Levelling (*or Leveling*) is a branch of surveying, the object of which is

1- To find the elevations of given points with respect to a given or assumed datum.

2- To establish points at a given or assumed datum.

The first operation is required to enable the works to be designed while the second operation is required in the setting out of all kinds of engineering works. Leveling deals with measurements in a vertical plane.

**Horizontal plane:**

A horizontal plane via a point refers to a plane tangential to the level surface at that point. So, it is perpendicular to the plumb line via the point.

**Horizontal line:**

It refers to a straight line tangential to the level line at a point. It is also orthogonal to the plumb line.

**Vertical line:**

** **This line is normal to the level line at a point. It is generally taken as the line determined with a plumb line