A plane table surveying one of the widely used types of surveying. A plane table alidade is a straight edge with some form of sighting device. Two types of alidade are used in plane table surveying, they are plain alidade and telescopic alidade.
Types of Alidade in plane table surveying
Figure above shows a simple form and used for ordinary work. 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 vanes is provided with a narrow slit while the other is open and carriers a hair or thin wire.
Both slits thus provide a definite line of sight that 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 ( known as a fiducial edge) of the alidade. It is essential to have the vanes perpendicular be the surface of the sheet. The alidade is not very much suitable in the hilly area since the inclination of the line of sight is limited. A string joining the tops of the two vanes is sometimes provided to use it when sights of considerable inclination have to be taken.
2. Telescopic alidade
A telescopic alidade is used when it is required to take inlined sights. Also, the accuracy and range of sights are increased by its use. It essentially consists of a small telescope with a level tube and graduated arc mounted on a horizontal axis.
The horizontal axis rests on an A-frame fitted with verniers fixed in position in the same manner as that in a transit. All the parts are finally supported on a heavy rule, one side of which is used as the working edge along which line may be drawn. The inclination of the line of sight can be read on the vertical circle. The horizontal distance on the staff kept at the point.
The elevation of the point can also be computed by using usual tacheometric relations. Sometimes, to facilitate calculation work, a Beaman stadia arc may be provided as an extra. Thus, the observer can very quickly and easily obtain the true horizontal distance from the plane table to a leveling staff placed at the point and the difference in elevation between them. These are the types of alidade used in plane table surveying.