BASIC TERMS AND DEFINITION REDUCED LEVEL RL Elevation

BASIC TERMS AND DEFINITION Ø Ø Ø REDUCED LEVEL (RL): Elevation of a point is the vertical distance above or below a datum (reference level) considered zero elevation. Levelling is the operation of determining the differences of elevation of points with respect to each other (on, above or below the surface of the earth) Level surface is the curved surface which is perpendicular to the direction of gravity and whose every point is equidistant from the centre of the earth, e. g surface of still water. Level line is any line lying on the level curved surface of the earth. Vertical line is the line joining the point on the earth surface to the centre of earth. It is the line denoted by plumb line.

Horizontal Plane is a plane tangential to the level surface at all point under consideration. It is perpendicular to the plumb line at a point of tangency. Ø Horizontal line is a line in the horizontal plane. It is straight line tangential ti the level line. Ø Axis of telescope is the line joining the optical centre of the objective lens to the optical centre of the eyepiece. Ø Line of sight is the line joining the intersection of the cross hairs to the optical centre of the objective lens and its contribution. It should be horizontal while making sights. Line of collimation is also the line of sight Ø

Axis of level tube is the imaginary line tangential to the longitudinal curve at its mid point. Ø Height of Instrument (HI) is the elevation (RL) of the plane of collimation when the instrument is leveled. Ø Back Sight (BS) is the staff reading taken on point of known elevation eg a BM or a change point. Ø Fore Sight (FS) is the staff reading taken on a point of unknown elevation between back sight and fore sight. Ø Intermediate sight is the staff reading on a point of unknown. Ø