Line levelling

 if the points A and B are widely separated, the height difference between them is determined by line levelling with target distances generally between 30 and 50 metres. 

pace out the distances between the instrument and the two staffs; they need to be about the same

 1- set up the instrument at S1.  

2-  set up the staff precisely vertically at point B; read off and record the height (backsight R).  

3- Set up the staff at the turning point 1 (ground plate or prominent ground point ) ; read off and record the height (foresight V).  12.   

4- set up the instrument at S2(the staff remains at the turning point 1).  

 5- Carefully rotate the staff at the turning point 1 so that it faces the instrument. 

 6- Read off the back sight and continue . 

the height difference between A and B is equal to the sum of the backsight and the foresight.