Both ends have angle scales of 0° to 5° that helps to point the north and south pole. A double-edged needle is pivoted in the compass which will freely move inside the compass. The trough compass will be 80 to 150 mm long and 30mm wide. Trough compass is used to mark the north pole on the drawing sheet. Using the help of plumb bob, the ground station point will be marked on the paper (with the help of pointer). The plumb bob and pointer will be on the same line. The upper arm has a pointer fixed at the end. Plumbing fork and plumb bob consist of a upper and lower arm. Plumbing Fork and Bob help to attain the levelling, centering and orientation of the plane table. The whole process of plane table surveying depends on the setup of the plane table in the right position. With the help of the object sight, we can able to point the ranging rods. It is movable in a vertical position mounted with a vertical scale. Here in telescopic alidade, we can do it.Īs you can see from the picture, a telescopic alidade will be mounted on a straight edge ruler to plot the line of sight. In simple alidade, we can’t move it vertically to observe it. On hill stations, we have to adjust the vertical eyesight where telescopic alidade comes in hand. Using both vanes, we plot the line of sight on board. One with narrow slit sight serves as eye vane, and the other wide slit attached with thin wire serves as object vane. Two sight vanes are mounted at two edges. It consists of two foldable vanes which are bevelled and graduated. One helps to view horizontal line sight, and the other helps to measure both horizontal and vertical lines of sight. The difference between the alidades is more like a difference between the tacheometer and theodolite. Based on the line of sight, there are two types of alidade. ![]() AlidadeĪlidade is a straight ruler kind of object. It can be rotated horizontally to place it according to the stations. ![]() The pad is attached to a tripod which allows adjusting the pad to the comfortable height position. ![]() Drawing Sheets & Other Tools Plane Table Survey TripodĪs the name suggests, a plane table pad is like an exam pad which helps to hold the drawing paper in place.
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