Definitions of Basic Terms:
Level Surface
Any level surface parallel to mean spherical surface of earth is called a level surface. This surface is normal to the direction of gravity (indicated by plumb bob). Every point on this surface is equidistant from centre of earth.
A plane tangential to level surface is called the horizontal plane at that point. Any line lying in the horizontal plane is a horizontal line as shown in Figure (a).
Figure(a) : Level Surface