SURVEYING
& LEVELLING
Lecture
Civil Engineering Dept.
Leveling
- Leveling is defined as the process of measuring the difference in height
between points on the surface of the earth.
Objectives of Leveling
1) To find the elevation of a given point with respect to the given or assumed
Datum.
2) To establish a point at a given elevation with respect to the given or
assumed Datum.
Uses
Leveling is done for the following
1) To prepare a contour maps
2) To determine altitudes of different points on a hill or to know the reduced
levels of different points on or below the surface of the earth.
3) To prepare a longitudinal section and cross sections of a project.
4) To prepare layout map for water supply, sanitary or drainage schemes.
Definitions
1. Level Surface or Level Line : Any surface parallel to the mean spheroidal of
the earth is called level surface. Such a surface is obviously curved. The
water surface of a still lake is considered as level surface.
2. Horizontal surface or Horizontal line : Any plane tangential to the level
surface at any point is known as horizontal surface. It is perpendicular to
the plumb line which indicates the direction of gravity.
3. Datum Surface : Datum surface is any level surface to which the elevations
of all points may be referred. The mean sea level is usually adopted as
datum.
4. Reduced Level : The reduced level of a point is its vertical distance above
or below the datum surface.
5. Bench marks (BM) : Bench marks are stable reference points the reduced
levels of which are accurately determined by leveling.
6. Back sight (BS) : This is the first reading taken with a leveling instrument
in a leveling operation.
7. Foresight (FS) : This is the last reading taken in a leveling operation.
8. Intermediate Sight (IS) : This is the reading taken between the back sight
and foresight in a leveling operation.
9. Turning Point or Change Point (CP) : A change point or turning point
indicates the shifting of the instrument. At this point, a FS is taken from
one setting and a BS is taken from the next setting.
10. Height of Instrument (HI) : The elevation of the line of sight established
by the instrument.
Line of collimation: It is an imaginary line passing through the intersection of
cross hairs at the diaphragm and the optical center of the object glass and its
continuation. It is also known as the line of sight.
Equipment for Leveling
The level, its tripod and the staff are all precision items of equipment upon
which the accuracy of the work is highly dependent.
Levels
A level is basically a telescope attached to an accurate leveling device, set
upon a tripod so that it can rotate horizontally through 360°.
There are three basic types of level
(a) Dumpy levels: The telescope is rigidly attached to a single bubble and the
assembly is adjusted either by means of a screwed ball-joint or by foot-
screws which are adjusted first in one direction, then at 90°.
(b) Tilting levels: This type of level is fitted with a circular bubble for
preliminary approximate leveling and a main bubble which is attached to the
telescope. For each observation the telescope tilted by a fine screw to bring
the main bubble to the center.
(c) Automatic levels
This more modern type of level is now in general use. It has a compensator
which consists of an arrangement of three prisms. The two outer ones are
attached to the barrel of the telescope. The middle prism is suspended by
fine wiring and reacts to gravity. The instrument is first leveled approximately
with a circular bubble; the compensator will then deviate the line of sight by
the amount that the telescope is self aligned.
Staves
The leveling staff is a box section of aluminum or wood, which will extend to 3
or 5 m in height. One face has a graduated scale attached for reading with the
cross-hairs of the level telescope. These faces can vary in pattern and
graduation; 5mm graduations should be the maximum for accurate leveling of
gauging stations.
Many staves used these days are of aluminum because of its durability.
Tilting Levels
Automatic
Level
DIGITAL LEVEL
Trimble DiNi Digital Level
Leica DNA03 Digital Level
Leveling
Staves
Leveling Process
1. First of all the instrument is placed at a suitable location from where
maximum number of observations can be taken.
2. Instrument should be perfectly leveled by the help of circular bubble.
3. If using a tilting level, the telescope is leveled with the help of a plate
bubble before taking every observation.
4. Eye-piece should be focused by holding a sheet of white paper in front of
the telescope.
5. Objective should be focused by pointing the telescope towards the staff.
6. Parallax should be eliminated.
7. Verticality of the staff should be verified with the help of vertical hairs
of the diaphragm.
8. After the staff reading, the position of the bubble should be verified. If
it is disturbed, the reading should be taken again.
Temporary adjustments of level
1. selection of suitable position
2. fixing level with tripod stand
3. Approximate levelling with legs of tripod
4. Perfect levelling with foot screws
5. Focusing the eye piece
6. Focusing the object glass
7. Taking the staff readings
Types of Leveling Operations
Simple Leveling :
When the difference between two points is determined by
setting the leveling instrument midway between the points, the
process is called simple leveling.
Differential Leveling
Also known as Compound Leveling or continuous leveling.
It is adopted when
- Points are at greater distance
- Difference of elevation between the points is large
- Obstacles are present between the points
Simple Leveling
Differrential Leveling
Fly Leveling
When Differential leveling is done in order to connect a BM
to the starting point of the project by only taking back and
fore sights. It is done to check the accuracy of the work.
ERRORS IN LEVELLING
There are a large number of potential sources of error in leveling.
1. Collimation Error
2. Error due to Earth Curvature
3. Error due to Refraction
Collimation Error:
Collimation error occurs when the collimation axis is
not truly horizontal when the instrument is level. The
collimation axis is tilted with respect to the horizontal
by an angle a.
Earth Curvature:
Due to the curvature of the Earth, the line of sight at
the instrument will deviate from a horizontal line as
one moves away from the level.
Refraction:
The variable density of the Earth's atmosphere causes a bending of
the ray from the staff to the level. The effect is illustrated in the
sketch below.
Concluded
