356 HYDROGRAPHIC MANUAL PaGE 274 



isolated or extending from a reef or point of land may exist between two lines of sound- 

 ings, even though these are closely spaced. A few crosslines in such places will con- 

 siderably diminish the chance of missing such an extension. 



Particular attention must be paid to the areas within the 10-fathoni curve where 

 the closest development is especially necessary. 



Extensive development may often be avoided if an area can be examined with the 

 wire drag (see 3663). 



356. Ranges, Bearings, and Sailing Lines 



In the project area all useful ranges, bearings, and other marks for clearing dangers 

 or passing close by them shall be noted. Ranges which are established by the United 

 States Coast Guard, and ranges, sailing lines, and courses which are recommended by the 

 hydrographer must be closely sounded over. One line of soundings shall be run along 

 the centerlines of channels and along ranges and recommended courses, and this line 

 shall be paralleled by one or more lines closely adjacent on each side of the centerline. 

 Where bars exist at the entrances to rivers these shall be sounded over thoroughly, and 

 the best com-se across them determined and sounded over as specified above. (See 

 also 3833.) 



In areas where there is little navigation and consequently few established aids to 

 navigation, leading and clearing lines are especially valuable to the mariner, and the 

 hydrographer should be constantly alert to select and recommend any that may be of 

 value. One of the most essential requirements for such marks is that they should be 

 readily recognized by a stranger from the given description. The two range marks 

 wliich. indicate the leading or clearing line should be a considerable horizontal distance 

 apart so that the range will be quite sensitive, but rear range marks which are high or 

 very distant should be avoided as they may often be obscured in hazy or cloudy weather. 



All ranges, bearings, courses, etc., recommended for use by the hydrographer shall 

 be described in the Descriptive Reports and shown on the sheets in accordance with 

 7845 and 8420. 



In the project area all prominent natural objects and natural ranges that may be 

 useful in determining deviations of ship magnetic compasses and in verifying gyro- 

 compasses shall be located and the azimuths of the ranges determined. 



The Chief of Party shall determine such azimuths from piers and wharves to 

 prominent natural objects, as are desired locally for use in setting gyrocompasses in the 

 true meridian. The Chief of Party shall inquire at each navy yard, naval establish- 

 ment, and commercial shipyard, and of each compass adjuster operating in the area, if 

 these are desired, and from which places. He shall cooperate by determining these and 

 shall report the results directly to the interested person or establishment. The results 

 need not be sent to the Washington Office, but mention of what was done should be 

 included in the season's report. 



357. Crosslines 



Crosslines are intended to disclose discrepancies resulting from the use of a faulty 

 plane of reference; discrepancies produced by surveying in unusual conditions of wind, 

 sea, and current; a temporary operational fault in the sounding apparatus; inadequacy 

 or weakness in the control; and incorrect reduction of the records. (See 7771.) 



