What is height of instrument and Rise & fall method of levelling?

Height of Instrument (HI) method: consider the readings taken in the operation shown below:

Reduce level can be computed using the following expression:

RL of a point = HI – FS = ( RLBM +BS) – FS

Arithmetic check: ΣBS – ΣFS= Last RL – first R

Where:

RL: Reduce Level

HI: height of instrument

FS: foresight

BS: Back sight

BM: Bench Marks

Rise and fall (RF) method: the rise or fall of the ground between successive rod stations is considered. For the same instrument set up,

If 2nd rod reading > first rod reading ⇒ fall

If 2nd rod reading < first rod reading ⇒ rise

Then RL of 2nd station = RL of first station – fall       or

RL of first station + rise