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# Definition of leveling and some vital terms used in surveying & leveling

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Levelling is the process of finding out the pertinent heights of points on the surface of the earth.

A levelling instrument known as “Level” and “Staff” is applied together with other equipments for leveling work.

Some vital terms in Surveying and Levelling.

Level Surface: It belongs to a surface that is normal to the direction of gravity at all points as denoted with a plumb line. As for instance, the surface of a still lake is considered as a level surface.

As the earth refers to an oblique spheroid, a level surface will not be plane rather it will be a cursed one. Each point on a level surface maintains equal distance from the mid point of the earth.

Level Line: A line that rests on a level surface is defined as a level line. It is normal to the plumb line at all points.

Horizontal Plane: A horizontal plane through a point belongs to a plane that is tangential to the level surface at the point. It is normal to the direction of gravity at that point.

Horizontal Line: Any line resting throughout in a horizontal plane is called as “horizontal line.”

Vertical Plane: A vertical plane refers to any plane that includes a vertical line. A vertical line at any point means a line normal to the level surface at that point. As for instance, a plumb line is a vertical line.

Datum Surface: It is an arbitrary surface regarding which the elevations of points are calculated and compared.

Elevation of a Point: Elevation of a point stands for the vertical distance over or under the datum. It is generally known as the reduced level (R.L) of the point. It may be positive or negative on that basis since the point is over or under the datum.

Line of Collimation: The line of collimation or the line of sight belongs to the line that connects the junction of cross-hairs to the optical center of the object glass and its continuation.

Axis of Telescope: The axis of a telescope belongs to the line that connects the optical center of the object glass to the center of the eye piece.

Axis of Bubble Tube: The axis of bubble tube or level tube belongs to the line tangential to the longitudinal curve of bubble tube at its centre point. It is horizontal when the bubble remains in centre. It is also known as “bubble line.”

Vertical Axis: The vertical axis stands for the line on which the telescope is revolved in a horizontal plane.

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