Flerf comment of the day:

All elevations and elevation angles are measured from and use a horizontal, straight reference plane. You can research “Fundamentals of Surveying”.

OK.

S.K. Roy: Fundamentals of Surveying. Second Edition 2010.

“1.5 The Shape and Size of the Earth. 

“Since in surveying we are mainly concerned with measurements on the surface of the earth, it is necessary to know as fully as possible the size and shape of the Earth. The surface of the earth is not of a regular shape because of presence of mountains in some parts and oceans in other. This surface is the topographical surface. The force and direction of gravity at each point varies with the shape of the topographical surface. The surface that is normal to the direction of gravity is defined as a geoid. It is the surface to which the waters of the oceans would tend to conform if allowed to flow into very narrow and shallow canals cut through the land. Geoid is very irregular and to help in mathematical computation a spheroid (which is obtained by rotating an ellipse about its minor axis) is assumed which nearly fits the shape of the Earth. Different countries have their own reference spheroid because they base their computations on the spheroid which fits the geoid with part of the earth’s surface in their respective countries. 

“Figure 1.2 shows the three surfaces. The angle between normal to geoid and normal to spheroid is known as _deflection of the vertical_ or _station error…_” 

And note this: 

“1.6 HORIZONTAL AND LEVEL DISTANCES. 

“A horizontal plane is perpendicular to the plumb line at a point but a level surface is at all points perpendicular to the local plumb line. The two surfaces are coincident at the instrument station but diverge with increasing distance from it due to the earth’s curvature. Hence there is a technical difference between a horizontal distance (HD) and a level difference (LD)…”