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All planetable topographic surveys shall be made according to the requirements 

 and instructions contained in Special Publication No. 144, Topographic Manual, except 

 as amended by section 23 of this Manual and by the project instructions. They shall 

 always be made on aluminum-mounted sheets unless otherwise specified. 



1214. Instructions for Hydrography 



The project instructions will ordinarily specify: 



(a) The limits of the area to be surveyed, the junctions to be made with prior surveys, and the 

 ■overlap required if a satisfactory check is not obtained at the junctions. 



(b) The scales of the hydrographic surveys in different parts of the area. For coastal surveys 

 the offshore limit of each scale is often defined by the depth of the water. Sometimes only the basic 

 scale for the inshore hydrographic surveys is specified and the Chief of Party will be expected to 

 select the appropriate offshore scales for an adequate systematic survey and development of the 

 -area, taking into consideration the available control. The Chief of Party is always authorized to 

 use larger scales for any small parts of the area for which he deems it essential, as for example, im- 

 portant small harbors and anchorages. 



(c) The maximum spacing between sounding lines for different parts of the area, sometimes only 

 in a general way. The limits of the zones may be defined by distances offshore or by depths. It is 

 to be noted that these are maximum spacings and the hydrographer is expected to reduce them if, 

 in his opinion, this is necessary for an adequate development of any part of the area. 



(d) The desirable direction or azimuth of the principal system of sounding lines to be run in the 

 area, although the hydrographer should use his judgment regarding this, in order that the area may 

 be most adequately and economically surveyed. 



(e) The percentage of crosslines to be run, if any, as a check on the accuracy and adequacy of 

 the survey. 



(/) The depth curves which are to be drawn on the boat sheet, to aid in determining whether 

 the area has been adequately surveyed and whether all indications of shoals and dangers have been 

 investigated. 



(g) That all critical depths and dangers to navigation on existing charts be proved or disproved. 

 Reported shoals that require investigation may be individually mentioned. 



(h) That certain areas which are periodically surveyed or have recently been surveyed by another 

 agency be omitted, if such survey appears adequate and a satisfactory check of the depths can be 

 obtained at the junctions. 



(i) Any special development which may be required to delineate certain submarine features more 

 completely than is otherwise specified for such depths. 



(j) The interval between recorded soundings when the depths are to be read visually from 

 an echo-sounding instrument. In the absence of such instructions at least as many soundings must be 

 recorded as are ordinarily inked on the smooth sheet (see 7724). 



(k) The number of R.A.R. (Radio Acoustic Ranging) returns considered adequate to fix a posi- 

 tion, where this method of control is used. 



(/) The required frequency of bottom characteristics. 



Photographic copies of all prior hydrographic surveys with which junctions are 

 required are furnished with the instructions, as well as photographic copies of the 

 most recent hydrographic surveys of the entire area, if the latter are believed to be of 

 value to the hydrographer. 



1215. Instructions for Tide 'Stations 



For the vertical control of the hydrographic surveys a knowledge of the height of 

 the tide above the sounding datum is necessary at all times during hydrographic surveys. 

 For this reason tide observations must be made at selected places within the project 

 area, both for use in reducing soundings to the sounding datum, and for use in deriving 

 data for the prediction of tides. 



