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have been prepared by the Washington Office for the use of this instrument shall be 

 followed. 



121 7. Current Observations 



Where current observations are to be made, the project instructions 'will specify: 

 (a) Wliere they are to be made; (6) the period of continuous observations required at 

 each station; and (c) the instrument to be used in obtaining the data. 



Current observations shall be made in accordance with the instructions contained 

 in Special Publication No. 215, Manual of Current Observations, except as amended 

 by the project instructions. 



1218. Miscellaneous Instructions 



Under a miscellaneous heading the project instructions will specify other details 

 of the operations. Among these are the scales to be used for the monthly and season's 

 progress sketches. On the monthly progress sketch the Chief of Party is required to 

 include the limits of the topographic and hydrographic sheets as these are determined 

 during the field season. The standard size of the annual progress sketch is specified. 

 If coast pilot notes are known to be required for the revision of or to supplement the 

 Coast Pilot of the area, this will be stated. The instructions may specify that special 

 reports be made regarding the use of certain new instruments or methods concerning 

 which information is desired. The instructions may require that a report of dangers 

 to navigation discovered be made elsewhere than to the Washington Office (see 8522). 

 Consultation with officials of certain organizations may be required. 



122. Offshore Limits 



The use of echo-sounding instruments now permits the extension of hydrographic 

 surveys far beyond the limits formerly considered practicable. Commercial and naval 

 vessels equipped with some of these instruments are now able to obtain soundings at 

 will in the greatest depths of the oceans, with the result that they are virtually always 

 "on soundmgs." Unless otherwise specified in the project instructions, hydrographic 

 surveys should be extended as far seaward as it is practicable to control adequately 

 the positions of the soundings. 



Modern hydrographic surveys have disclosed numerous offshore submarine features, 

 which are of incalculable value on the published charts for use as landmarks, from 

 which vessels with echo-sounding instruments navigating these areas may determine 

 their approximate positions when approaching a coast. On the continental slopes of 

 the Atlantic and Gulf Coasts of the United States many submarine valleys have been 

 discovered in recent years. On the Pacific Coast of the United States submarine 

 canyons have been discovered, and submarine mountains have been found here and 

 in the offshore waters of the Alaska Coast. It is considered desirable that the hydro- 

 graphic surveys be carried far enough seaward to survey such features completely and 

 adequately for charting purposes. 



123. Survey Scales 



The scale of a map is the proportion by which distances on the map represent 

 respective distances on the earth. The scale of a hydrographic survey is that scale at 

 which the positions of the soundings in the area are plotted on the hydrographic sheet. 



The terms fractional, natural, numerical, and linear are used more or less inter- 



