Leveling operations

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.
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.
Level line: A level line is a line lying in a level 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.
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 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.
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.
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 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.
Methods of levelling
Three principle methods are 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, 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.
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.
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 Instruments
The instruments commonly used in direct levelling
are:
1. A level
2. A levelling staff