This document provides an overview of leveling, including key terms, types of benchmarks, classifications of leveling, and sources of errors. It defines leveling as measuring the relative heights of points on Earth's surface. Key terms include level surface, horizontal line, and datum surface. There are four main types of benchmarks: GTS, permanent, arbitrary, and temporary. Classifications of leveling include simple, differential, profile, cross-sectional, fly, check, and reciprocal leveling. Sources of errors in leveling come from instruments, refraction, and personal factors.

1. LEVELLING

2. INTRODUCTION
• An art of measuring the relative heights or elevations of the points or objects on the
surface of the earth is called as levelling.
• Levelling deals with the measurements in vertical plane.
• It can also defined as an operation for the measurements in vertical plane.
• Levelling is an operation for the measurement of the difference in the elevations
between points above some given plane or surface known as datum surface which
may be purely an arbitary one for small surveys but generally taken as the mean sea
level (m.s.l.) for major surveys.
• Datum means any surface of which exact elevations are known.
• The mean sea level is often considered or referred to as the datum all over the
world.

3. IMPORTANT TERMS IN LEVELLING
• Level surface-
Any surface parallel to the mean spheroidal of
earth called level surface.
• Level line-
A line lying in a level surface normal to the plumb
line at all points called level line
• Horizontal line-
A straight line tangential to the level line and
perpendicular to the plumb line is called as
horizontal line.
• Horizontal surface-
Any surface tangential to the level surface at a
given point called as horizontal surface.
• Vertical line-.
A line at any point normal to the level surface
through that point called as vertical line. It is called
as plumb line at that point.

4. • Datum surface-.
An arbitrarily assumed level surface from which vertical surface from which vertical
distances are measured is called as datum surface. The mean sea level at Karachi is
considered as the datum surface for G.T.S. Bench mark in India.
• Mean sea level-
Mean sea level is the average height of sea for all stages of the tides.
• Parallex-
If the image formed by objective lens is not in the same plane with cross-hairs, any
movement of the eye is likely to cause an permanent movement of the image with
respect to the cross-hairs. This is parallel.
• Bench mark-.
It is a fixed reference point whose elevation with respect to some datum is known. It
is noted that any levelling work is started from bench mark (B.M.)
• Reduced level-
It is vertical distance of a point measured above or below the datum is calculated by
following two methods- 1) Height of collimation method. 2) Rise and fall method

5. TYPES OF BENCH MARK
1. G.T.S. Bench mark-
a) These bench marks are established by the survey of India department
throughout the country with very high precision with reference to mean sea
level considered as zero level at Karachi in Pakistan.
b) The position of G.T.S. Bench marks & elevations are shown on the G.T.S. Maps
by survey of India department.
c) Levelling work carried out with reference to G.T.S. Bench marks gives elevations
of points with respect to mean sea level.
2. Permanent bench marks-
a) These are permanent reference points established with reference to G.T.S.
Bench mark on permanent object like bridge parapets, culverts, well, plinth of any
government buildings.
b) The location & reduced level of this bench mark is usually marked on top of the
permanent object.

6. Arbitary bench marks-
a) When small ordinary levelling work is to be carried out or when permanent bench
mark are not nearby the place where survey is to be carried out, then to start the
levelling work any prominent object like plinth or steps of the building etc. is chosen
as bench mark & its elevation is assumed arbitrarily.
b) The elevation of points obtained in this type of levelling work are elevations with
respect to mean sea level as the elevation of B.M. Is assumed arbitrarily
Temporary bench mark-
a)The survey for big projects normally continues for a number of days. Hence at the
end of day when levelling work of that day has to be stopped, then any permanent
object is chosen on which the work is stopped & can be started further on the next
day. Then these types of bench mark chosen is known as temporary bench marks.
b) With reference to this temporary bench mark, the work that is carried forward on
the next day & so on.
3)
4)

7. CLASSIFICATION OF LEVELLING
1. Simple levelling.
2. Differential levelling.
3. Profile levelling.
4. Cross sectional.
5. Fly levelling.
6. Check levelling.
7. Reciprocal levelling.
8. Trignometrical levelling

8. A) SIMPLE LEVELLING
1. When we want to find the difference
in elevation between the points which
are visible from same position of the
instrument station, such levelling
called as simple levelling.
2. This method is commonly used.
3. No change point is taken for this
method.

9. B) DIFFERENTIAL LEVELLING
Differential levelling is used when it is
required to find the difference in
elevation between the two points having
considerable distance between them.
This method is adopted for the following
two cases:
1. When the difference in elevation
between the points is much more.
2. When there is some obstacles such as
hill, tree, heap, crest,trough in ground
or any object come in the way of
levelling.

10. C) PROFILE LEVELLING
Profile levelling is the process of
determining the elevations of points at
measured intervals along a fixed line such
as the centre line of railway, highway,
canal or sewer.
It is also known as longitudinal
sectioning.

11. D) CROSS SECTIONS• The lines of cross sections are in general
perpendicular to the longitudinal section
line.
• The purpose of cross sectioning is to
furnish the engineer with sufficient
information regarding the levels of ground
on either side of the longitudinal section to
enable distance along the centre line. The
length of cross-section may be run at closer
intervals to outline the features of road,
nala etc. Electro refining
• The cross sections are numbered
consecutively from the commencement of
the centre line & are set out at right angles
to the main line of section with the chain &
tape. The cross staff or the optical square &
the distances are measured left & right
from the centre peg. The length of cross-
section depends upon nature of work

12. E) FLY LEVELLING
• Fly levelling is the process of
determining the elevations of points
which are certain distance apart.
• A series of level is taken from one
bench mark to the other.
• It is used to establish bench mark it is
also known as differential levelling or
compound levelling.

13. F) CHECK LEVELLING
• It is the process of running levels for the purpose of checking series of levels.
• A good check is obtained if the levelling line closes on itself or on the benchmark of
known elevation, however, such closure does not check any reading on a
intermediate point, but an error at an intermediate point is isolated and cannot be
forward.
• Also at end of each day’s work, a line of levels is run, returning to the starting point
of that day with a view to checking the work done on that day.

14. G) RECIPROCAL LEVELLING
• When it is necessary to carry levelling
across a river or any obstacle a long
sight between two points so situated
that no place for the level can be found
from which the lengths of foresight and
backsight will be even approximately
equal.
• Special method i.e. Reciprocal levelling
must be used to obtain accuracy and
eliminate the following errors-
1) curvature & refraction.
2) Error in instrument adjustment.

15. H) TRIGNOMETRICAL LEVELLING
• In trignometrical levelling find out the
vertical distance between two points by
taking the vertical angular observations
and the known distances. The known
distances are either assumed to be
horizontal or the geodetic lengths at the
mean sea level(MSL). The distances
are measured directly(as in the plane
surveying) or they are computed as in
the geodetic surveying.

16. ERRORS IN LEVELLING( SOURCES)
Instrumental
1. Imperfect adjustment
2. Sluggish bubble
3. Faulty focussing tube
4. The levelling staff
Refraction
1. Earth’s curvature
2. Atmospheric refraction
3. Variations in temperature
4. Settlement of tripod
5. Wind vibration
Personal
1. Mistakes in manipulation
2. Mistake in reading rod
3. Errers in sighting
4. Mistakes in recording

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