Engineering Surveying Exam 2015 Download - Click Here Terms in Levelling Level surface The surface which is parallel to the mean spheroid surface of the earth is known as Level Surface. A surface over which water will not flow (equipotential) The direction of gravity is always normal to a level surface Horizontal surface A surface tangential to the level surface as any point is known as horizontal surface. Horizontal line A line lying on the horizontal surface , is known as the horizontal line. Vertical line The direction indicated by the direction of gravity is known as the vertical line. Vertical plane A plane containing vertical line is a Vertical plane. V.Datum A reference surface to which all the heights of the points were referred Can be arbitrary or a National height datum In Sri Lanka we use MSL as the National Datum for heights The surface which defines the MSL is Mean Sea Level
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