Levelling
The levelling is a branch of serveying which is done to find the elevations of given points with respect to a given or assumed datum i.e. Mean Sea Level (MSL), and also to establish points at a given elevation or at different elevations with respect to a given or assumed datum. It deals with measurements in a veritcal plane.
Level Surface: - A level surface is defined as a curved surface which at each point is perpendicular to the direction of gravity at the point.
Level Line: - A level line is a line lying in a level surface. It is normal to the plumb line at all points.
Horizontal Plane: - Horizontal plane through a point is a plane tangential to the level surface at that point. It is, therefore, perpendicular to the plumb line through the point.
Horizontal Line: - It is straight line tangential to the level line at a point. It is also perpendicular to the plumb line.
Vertical Line: - It is a line normal to the level line at a point. It is commonly considered to be line defined by a plumb line.
Datum: - Datum is any surface to which elevations are referred. The mean sea level affords a convenient datum world over, and the elevations are commonly given as so much above or below sea level.
Reduced Level or Elevation: - The elevation of a point on or near the surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum.
Vertical Angle: - It is an angle between two intersecting lines in a vertical plane. Generally, one of these lines is horizontal.
Mean Sea Level: - It the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years.
Bench Mark (B.M.): - It is a relatively permanent point of reference whose elevation with respect to MSL is known. It is used either as a starting point for levelling or as a point upon which to close as a check.
No comments:
Post a Comment