and degree of accuracy needed, the surveying is primarily divided into two divisions:

1. Plane Surveying and 2. Geodetic Surveying.

1. PLANE SURVEYING

Earth is a compressed spheroid of radius 6371 km, like an orange. A line connecting any two points on the surface of Earth is an arc of a circle. When the survey conducted is on a limited area, the length between the arc and the chord connecting any two points is small. The difference between an arc of 18.5 km long and the corresponding chord is only less than 10 mm. Hence, for ordinary survey work, Earth surface is assumed plane. Curvature effect is neglected.

Plane Surveying is the surveying in which the area surveyed is comparatively small, the effect of curvature is not considered and the surface of Earth is assumed flat or plane. This survey is conveniently adopted when the area is less than 250 sq km. Plane surveying is used for the layout of buildings, highways, railways, canals, dams, bridges, etc.

2. GEODETIC SURVEYING or TRIGONOMETRICAL SURVEYING

When the area surveyed is considerably large to include the curvature of the Earth and degree of accuracy is more, the survey is called Geodetic Surveying. It is also known as Trigonometrical Surveying, as it involves the knowledge of trigonometry.

See Fig. 1. Since geodetic surveying extends over large areas, any line AC connecting two points A and C on the Earth's surface is considered as an arc AC. The angle between any two such arcs CA and CB is treated as a Spherical Angle.

Geodetic surveying is conducted by the Great Trigonometric Survey (GTS) Department of India to locate the widely distant control points.