Definitions of teams in Levelling
Levelling (or Leveling) is a branch of surveying, the object of
which is: i) to find the elevations of given points with respect to a given or
assumed datum, and ii) to establish points at a given or assumed datum. The
first operation is required to enable the works to be designed while the second
operation is required in the setting out of all kinds of engineering works.
Levelling deals with measurements in a vertical plane.
which is: i) to find the elevations of given points with respect to a given or
assumed datum, and ii) to establish points at a given or assumed datum. The
first operation is required to enable the works to be designed while the second
operation is required in the setting out of all kinds of engineering works.
Levelling deals with measurements in a vertical 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. The surface of a still water is a truly level surface. Any surface
parallel to the mean spheroidal surface of the earth is, therefore, a level
surface.
surface which at each point is perpendicular to the direction of gravity at the
point. The surface of a 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. It is, therefore, normal to the plumb line at all points.
surface. It is, therefore, 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.
plane tangential to the level surface at that point. It is, 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.
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 the line defined by a plumb line.
a point. It is commonly considered to be the line defined by a plumb line.
Datum: Datum is any surface to which elevation
are 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 is often
more convenient, however, to assume some other datum, specially, if only the
relative elevation of points are required.
are 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 is often
more convenient, however, to assume some other datum, specially, if only the
relative elevation of points are required.
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. The difference in elevation between two points is the
vertical distance between the two level surface in which the two points lie.
surface of the earth is its vertical distance above or below an 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.
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 is derived by averaging the
hourly tide heights over a long period of 19 years.
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: It is a relatively permanent point of
reference whose elevation with respect to some assumed datum is known. It is
used either as a starting point for levelling or as a point upon which to close
as a check.
reference whose elevation with respect to some assumed datum is known. It is
used either as a starting point for levelling or as a point upon which to close
as a check.
Methods of levelling
Three principle methods are used
for determining differences in elevation, namely, barometric levelling,
trigonometric levelling and spirit levelling.
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, therefore, may be used and the readings observed at different points
would yield a measure of the relative elevation of those points.
use of the phenomenon that difference in elevation between two points is
proportional to the difference in atmospheric pressures at these points. A
barometer, therefore, may be used and the readings observed at different points
would yield a measure of the relative elevation of those points.
At a given point, the
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.
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)
Trigonometric or Indirect
levelling is the process of levelling 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 be computed from proper
trigonometric relations. In a modified form called stadia levelling, commonly
used in mapping, both the difference in elevation and the horizontal distance
between the points are directly computed from the measured vertical angles and
staff readings.
levelling is the process of levelling 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 be computed from proper
trigonometric relations. In a modified form called stadia levelling, commonly
used in mapping, both the difference in elevation and the horizontal distance
between the points are directly computed from the measured vertical angles and
staff readings.
Spirit Levelling
(Direct Levelling)
It is that branch of
levelling in which the vertical distances with respect to a horizontal line
(perpendicular to the direction of gravity) may be used to determine the
relative difference in elevation between two adjacent points. A horizontal
plane of sight tangent to level surface at any point is readily established by
means of a spirit level or a level vial. In spirit levelling, a spirit level
and a sighting device (telescope) are combined and vertical distances are measured
by observing on graduated rods placed on the points. The method is also known
as direct levelling. It is the most precise method of determining elevations
and the one most commonly used by engineers.
levelling in which the vertical distances with respect to a horizontal line
(perpendicular to the direction of gravity) may be used to determine the
relative difference in elevation between two adjacent points. A horizontal
plane of sight tangent to level surface at any point is readily established by
means of a spirit level or a level vial. In spirit levelling, a spirit level
and a sighting device (telescope) are combined and vertical distances are measured
by observing on graduated rods placed on the points. The method is also known
as direct levelling. It is the most precise method of determining elevations
and the one most commonly used by engineers.
Levelling Instruments
The instruments commonly used
in direct levelling are:
in direct levelling are:
- A level
- A levelling staff
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