It consists of a 80 to 150 mm long and 30 mm wide box carrying a freely suspended needle at its centre (Ref. To get perfect level, spirit level should show central position for bubble tube when checked with its positions in any two mutually perpendicular directions. Spirit LevelĪ flat based spirit level is used to level the plane table during surveying (Fig.5). The plumb bob helps in transferring the ground point to the drawing sheet and vice versa also.įig. 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 upper arm is provided with a pointer at the end while the lower arm is provided with a hook to suspend plumb bob. Plumbing fork is a U-shaped metal frame with a upper horizontal arm and a lower inclined arm. Plumbing Fork and Plumb Bobįigure 4 shows a typical plumbing fork with a plum bob. By providing telescope the range and the accuracy of line of sight is increased.įig. If inclined sights are required vertical graduation helps in noting the inclination of the line of sight. If horizontal sight is required bubble in the level tube is kept at the centre. The telescope is provided with a level tube and vertical graduation arc. The line of sight through the telescope is kept parallel to the bevelled edge of the ruler. It consists of a telescope mounted on a column fixed to the ruler. Plain alidade is not suitable in surveying hilly areas as the inclination of line of sight in this case is limited.įig: 2 – Plane Alidade Telescopic Alidade: 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. 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. A sight vane is provided at each end of the ruler. Depending on the type of line of sight there are two types of alidade:įigure 2 shows a typical plain alidade. Always this edge is used for drawing line of sight. One edge of the ruler is bevelled and is graduated. It is a straight edge ruler having some form of sighting device. Drawing sheets and accessories for drawing.Drawing Sheet and Accessories for DrawingĮquipments for Plane Table Surveying The following accessories are required to carry out plane table survey:
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