| Introduction |
In a clear night, the sky appears as a vast hollow hemisphere with its interior studded with innumerable stars. On observing the sky for some duration it appears that the celestial bodies are revolving around the earth with its centre at the position of the observer. The stars move in a regular manner and maintain same position relative to each other. Consequently, terrestrial position or direction defined with reference to celestial body remains absolute for all practical purposes in plane surveying. Thus, the absolute direction of a line can be determined from the position / direction of a celestial body. |