Glossary of Module 4 |
R.L. (Reduced Level) : The R.L. of any point represents its elevation (above or below0 with respect to mean sea level as datum. Axis of telescope : line joining the optical center of the objective to the center of the eyepiece. Axis of level tube : an imaginary line tangential to the longitudinal curve of the tube at its mid-point. Back sight : Staff reading taken on a point of known elevation. Bench Mark : A fixed point of known elevation above the datum . Datum : The level surface (real or imaginary) used for reference is called a datum. Elevation : of a point/ station, on or near the surface of the earth, is its vertical distance above or below a datum . Height of instrument : Elevation of the line of sight of the telescope. Horizontal plane : a plane always perpendicular to one considered plumb line. It is tangent to a level surface. Horizontal line : is a line always perpendicular to one considered plumb line. It is tangent to a level line. It lies in Horizontal plane. Horizontal distance : In plane surveying, distance measured along a level line termed as horizontal distance. Intermediate sight : Staff reading on a point, other than back sight and fore sight, of whose elevation is to be determined. Level surface : A level surface is an equipotential surface of the earth's gravity field. It is a curved surface and every element of which is perpendicular to plumb line. A body of still water provides the best example of a level surface. Level line : is a curved line normal to plumb lines at all points. Thus, it lies in a level surface. Line of collimation : Line joining the intersection of the cross-hairs to the optical center of the objective and its continuation. It is also know as Line of sight. Line of sight : is defined as the intersection of the cross hairs and the optical centre of the objective lens. Mean Sea level : Mean sea level (MSL) at a location is obtained by averaging the height of the surface of the sea for all stages of the tides for a period of 18.6 years. It is used as a vertical datum level surface. For India , MSL of Mumbai port is considered as reference surface for finding vertical height. Plumb line : is the direction of the lines of force of earth's gravity field. In plane surveying, it is defined by the direction of a freely suspended plumb-bob. Relative elevation : Difference in elevation between two points is the vertical distance between the level lines passing through the points. Turning point : is an intermittent/ intervening point between set up of stations at which both fore sight and back sight readings are taken. Vertical plane : is a plane along a plumb line. It is tangent to a Horizontal plane Vertical line : is a line along a plumb line. It is tangent to a Horizontal line . It lies in Vertical plane. Vertical distance : is the distance measured along a plumb line . Vertical angle : is an angle between two intersecting straight lines in a vertical plane . |