**Levelling (or Leveling) is a branch of surveying**-Types Of Leveling.

**The objects of which is:-**

- To find the elevations of given points with respect to a given or assumed datum, and
- To establish points at a given or assumed datum.
- The first operation required to enable works to design while the second operation required in the setting out Of all kinds of engineering works.
- Leveling deals with measurements in a vertical plane.

**Level surface:-**

A level surface defined as a curved surface and at each point is perpendicular to the direction of gravity at the point.

The surface of still water is a truly level surface.

Any surface parallel to the mean spheroidal surface of the earth is, therefore a level surface.

**Level line:-**

A level line is a line lying in a level surface.

Therefore, normal to the plumb line at all points.

**Horizontal plane:-**

A horizontal plane through a point in a plane tangential to the level surface at that point.

Therefore, perpendicular to the plumb line through the point.

**Horizontal line:-**

It is a 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 surface to which elevation is referred.

The mean sea level affords a convenient datum world over, and elevations are commonly given as so much above or below sea level.

It often more convenient, however, assume some other datum.

especially, only relative elevation of point required.

**Elevation:-**

The elevation of a point on or near the surface of the earth is the vertical distance above or below the arbitrarily assumed level surface or datum.

The difference in elevation between two points is the vertical distance between the two-level surface in which the two points lie.

**Vertical angle:-**

Vertical angle is an angle between two intersecting lines in a vertical plane. Generally, one of these lines is horizontal.

**Mean sea level:-**

It is the average height of the sea for all stages of the tides.

At any particular place, it derived by averaging the hourly tide heights over a long period of 19 years.

**Benchmark:-**

It relatively permanent point of reference whose elevation with respect to the some assumed datum is known.

It used either as a starting point for leveling or as a point upon which to close as a check.

**Methods of levelling**(Types Of Leveling)

Three principal methods used for determining differences in elevation, namely, barometric levelling, trigonometric levelling and spirit levelling.

**Barometric levelling**

Barometric levelling makes use of the phenomenon that difference in elevation between two points is proportional to the difference in atmospheric pressures at these points.

A barometer may used and the readings observed at different points

That would yield a measure of the relative elevation of those points.

At a given point atmospheric pressure doesn’t remain constant in the course of the day even in the course of an hour.

The method is, therefore, relatively inaccurate and is little used in surveying work except on reconnaissance or exploratory survey.

**Trigonometric Levelling (Indirect Levelling)**-Types Of Leveling

Trigonometric or Indirect leveling is the process of leveling in which the elevations of points are computed from the vertical angles

And horizontal distances measured in the field

Just as the length of any side in any triangle can computed from proper trigonometric relations.

In a modified form called stadia leveling.

Commonly used in mapping.

Both the different elevations and the horizontal distance between the points directly computed from the measured vertical angles and staff readings.

**Spirit Levelling (Direct Levelling)**–**Types Of Leveling**

Spirit Leveling is a branch of leveling in which the vertical distances with respect to the horizontal line may be used to determine the relative difference in elevation between two adjacent points.

A horizontal plane of sight tangent to the level surface at any point readily established by means of a spirit level or a level vial.

A spirit level and sighting device (telescope) combined and vertical distances measured by observing on graduated rods placed on the points.

The method also known as direct leveling.

It is the most precise method of determining elevations and the one most commonly used by engineers.

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