Back Sight (BS):

It is the first staff reading taken at any setup of the instrument.

BS is always taken on the point of known R.L i.e. on the Bench Mark BM.

Fore Sight (FS):

It is the Last staff reading taken at any setup of the instrument.

Intermediate Sight (IS):

It is any other staff reading between BS and FS in the same  setup of the instrument.

Change Point (C.P):

It is Point at which both BS and FS is taken.

CP indicates the shifting of the instrument.

Height of Instrument (H.I.):

The RL of the line of collimation is called as Height of Instrument (HI).

HI = RL of BM + BS

All of the Above Points are performed and explained in simple Hindi language in following video.

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