What is Levelling?
The art of determining the relative heights of differnt points on or below the surface of the earth is known as Levelling.
Why Levelling is done?
- To determine the relative heights of different points on the earth.
- To determine the undulation of ground surface.
- To prepare contour map
- To determine altitude or RL of different points.
What is a Datum?
Datum are the imaginary level surface or level line from which the vertical distances of different points are measured.
What is Reduced Level( RL)?
Reduced level (RL) is the vertical distance of a point anove or below the datum line .
What is meant by Mean Sea Level (MSL)?
MSL is the average elevation of the surface of the sea.
What is Bench Mark?
These are the fixed points or marks of known RL determined with reference to the datum line.
What is meant by Line of Collimation?
Line of Collimation is the imagionary line pasing through the intersection of the cross hairs of the diaphragm and the optical center of the object glass and its continuation.It is also known as line of sight.
Apparent movement of the image relative to the cross hairs is known as parallax.