Datum

Since the actual ground surface of earth is undulating, one reference line has to be decided to obtain the relative heights of points on ground on the surface of earth in the plot of area surveyed. This arbitrarily decided level surface is known as datum surface. The heights of various points in surveyed area are measured along with reference to this stages surface. The datum was fixed as mean sea level at Karachi during Great Triangulation Survey (GTS). This datum is still being used for benchmarking in all precision surveys.