In this article, we will discuss the tangent clinometer.
Indian pattern clinometer (Tangent Clinometer) is used for determining the differences in elevation between points and is specially adapted to plane tabling.
The clinometer is placed on the plane table which is levelled by estimation.
The clinometer consists of the following:
(1) A base plate carrying a small bubble tube and a levelling screw. Thus, the clinometer can be accurately levelled.
(2) The eye vane carrying a peephole. The eye vane is hinged at its lower end to the base plate.
(3) The object vane having graduations in degrees at one and a tangent of the angles to another side of the central opening.
The object vane is also hinged at its lower end to the base plate. A slide, provided with a small window and horizontal wire in its middle, can be moved up and down the object vane by a rack and pinion fitted with a milled head.
The line of sight is defined by the line joining the peephole and the horizontal wire of the slide.
When the instrument is not in use, the vanes fold down over the base.
2. Uses of Tangent Clinometer with a Plane Table
1. Set the plane table over the station and keep the Indian pattern clinometer on it.
2. Level the clinometer with the help of the levelling screw.
3. Looking through the peephole, move the slide of the object vane till it bisects the signal at the other point to be sighted. It is preferable to use a signal of the same height as that of a peephole above the level of the plane table station.
4. Note the reading, i.e tangent of the angle, against the wire. Thus,
the difference in elevation between the eye and the object = distance x tangent of vertical angle = d tanα.
The distance d between the plane table station and the object can be found from the plan. The reduced level of the object can thus be calculated if the reduced level of the plane table station is known.
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