Levelling, Level surface mean sea level (msl) ,level line- ,horizontal plane- ,horizontal line- ,plumb line ,mean sea level (msl) ,bench mark ,height of instrument (hi)

1. By Surjeet Singh Rana

2.  Leveling - It is defined as an activity which is performed
to determine the elevations of given points with respect
to a given or assumed datum.
 Level surface- A level surface is a CURVED surface,
which at each point is PERPENDICULAR to the direction
of gravity at that point. "StillWater" is an example of
natural level surface.
 Level line- A line lying on the level surface. Hence it is
also perpendicular to the plumb line at each point.
 Horizontal plane- It is a plane which isTANGENTIAL to
the level surface at a point. It is, therefore, perpendicular
to the plumb line at that point.

3.  Horizontal line- It is a straight line tangential to the level
line at a point. It is also perpendicular to the plumb line.
 Plumb line - It is also called asVertical line. It is an
imaginary line that point towards the gravity of Earth.
 Datum - It is a reference line or surface, with respect to
which vertical distance or elevations are taken.
 Mean Sea Level (MSL) - It is a Datum. 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.

4. 1. Station
2. BS FS IS
3. Height of the Instrument
4. Benchmark, Elevation, Reduced level
5. Line of sight
6. Line of collimation

5.  BENCH MARK – (BM) – B.M. is a fixed reference point of known elevation. It may
be of the following types.
i) GTS Bench mark (Geodetic Triangulation Survey) :
These Bench marks are established by national agency like Survey of India. They
are established with highest precision. Their position and elevation above MSL is
given in a special catalogue known as GTS Maps ( 100 km. interval).
ii) Permanent Bench Mark : They are fixed points of reference establish with
reference to GTS Bench mark (10 km. interval).
iii) Arbitrary Bench mark :These are reference points whose elevations are
arbitrarily assumed. In most of Engineering projects, the difference in elevation is
more important than their reduced levels with reference to MSL as given in a
special catalogue known as GTS Maps ( 100 Km. interval).
Line of Collimation : It is the line joining the intersection of the cross hair and
the optical center of the objective and its extensions, it is also called line of sight
or collimation.

6. The elevation of the line of sight with respect to assumed datum is known as height of
Instrument(HI).
Back sight : (B.S.) - The first sight taken on a leveling staff held at a point of known
elevation. B.S. enables the surveyor to obtain HI +sight i.e. Height of Instrument or
line of sight.
Fore Sight : (F.S.) – It is the last staff reading taken from a setting of the level. It is also
termed as minus sight.
Fore sight is the sight taken on a leveling staff held at a point of unknown elevation to
ascertain the amount by which the point is above or below the line of sight. This is
also called minus sight as the foresight reading is always subtracted from height of
Instrument.
Change Point (CP) : The point on which both the foresight and back sight are taken
during the operation of leveling is called change point.
Intermediate Sight (IS) :The foresight taken on a leveling staff held at a point between
two turning points, to determine the elevation of that point, is known as intermediate
sight.
It may be noted that for one setting of a level, there will be only one back sight and
one foresight but there can be any number of intermediate sights.

7. i) Dumpy level
ii)Tilting level
iii)Automatic level
iv) DigitalAuto level

8.  Dumpy level : It is simple compact and stable. The
telescope is rigidly fixed to its support therefore cannot be
rotated about its longitudinal axis. A long bubble tube is
attached to the top of telescope. Dumpy literally means
short and thick.The different parts of levelling instrument
are,
 (a)Telescope
 (b) Eye-piece
 (c) focussing knob
 (d) level tube
 (e) cross bubble
 (f) foot screws
 (g) levelling head
 (h) diaphragm
 (i) ray shade

9.  Tilting level : It consists of a telescope attached with a level tube which
can be tilted within few degrees in vertical plane by a tilting screw.
 The main peculiarity of this level is that the vertical axis need not be truly
vertical, since the line of collimation is not perpendicular to it.
 The line of collimation, is, however, made horizontal for each pointing of
telescope by means of tilting screw. It is mainly designed for precise
levelling work.

10. A dumpy level, builder's auto level, leveling instrument, or automatic level is an optical
instrument used to establish or verify points in the same horizontal plane.
It is used in surveying and building with a vertical staff to measure height differences and to
transfer, measure and set heights.
Automatic levels are widely used by surveyors, engineers, and construction professionals
to precisely measure horizontal planes and angles.

11. Leveling staff in a straight rectangular rod having graduations the foot of the staff
representing zero reading.The purpose of a level is to establish a horizontal line of
sight.The purpose of leveling staff is to determine the amount by which the
station (foot of the staff ) is above or below the line of sight.
Leveling staffs may be classified into two classes.
 Self reading staff
 Target staff
Self reading staff: is the one which can be read directly by the instrument man
through the telescope.
Target staff: on the other hand, contains a moving target against which the reading
is taken by a staff man.There are usually three forms of self reading staff
 Solid staff
 Folding staff
 Telescopic

12. •Simplelevelling
•Differentiallevelling
•Fly levelling
•Profilelevelling
•Cross sectionallevelling
•Reciprocallevelling
There are two methods for obtaining the
elevations at different points:
- Height of instrument (or planeofcollimation)method
-Rise and fallmethod