Levelling - Definition - Part 1

Vertical Line - A line that follows the direction of gravity as indicated by a plumb line

Level Surface - A curved surface that at every point is perpendicular to the local plumb line. Level surface are approximately spheroidal in shape. A body of still water is the best example.

Level line - A line in a level surface - a curved line

Leveling - Introduction

Leveling - applied to any of the various processes by which elevation points are determined.

Leveling commonly used to:-
1. Design highways, railways, canals and other facilities having grade lines
that best conform to existing topography.
2. Lay out construction project according to planned elevations
3. Calculate volume of earthwork
4. Investigation drainage characteristic of an area
5. Develop map showing general ground configuration
6. Earth study subsidence and crustal motion

Source: Wolf, P. R. & Ghilani, C. D. (2002). Elementary Surveying: An Introduction to Geomatics. (10th ed.). Prentice Hall, US.