Answer: D. Horizontal axis in a vertical plane
In transit theodolite telescope is rotated through horizontal axis in a vertical plane.
Transit Theodolite: A theodolite is said to be a transit one when its telescope can be revolved through 180° in a vertical plane about its horizontal axis, thus directing the telescope in the exact opposite direction.
Types of Theodolite:
1. Transit Theodolite: A transit Theodolite is the one in which the telescope mounted in the instrument can be revolved through a complete revolution about its horizontal axis, in a vertical plane.
2. Non-Transit Theodolite: It is the opposite of Transit Theodolite. In this type of Theodolite, the telescope can not be revolved through a complete revolution about its horizontal axis, in a vertical plane. It can be rotated to a certain extent to take vertical angles.
Theodolites can also be classified into two categories on the basis of the Scale used in theodolite as:
Vernier Theodolite: it is fitted with a Vernier Scale. Vernier Theodolites are most commonly used in normal Surveying operations.
Micrometer Theodolite: Fitted with a micro micrometer.
The size of Theodolites is defined according to the diameter of its main scale, such as a 10 cm theodolite which means that the diameter of its main scale is 10cm.
In a survey, 8 cm to 12 cm theodolites are generally used. Hence, in transit theodolite telescope is rotated through horizontal axis in a vertical plane.
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