• Height of Instrument :

It is the method which deals with the obtaining of RL of the line of collimation by adding BS reading of a known RL point. Thus, RL of the line of collimation is called the Height of Instrument. It is always measured from the benchmark. Hence the benchmark is the point on the sea derived from mean sea level.

– method to calculate RL;

height of collimation (H of C) = reduced level(R.L.) + backsight (B.S.)

reduced level (R.L.) = height of collimation (H of C) – foresight (F.S.)

reduced level (R.L.) = height of collimation (H of C) – intermediate sight (I.S.)

• Rise & fall method :

It is the method which gives the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. Rise and fall are indicated by the difference in staff reading. After that RL is found by adding the rise to or subtracting the fall from the reduced level of the preceding point.

– method to calculate RL;

fall (F) = foresight (F.S.) – backsight (B.S.)

fall (F) = intermediate sight (I.S.) – backsight (B.S.)

rise (R) = backsight (B.S.) – foresight (F.S.)

reduced level (R.L.) = precede reduced level  + rise (R)

reduced level (R.L.) = precede reduce level – fall (F)