The operation involved in placing the vertical axis of the instrument exactly over the station mark is known as centring. First, the approximate centring of the instrument is done by moving the tripod legs radially or circumferentially as per need of the circumstances.

It may be noted that due to radial movement of the legs, plumb bob gets shifted in the direction of the movement of the leg without seriously affecting the level of the instrument. On the other hand, when the legs are moved side ways or circumferentially, the plumb does not shift much but the level gets affected. Sometimes, the instrument and the tripod have to be moved bodily for centring. It must be noted that the centering and leveling of instrument is done recursively. Finally, exact centring is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the shifting head is loosened and the upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark. After the exact centring, the screw clamping ring gets tightened.

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