HYDROGRAPHY 51 



WIRE-DRAG SURVEYS (0-6a; Q-9-9a) 



Areas in Alaska which have been swept to a safe depth by wire drag, but which have not 

 been covered by adequate hydrographic surveys, are indicated on the charts by a green over- 

 print. When time permits, the green tint may be added to other charts of important areas 

 covered by wire drag. 



Soundings on shoals or rocks discovered with the wire drag and not yet transferred to a 

 contemporary hydrographic sheet must not be overlooked when correcting or compiling a 

 chart. 



The maximum cleared depth over a rock, wreck, obstruction, or shoal shall be charted 

 on other than a Wreck Chart only when it is within 3 feet of the depths found by other means 

 in general depths of 18 feet, and withia 6 feet in greater depths. Descriptive terms Rk, Wk, 

 Obstr) are to be charted only when existence of the object is indicated by a field examination 

 such as by a hang or sounding, thus: 



i2i ili lIL. 



Rk Wreck Obstruction 



The maximum clearance over small, shoal areas, such as along the Maine coast, is to be 

 shown by the bracket symbol and bottom characteristic (if available) thus: 



^ ill 



hrd 



If no bottom characteristic is available, the descriptive term "Shoal" is to be used, i.e., 



,21. 

 Shoa/ 



The charted abbreviation note shall read: 



,21. Wreck, rock, obstruction, or shoal swept clear to the depth indicated. 



II 



Soundings 



PLANE OF REFERENCE FOR SOUNDINGS 



The plane of reference for soundings along the Atlantic and Gulf coasts of the United 

 States and Puerto Rico is the mean of aU low waters (Mean Low Water) ; except that where 

 tides on the Gulf Coast and south coast of Puerto Rico are chiefly diurnal, Mean Low Water is 

 based on a mean of the lower of the daily low waters. Along the Pacific coast of the United 

 States, Alaska, and the Hawaiian Islands, it is the mean of aU lower low waters (Mean Lower 

 Low Water). Care must be exercised to see that foreign charts used in compilation are on 

 these planes. 



