a level line or level surface is one which at all point is
normal to the direction of the force of gravity as defined
by a freely suspended plumb - bob . as already indicated
in chapter 1 in the discussion of the geoid, such
surfaces are ellipsoidal in shape .
thus in figure 3.1 the difference in level between A and B
is the distance A,B provided that non - parallelism of
level surfaces is ignored