390 HYDROGRAPHICAL SURVEYING [chap. xvii. 



5. The longest calculated side, with its Mercatorial bearing 

 and half convergency when the latter is appreciable. 



6. Whether the topography has been sketched on the ground 

 or from the ship. 



7. Reference to tidal observations, the datum or level to 

 which the soundings have been reduced, referred to a per- 

 manent mark on shore wherever tliis is possible, together with 

 any similar information that may be useful as a record. 



In home waters the connection of the datum for soundings 

 with the Ordnance Survey datum should be stated. 



8. A list of the most conspicuous objects recommended for 

 use in fixing, in order that they may be suitably delineated on 

 the chart. 



The name of the officer who has drawn the chart should 

 always be stated in the lower left-hand corner. 

 Trans- It is scarcely necessary to describe the construction of a 



Merca- Mercator's chart, as every naval officer learns it as part of his 

 tor s Pro- education. 



To redraw a survey on Mercator's projection similar meridians 

 and parallels must be draw^n on both charts, and enough of them 

 to make the parallelograms formed by them small enough to 

 reduce the discrepancy between the shape of any parallelograms 

 on either chart uo as little as possible. The soundings, coast- 

 line, etc., in each parallelogram of the gnomonic chart are then 

 transferred to the same parallelogram of the Mercator, by 

 latitude and longitude of each detail. 



jection. 



