Correct Answer - Option 3 : both back sights and foresights

i. Arithmetic check for Collimation Method:

Sum of B.S. – Sum of F.S. = last R.L. – First R.L.

ii. Arithmetic check for **Rise and Fall Method:**

**Sum of B.S. – Sum of F. S.** = Sum of rise – Sum of fall = last R. L. – first R.L.

__Important Points:__

Other sources of errors in levelling which are not eliminated by arithmetic checks are:

1. Instrument not correctly levelled.

2. Telescope not correctly focused.

3. The wrong crosshair reading recorded (e.g. top instead of the middle).

4. Staff incorrectly read or not held vertical.

5. Staff incorrectly booked.