The most commonly used plane table is shown in Fig. 14.1. It consists of a well seasoned wooden table top mounted on a tripod. The table top can rotate about vertical axis freely. Whenever necessary table can be clamped in the desired orientation. The table can be levelled by adjusting tripod legs.
The following accessories are required to carry out plane table survey:
2. Plumbing fork with plumb bob.
3. Spirit level
4. Trough compass
5. Drawing sheets and accessories for drawing.
It is a straight edge ruler having some form of sighting device. One edge of the ruler is bevelled and is¬†graduated. Always this edge is used for drawing line of sight. Depending on the type of line of sight
there are two types of alidade:
(a) Plain alidade (b) Telescopic alidade
Plain Alidade: Figure 14.2 shows a typical plain adidate. A sight vane is provided at each end of the ruler. The vane with narrow slit serves as eye vane and the other with wide slit and having a thin wire at its centre serves as object vane. The two vanes are provided with hinges at the ends of ruler so that when not in use they can be folded on the ruler. Plain alidade is not suitable in surveying hilly areas as the inclination of line of sight in this case is limited.
Telescopic Alidade: It consists of a telescope mounted on a column fixed to the ruler [Fig. 14.3].
The line of sight through the telescope is kept parallel to the bevelled edge of the ruler. The telescope is¬†provided with a level tube and vertical graduation arc. If horizontal sight is required bubble in the level¬†tube is kept at the centre. If inclined sights are required vertical graduation helps in noting the inclination¬†of the line of sight. By providing telescope the range and the accuracy of line of sight is increased.
Plumbing Fork and Plumb Bob
Figure 14.4 shows a typical plumbing fork with a plum bob. Plumbing fork is a U-shaped metal frame¬†with a upper horizontal arm and a lower inclined arm. The upper arm is provided with a pointer at the¬†end while the lower arm is provided with a hook to suspend plumb bob. When the plumbing fork is kept¬†on the plane table the vertical line (line of plumb bob) passes through the pointed edge of upper arm.¬†The plumb bob helps in transferring the ground point to the drawing sheet and vice versa also.
A flat based spirit level is used to level the plane table during surveying (Fig. 14.5). To get perfect level,¬†spirit level should show central position for bubble tube when checked with its positions in any two¬†mutually perpendicular direction.
It consists of a 80 to 150 mm long and 30 mm wide box carrying a freely suspended needle at its centre¬†(Ref. Fig. 14.6). At the ends of the needle graduations are marked on the box to indicate zero to five¬†degrees on either side of the centre. The box is provided with glass top to prevent oscillation of the¬†needle by wind. When needle is centred (reading 0‚Äď0), the line of needle is parallel to the edge of the¬†box. Hence marking on the edges in this state indicates magnetic north‚Äďsouth direction.
Drawing Sheet and Accessories for Drawing
A good quality, seasoned drawing sheet should be used for plane table surveying. The drawing sheet¬†may be rolled when not in use, but should never is folded. For important works fibre glass sheets or¬†paper backed with thin aluminium sheets are used.
Clips clamps, adhesive tapes may be used for fixing drawing sheet to the plane table. Sharp hard¬†pencil, good quality eraser, pencil cutter and sand paper to keep pencil point sharp are other accessories¬†required for the drawing work. If necessary, plastic sheet should be carried to cover the drawing sheet¬†from rain and dust.