The imaginary level surface with reference to which vertical distances of the points are measured is called as:

Explanation:

Level surface:

• A level surface is the equipotential surface of the earth's gravity field. It is a curved surface and every element of which is normal to the plumb line.
• A body of still water provides the best example of a level surface.

Horizontal plane/ surface: 

• A horizontal plane is a plane that is perpendicular to the plumb line. It is also tangential to a level surface at a particular point.

Mean sea level: 

• It is the altitude of the earth's surface above sea level. MSL adopted in India is located in Mumbai.

Collimation surface: 

• It is an imaginary surface passing through the intersection of the crosshairs at the diaphragm and the optical center of the object-glass and its continuation.

Datum surface: 

• It is a level surface from which the vertical distances are measured for any point in the field