Distance Between Two
    Points
 Classes of Level
 Precision of Levelling
 Two Peg Test
 Field Procedures
 Procedural Rules
 Reduction Methods
 Adjustment of Misclosure

Height Difference Between Two Points - Levelling

Basic Principals

Levelling is the process by which differences in height between two or more points can be determined. Its purpose may be to provide heights or contours on a plan, to provide data for road cross-sections or volumes of earthworks, or to provide a level or inclined surface in the setting out of construction works.

Definitions

A level surface

This is a surface such that the direction of gravity is normal to it at all points. Since gravity is varible over the earth's surface, this surface will be irregular.

A horizontal surface

This will form a tangent to the level surface at one point. For most practical purposes using sights of less than 150m the horizontal line approximates very closely to the level line, both being lines on their respective surfaces.

Datum surface

This is an arbitrary level surface to which the heights of all points are referred. This may be the National Datum (Australian Height Datum) or local datum point established on a construction site.

Line of Sight

This is the optical line produced by the telescope of the instrument used for sighting. In this particular case the line is approximately horizontal.