Aquaculture Engineering

Plane table surveying - Instruments - I

Unit 1- Surveying

Plane table surveying
Instruments used for plane table surveying

The plane table
The plane table consists of a small drawing board mounted on a light tripod in such a way that the boards can be rotated about the vertical axis and can be clamped in any position.There are three types of plane table,

• Traverse table
• Johnson table
• Coast survey table

Alidade
A plane table alidade is a straight edge with some form of sighting devices. It generally consists of a metal or wooden rule with two vanes at the ends. The two vanes or sights are hinged to fold down on the rule when the alidade is not in use. One of the vane is provided with a narrow slit, while the other is open and carries a hair or thin wire. Both the slits thus provide a definite line of sight which can be made to pass through the object to be sighted. The alidade can be rotated about the point representing the instrument station on the sheet so that the line of sight passes through the object to be sighted. A line is then drawn against the working edge of the alidade. The alidade is not very much suitable on hill area since the inclination of the line of sight is limited.
There are two types of alidade.

• Plane alidade
• Telescopic alidade.

Plumbing Fork
The plumbing fork used in large scale work, is meant for centring the table over the point or station occupied by the plane table when the plotted position of that point is already known on the sheet. Also in the beginning of the work, it is meant for transferring the ground point on to the sheet so that plotted point and the ground station are in the same vertical line. The fork consists of a hair pin-shaped light metal frame having arms of equal length in which a plumb bob is suspended from the end of lower arm. The fitting can be placed with the upper arm lying on the top of the table and the lower arm below it, the table being centered when the plumb bob hangs freely over the ground mark and the pointed end of the upper arm coincides with the equivalent point on the plan.