LEVELLING

Leveling is a major process in the construction field. Leveling is concerned with the determination of the differences in elevation of two or more points between each other or relative to some given datum. Leveling is used in taking excavation levels, checking top levels, culvert construction, etc…

Some Definitions Datum

This made be purely arbitrary but for many purposes, it is taken as the Mean sea level. (MSL)

v  Bench Mark (BM)

A fixed reference point of known elevation.

v  Temporary Bench Mark (TBM)

Any mark fixed by an observer for reference purposes.

v  Backsight (BS)

The first staff station of an instrument station is referred to as Back sight.

v  ForeSight (FS)

The last staff reading of an instrument station is referred to as fore sight.

v  Intermediate sight (IS)

Any staff reading between backsight and foresight is referred to as Intermediate sight.

v  Changing Point (CP)

It is a staff station on which two sight FS & BS are taken and is used for the purposes of changing the position of the instrument.

v  Reduced Level (RL)

Height of a point with respect to a known Datum.

v  Mean Sea Level (MSL)

This is the level at some selected station on the coast and is the usual datum to which heights are referred. This is the Mean high of the Sea during an indefinitely long period. The accepted MSL is the mean of the hourly height, extending over 65 years.

v  Line of Collimation

An imaginary line passing through the intersection point of crosshairs and the center point of the objective lens is called the Line of Collimation.

v   Longitudinal section & Cross section

A Longitudinal section follows some predetermined line (not necessarily straight) usually the centerline of the proposed work (road, canal, etc.) This enables us to study the relationship between the existing ground surface and the level of the Network along the direction of the centerline. The additional information required for the design of work can be obtained by means of cross-section taken at right angles to the centerline at suitable intervals, and extending to a sufficient length to cover the boundaries of the work.