Page 275 hydrography 3571 



Crosslines shall be run for the purpose of verifying the accuracy of the survey and 

 the control under the following conditions: 



(a) All launch and small-boat hydrography shall be verified by crosslines to the extent of 8 to 10 

 percent of the principal system of lines exclusive of development. 



(b) All ship hydrography in the Gulf of Mexico, on the Atlantic Continental Shelf south of Cape 

 Cod, and in other areas of fairly regular bottom shall be verified by crosslines to the extent of 5 to 6 

 percent of the principal system of lines exclusive of development. 



(c) In all areas where the control is so weak as to prevent the drawing of the depth curves with 

 confidence, the principal system of lines shall be supplemented by crosslines to the extent of 8 to 10 

 percent, to aid in adjusting the principal system of lines. 



(d) Except as specified in (c), crosslines need not be run in areas of ship hydrography in compara- 

 tively irregular submarine relief because in such areas they are of very little value for checking 

 purposes. 



Crosslmes need not be run normal to the principal system of lines. Any angle of 

 crossing of 45° or greater may be considered satisfactory. Crosslines should be planned 

 to obtain a uniform distribution as economically as practicable. When practicable, 

 crosslines should be run under conditions of tide, wind, and weather different from 

 those obtaining when the principal system of lines was run and using different control. 



Crosslines should never be run prior to the completion of the main system of 

 sounding lines since the purpose of the crosslines is to check the principal liiies. 



3571. Discrepancies at Crossings 



Discrepancies at crossings may be systematic or accidental. They should be recog- 

 nized as evidence of some fault in the apparatus, method, or record, requiring a study 

 to discover its source and to indicate the most probable correction — possibly a re- 

 examination in the field. If the study does not result in an actual correction of one of 

 the lines it may plainly show good reason for the rejection of one and consequently 

 warrant the adoption of the other. 



The allowable discrepancies in depths in any given area should be based on the 

 amount of horizontal displacement corresponding to the differences in depth, rather 

 than a percentage of the depth. In comparatively even bottom a difference of 2 or 3 

 feet may be excessive because of the lateral displacement of the depth curve. On the 

 other hand, in areas of more irregular bottom, a difference of several fathoms may be 

 readily allowable since this may not affect the position of the depth curve appreciably. 

 It must be borne in mind that a discrepancy at a crossing may be due to either a differ- 

 ence in soundings or an error in horizontal position. In any study of discrepancies 

 this fact must be taken into consideration. In general, in the lesser depths the differ- 

 ences at crossings should average not more than 5 percent of the depth and in greater 

 depths not more than 2 percent of the depth. When an examination of the data does 

 not disclose a reasonable explanation of differences greater than these nor a method of 

 adjusting them with confidence, the work should be revised in the field. (See also 7771.) 



36. DANGERS AND SHOALS 



A hydrographic survey may not be considered complete and adequate until there 

 is reasonable assurance that all dangers to navigation and shoals existing in the area 

 have been found and the least depths on them determined. 



