BASIC MEASUREMENTS IN SURVEYING

Surveying is concerned with the fixing of position whether it be control points or points of topographic detail and, as such, requires some form of reference system. The physical surface of the Earth, on which the actual survey measurements are carried out, is not mathematically definable. It cannot therefore be used as a reference datum on which to compute position. Alternatively, consider a level surface at all points normal to the direction of gravity. Such a surface would be closed and could be formed to fit the mean position of the oceans, assuming them to be free from all external forces, such as tides, currents, winds, etc. This surface is called the geoid and is defined as the equi potential surface that most closely approximates to mean sea level in the open oceans.An equi potential surface is one from which it would require the same amount of work to move a given mass to infinity no matter from which point on the surface one started. Equipotential surfaces are surfaces of equal potential; they are not surfaces of equal gravity. The most significant aspect of an equipotential surface going through an observer is that survey instruments are set up relative to it. That is, their vertical axes are in the direction of the force of gravity at that point. A level or equipotential surface through a point is normal, i.e. at right angles, to the direction of gravity. Indeed, the points surveyed on the physical surface of the Earth are frequently reduced, initially, to their equivalent position on the geoid by projection along their gravity vectors.
The reduced level or elevation of a point is its height above or below the geoid as measured in the
direction of its gravity vector, or plumb line, and is most commonly referred to as its height above or below
mean sea level (MSL). This assumes that the geoid passes through local MSL, which is acceptable for most
practical purposes. However, due to variations in the mass distribution within the Earth, the geoid, which
although very smooth is still an irregular surface and so cannot be used to locate position mathematically.