PLANE TABLE SURVEYING

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PLANE TABLE SURVEYING
Plane table is a graphical method of surveying in which the field works and the plotting is done simultaneously. It is particularly adopting in small mapping. Plane table surveying is used for locating the field computation of area of field.
Merits:
1.      It is most suitable for preparing small scale map or surveying small area.
2.      It is most rapid method.
3.      Field book is not necessary.
4.      No great skill is required for satisfactory map.
5.      It is particularly suitable for magnetic area where prismatic compass is not reliable.
6.      Contour and irregular object may be represented accurately.
7.      It is less costly.

Demerits:
1.      Plane Table Essentially a tropical instruments.
2.      It is not suitable to work in wet climate.
3.      There are several accessories to be carried out and therefore they are likely to be lost.
4.      It is not suitable for accurate work.
Parts of plane Table:
Plane table essentially consist of
1.      Drawing board mounted on tripod.
2.      Alidade.
1. Drawing board mounted on tripod
A sheet of drawing paper, called plane table sheet is fastened to the  board. Board is made up of well seasoned wood such as teak of size 40x30 to 75x60cm. it had plane and smooth top. It is mounted on a tripod in manner that it can be leveled. Leveling up of the table is done by shifting the legs of tripod. Some tripod provided with leveling screw or by ball and socket head for accurate leveling.
2. Alidade:
Alidade consists of two vertical sight vane fitted at end the end of straightedge. The straight edge ruler usually made of brass or teak wood graduated beloved edge. One of the sight veins is provided with narrow slit and the other with a central vertical wire or hair. Beveled working edge alidade is called fiducial edge.
Accessories:
1.      A through campus for marking the direction magnetic meridian on paper.
2.      Sprit level for leveling the table.
3.      Forked plumb for centering the table.
4.      Water proof cover to protect the sheet from rain.
Centering:
It is the process of keeping the table over the station that the point on the paper representing the station being occupied is vertically over the point on the ground. It is done by forked plumb bob.
Orientation:
When the table has to be set up at more than one station it is necessary that it is be oriented so that the lines on the paper remain parallel to the lie which they represent on the ground. So orientation is “the process of keeping the table to the position which is occupied at the first station”.
Orientation is done by two methods:
1.      By use of the magnetic needle.
2.      Orientation by back sighting.
Orientation by the magnetic needle:
To orient the table at any subsequent station, the through compass(or circular box compass) is placed along the line representing the magnetic meridian which has been drawn on the paper at the first station, and the board is then turned until the ends of the needle o\are opposite the zeros of the  scale. The board is then clamped in position. It is suitable for rough small scale mapping.
Orientation by back sighting:
This is the most accurate method of orientation and is always be preferred. Suppose a table is set up over station Q on the line PQ which ahs been previously drowned as PQ from station p. The alidade is placed along the line QP and board then turned until the line of sight bisects the ranging rod at P. Board is then properly clamped


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