Fundamentals of Surveying – Plane and Geodetic Surveying Explained

📌 Fundamentals of Surveying

🧭 Introduction

Surveying may be defined as the method of making measurements of the relative positions of, natural & man-made features on earth’s surface and the presentation of this information either graphically or numerically.

The commonest methods of presentation are by means of a Plan or Map. Both Plans and Maps are the graphical representations of the features on a horizontal plane.

Plan is a large scale representation whereas Map is a small scale representation.

Height information can be added either as spot heights, which are individual heights of points, or as contours which give a less detailed but better visual representation of the area.

🌍 Plane & Geodetic Surveying

Surveying is divided primarily into Geodetic surveying & Plane surveying.

In Geodetic surveying, large areas of earth’s surface are involved and the curvature of earth is taken into account.

In Plane surveying, relatively small areas are under consideration, and it is assumed that the earth’s surface is flat.

In Plane surveying, measurements plotted will represent the projection on the horizontal plane of the actual field measurements.

For example, AB is plotted as AB’.

A horizontal plane is normal to the direction of gravity (as defined by a Plumb bob at that point).

However, such a plane will in fact be tangential to the earth’s surface at that point.

Thus, if a large area is considered, the discrepancy will become apparent between the area of the horizontal plane and the actual curved area of the earth’s surface.

In the above figure if actual area is ABC, the projected area will become A’B’C’.

Note:
That Arc AC will be projected as Chord A'C' represented by dotted line. If Arc AB = 18.5 km then Chord A'B' will be 1.52 cm shorter than Arc AB.

📘 Additional Details

• Length AB, BC & CA in Geodetic surveying are determined using spherical trigonometry, whereas lengths A’B’, B’C’ & C’A’ are determined in plane surveying using plane trigonometry.

• For Survey up to 195.5 km² in area, this discrepancy is not serious and therefore plane surveying will be adequate. However precautions are required when connecting such survey to control points established and co-ordinated by geodetic surveys.

• Plane surveys are done for engineering projects such as factories, bridges, dams, location & construction of canals, highways, railways etc.

• Geodetic surveying is done for fixing widely spaced control point, which may afterwards be used as necessary control points for fixing minor control points for plane survey.

• Geodetic survey is carried out by Department of National Survey of India.

Note:
Control points are points of known co-ordinates. It is used as a reference for taking other measurements.