268 



HYDROGRAPHICAL SURVEYING [chap. xt. 



Simple 

 Levelling. 



LEVELLING. 



Levelling with a staff is not very much required in marine 

 surveying. Ascertaining the height above the sea of the fixed 

 mark used for reference for the tidal datum is the purpose 

 for whicli it is most used, but it is also required to find the 

 height of the base of a lighthouse, etc. 



This is called simple levelling, and gives us the height 

 between the two required points only, without any regard to 

 distance. 



A levelling staff and level is usually supplied to surveying 

 ships ; but a theodolite and marked boat-hook or pole will 

 answer the purpose, if we have not got the regular apparatus. 



Holding the staff at high-water mark, we place the instru- 

 ment (level or theodolite) so far up the slope that we shall, 

 when it is carefully levelled by the level attached to the tele- 

 scope, read off near the top of the staff. The reading of this, 

 called the hack station, being taken, the staff is taken above 

 us, and planted so that we can read just above zero of the 

 staff, which is now at the fore station. The theodolite is now 

 moved up the hill until we shall again, when levelled, read 

 near the top of the staff ; this will be another observation of 

 hack station, and so on until the levelled telescope reads the 

 staff on the spot whose height we want. 



