Geodesy deals with the precise size, shape and gravitational field of the earth. The actual physical surface of the earth is very irregular. A more uniform surface is the geoid which is the equipotential surface which coincides with mean sea level (MSL). This too is irregular in shape in the sense that it cannot be represented by a finite mathematical formula, due to variations of gravity. In dealing with precise dimensions and the shape of the earth it is necessary to adopt a reference surface on which mathematical computations can be carried out and variations of the geoid from this reference surface could be indicated by elevations and depressions, termed Geoidal undulations above and below the reference surface. The mathematical surface which best fits the geoid is an ellipsoid of rotation. The dimensions of the ellipsoid vary according to the one selected by each country and for Sri Lanka the dimensions of the semi-major and minor axes are 6377276.345 metres and 6356075.413 metres respectively which is the Everest Figure 1830 of the Earth used in the Survey of India.
Observations of angles and distances are carried out on the earth's physical surface. They have first to be reduced to the ellipsoid before computations can commence. Computations are done using the dimensions of the ellipsoid adopted and appropriate formulae.