The Plane Table surveying is one
of the fastest and easiest methods of surveying. This method of surveying in
which the fieldwork and plotting are done simultaneously called plane
tabling. The principle of plane tabling is parallelism, meaning that the rays
drawn from stations to objects on the paper are parallel to the lines from the
stations to the objects on the ground.
It is commonly employed for small and medium scale mapping,
but this survey is recommended for work where great accuracy is not required.
It is less costly than a theodolite survey. It is very advantageous in
magnetic areas, but it is not suitable for work in a wet climate. As the fitting
and fixing arrangement of this instrument is not perfect, most accurate work
cannot be expected.
ACCESSORIES OF PLANE TABLE SURVEY:-
Some basic surveying instruments are required for plane
table surveying.
The Plane Table – The plane table is drawing board of size
750×600mm. The plane table made of well-seasoned wood like pine, teak etc. The
top survey of the table is extremely smooth and well levelled. The bottom
surface consists of a threaded circular plate for fixing the table.
The Alidade –
There are two types of alidade
a) Plain
and
b) Telescopic
Plain Alidade – The plain alidade consists of a metal or
wooden and it has two edges. One straight and other is bevelled and is known
as the fiducial edge. It consists of two vanes at both ends which are hinged
with the ruler. One known as the ‘object vane’ is open and fine thread between
the top and bottom silt; the other is called the ‘sight vane’ and provide with
the narrow silt.
Telescopic Alidade – Where the plain alidade took only
horizontal sights, this telescopic alidade consists of a telescope meant for
inclined sight or sighting distant objects clearly. It is used for increasing the
accuracy of the sight taken. The alidade has no vanes at the ends but is
provided with the fiducial edge.
Spirit Level:- In-plane table survey, sprit level is very
essential instrument. It contains a small bubble of sprit. A bubble (mercury
is usually used) is put inside the tube. The bubble is visible on the top.
The Compass:-
There
are two compass -
a) The Trough Compass – The trough compass is
rectangular box made of non- magnetic metal containing a magnetic needle pivoted
at the centre.
b) The
Circular Box Compass – It containing a pivoted magnetic needle at the centre.
The compass is meant for marking the north direction of the map.
U-fork or Plum Bob – Plum bob is a U-shaped metal strip that
has two different types of arm. The top arm is pointed and the bottom arm
carries a hook for suspending a plumb bob. The used of plumb bob for centring.
Ranging Rod – It’s a common instrument for all surveying,
used for marking the position of different surveying stations and ranging for
straight lines.
PROCEDURE OF SETTING UP:-
In order to set up a plane table at a station, the following
four operations are performed –
Levelling – The table is fixed on over a station point and
at a convenient height. The tripod stands are well over the station. The table
is levelled by chacking the sprit level at different corners and various points
on the table. When the table is levelled, then the levelling screws or by
simply adjusting the legs.
Centring – U fork and plumb bob is used for centring the
table over the station. Let a point P is selected on the sheet to represent
station P on the ground. The pointed end (at the upper hand) of the plumbing
fork is kept on the paper and at the other end, plumb bob is fixed. The station
P by turning the table clockwise or anticlockwise or adjusting the legs. This
processed is known by Centring.
Making the north-line – The compass is adjusted exactly the
right-hand top corner with its north end approximately towards the north. The
compass needle exactly coincides with the 0-0 mark, now the line representing
the north line. It should ensure that the table is not turned.
Orientation – The process by which the position occupied by
the board at various survey stations are kept parallel is known orientation. In-plane table survey orientation is the most important parts of set up.
Orientation must be done when the plane table is set at more than one station.
Orientation may be done by two following processed-:
Magnetic needle – This method is suitable when local
attraction is not suspended in the area and when it is not possible to bisect
the previous station from the new station.
Backsight – This method is more accurate and is always
preferred. In this method, a particular line drawn from the previous station is
drawn again from the new station. This process is called a backsight method.
Video Link:- Plane table survey
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