Description
Plane Table surveying accessories (Alidade, plumbing fork & Bob, Trough Compass, Spirit Level)
Alidade
It consists of two foldable vanes which are bevelled and graduated. Two sight vanes are mounted at two edges. One with narrow slit sight serves as eye vane, and the other wide slit attached with thin wire serves as object vane. Using both vanes, we plot the line of sight on board.
Plumbing Fork & Bob
The whole process of plane table surveying depends on the setup of the plane table in the right position. Plumbing Fork and Bob help to attain the levelling, centering and orientation of the plane table.
Plumbing fork and plumb bob consist of a upper and lower arm. The upper arm has a pointer fixed at the end. The lower arm has a plumb fixed under. The plumb bob and pointer will be on the same line. Using the help of plumb bob, the ground station point will be marked on the paper (with the help of pointer).
Trough Compass
Trough compass is used to mark the north pole on the drawing sheet.
The trough compass will be 80 to 150 mm long and 30mm wide. A double-edged needle is pivoted in the compass which will freely move inside the compass. Both ends have angle scales of 0° to 5° that helps to point the north and south pole.
The compass will be protected by the glass to prevent the wind blow.
Spirit Level
Spirit Level used to level the plane table tripod each time. The mercury bubble on the spirit level helps to ensure that the plane table is mounted at the correct level.
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