Levelling
Levelling may be defined as the
art of determining the relative height or elevants of points or
objects on the earth’s surface.
Instruments used for leveling:
ü Level
ü Levelling
Staff
Level: The purpose of a level is to
provide a horizontal line of sight.
Parts:
ü A
telescope to provide line of sight
ü A level
tube to make the line of sight horizontal
ü A
leveling head to bring the bubble in its centre of run
ü A tripod
to support the instrument
Dumpy level:
The telescope is rigidly fixed
with the support and therefore, can neither be rotated about the longitudinal axis,
nor can it be removed from its support. A long bubble tube is attached to the
top of the telescope. The leveling head generally consists of two parallel
plates with either three foot screws of four foot screws. The upper plate is
known as tribrach and the lower plate is known as trivet which can be screwed
on a tripod
Levelling staff:
A leveling Staff is a straight
rectangular rod having graduations, the foot of the staff representing zero
reading. There are various type of graduated staff available and only one type
is described here.
Folding type of 4m Levelling Staff:
It consists of two wooden pieces
each of 2m of length with the joint assembly. The thickness and width of staff
are respectively 18m and 75mm. The folding joint is of the detachable type with
locking device at the back.
Hence the stad can be made into
two parts for easy handling. When two pieces are locked together, the two
pieces form a straight rigid leveling staff of length equal to 4m.
Principle of leveling
When the level is set up
correctly and leveled the line of collimation will be horizontal telescope is
rotated about is its vertical axis it will revolve in a horizontal plane known
as the plane of collimation and therefore, all staffs readings taken with the
will be the vertical measurements made downwards from this plane.
To find by how much amount the
line of sight is above the bench mark and To ascertain by how much amount the
next point is below or above the
line sight. Height of instrument=
Elevation of B.M + Back sight Elevation of pt B = Height of instrument –
Foresight
reduction level Height of
instrument method
In this method, the height of
instrument is calculated for ach setting of instruct, and then the elevation or
reduced level of the turning point is calculated with respect to the height of
instrument
Rise and fall method:
In rise and fall method the
difference of level between consequence point is found comparing staff readings
on the two points of same setting of the instrument
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