Plane table survey
Plane table survey is that type of survey in which the measurement of survey lines of the transverse a of the transverse and their plotting to a suitable scale are done simultaneously on the field.It consists of:
ü Drawing Board mounted on a tripod
The drawing board is mad of well seasoned wood such as teak and its size varies from 400mm x 300mm to 750mm x 600 mm. the board is mounted on a tripod with ball and socket arrangement which allow the board to be leveled and roatated about the vertical axis.
The open slight alidade The telescope alidade
The open slight alidade consists of a flat rectangular piece of wood of about, 25mm width, 12.5mm thickness and length varying from 200 to 500 mm The leveled edge of the alidade is called finducial edge. Its edge is graduated and serves as a scale. Vertical sight vanes are mounted at both ends. One of the sight vanes is proved with a narrow slit and the other with a central vertical wire. The line of sight is in the same vertical plane as the finducial edge The telescope aliadade consists of a telescope mounted on a horizontal axis the ends of which are supported on standards. A circular level or two spirit levels are attached to the base of the telescope to level the plane table.
Fixing the table to the tripod stand
The table is set up at a convenient height say 1m above ground. The legs of the stand are spread apart and firmly fixed into the ground. The table is then centered. This means that the point plotted on the sheet corresponding to the station occupied should be exactly above the station on the ground. This is done by means of the plumbing fork.
The table is then leveled either by ordinarly tilting the board or by ball and socket arrangement.
Orientation is the process of putting the plane table into some fixed direction so that line representing a certain direction on the drawing sheet is paralle to that direction on the ground.
Two methods adopted:
Orientation by magnetic needle Orientation by back sighting
Methods of plane table survey