WYE Level ( Y Level ) in Levelling
The difference between the dumpy level and the wye level ( Y level ) is that in the former case the telescope is fixed to the spindle while in the wye level, the telescope is carried in two vertical wye supports. The wye supports consists of curved clips. if the clips are raised, the telescope can be rotated in the wyes, or removed and turned end for end.
When the clips are fastened, the telescope is held from turning about its axis by a lug on one of the chips. The bubble tube may be attached either to the telescope or to the stage carrying the wyes. In the former case, the bubble tube must be of reversible type.
The figure below shows the essential features of wye level ( Y level ). The levelling head may be similar to that of a dumpy level. In some cases, the instrument is fitted with a clamp and fine motion tangent screw for controlled movement in the horizontal plane.
The wye level has an advantage over the dumpy level in the fact that the adjustments can be tested with greater rapidity and ease. However, the adjustments do no have a longer life and are disturbed more frequently due to a large number of movable parts