Surveying is the art of mapping and plotting ground condition on paper.

Bench-Marks(BM):

These are fixed points. RL determined with reference to the line. These are very important marks. The serve as reference points for finding the RL of new points or for conducting levelling operations in projects involving roads and railways.

Backsight Reading (BS):

This is the first staff reading taken in any setup of the instrument after the levelling has been perfectly done. This reading is always taken on a point of known R.L, i.e: on a bench-mark or chainage point.

 Forsight Reading(FS):

It is the last staff reading in a setup of the instrument.

 Intermediate sight Reading(IS):

It is the any other staff reading between the BS and FS in the same setup of the instrument.

Different types of Levels:

The following are the different types of level:

1-   The dumpy level.

2-   The wye level (Y level).

3-   The automatic level.

H.I = B.M + B.S
R.L = H.I - I.S / F.S

Levelling:

The art of determining the relative heights of different points on or below the surface of the earth is known as Levelling. Thus leveling deals with measurement in the vertical plane.

Object and use of Levelling:

Object: The aim of levelling is to determine the relative heights of different objects on or below the surface of the earth and to determine the undulation of the ground surface.

Uses: Levelling is done for the following purposes:

-To prepare a longitudinal section and cross section of a project (roads, railways, irrigational canalset).

-To prepare a layout map for water supply sanitary or drainage schemes.

-To know the reduced levels of different points.

Datum surface or Line: 

This is an imaginary level surface or level line from which the vertical distance of different points (above or below this line) are measured.

Reduced Level(RL):

The vertical distance of a point above or below the datum line is known as the Reduced Level(RL) of that point. The RL of a point may be positive or negative according as the point is above or below the datum line.