3624 HYDROGRAPHIC MANUAL PaGE 280 



single stalk, must be investigated. In addition to the usual data a statement should 

 be made in the Record as to the value of the kelp for marking the spot; whether the 

 kelp is visible at all stages of tide and at what distances, or whether it tows under so 

 as to be nearly invisible at times. (See also 7864.) 



Thick extensive beds of kelp, especially those bordering the shores, through which 

 it is not practicable to navigate, need not be thoroughly surveyed nor examined. 

 The outer limit of the area must be defined by fixes and soundings, and a few widely 

 spaced lines or scattered detached soundings should be taken throughout the area to 

 give an idea of the general depth. The area should be outlined on the sheet by symbol 

 and contain the legend "heavy kelp, not navigable, not thoroughly surveyed" (see 

 also 367). 



c. From fish and gulls. — Schools of fish are habitually found in the vicinity of 

 shoals. In ocean areas, where a shoal might exist, a school of fish seen near the surface 

 in one location should be considered evidence of a shoal and be noted for future inves- 

 tigation. Likewise, flocks of gulls, which feed on the fish, are similar evidence when 

 they are seen circling over and feeding in one locality. 



3624. Evidence From Air Photographs 



Air photographs of water areas being surveyed often furnish valuable information 

 to the hydrographer. Any variation in the general appearance on the photograph of 

 water areas may mean a difference in depth or character of bottom important enough 

 to be shown on the hydrographic sheet. Such differences in appearance have resulted, 

 upon investigation, in the detection of dangerous sunken rocks between sounding lines 

 previously considered adequate to furnish indications of such dangers. Occasionally, 

 as in flat white sand bottom with numerous patches of grass, the differences appearing 

 in the photographs may be too complicated and unimportant to warrant investigation 

 in detail. 



Small spots, scratches, or changes in tone which appear on only one photograph 

 may be due to defects in manipulation or materials of the photographic process. But 

 if a similar difference in the same locality appears in each of two or more overlapping 

 photographs, it is practically certain to indicate differences in bottom or currents. 

 Unless the water is muddy or the water surface too rough, air photographs will show 

 the pattern of shoals or channels in depths to approximately 2 fathoms, and under the 

 most favorable conditions, to much greater depths. A careful study of the photo- 

 graphs of a surveyed area will enable the hydrographer to interpret photographs of 

 similar areas. The indications may vary with the locality from darker rocks or vegeta- 

 tion on shoals in generally lighter colored bottom, to light colored rocks projecting 

 above darker general depths. 



The surface of the water as photographed may also indicate areas requiring inves- 

 tigation. Tide rips, swirls, breakers, differences in a regular wave pattern, slicks, and 

 kelp may at times be more readily detected on the air photographs with their elevated 

 view than from the surface. 



The spots selected for investigation may usually be transferred to the boat sheet 

 by one of the methods described in 2394(B) for transferring supplemental stations 

 from photographs. This advance information is particularly valuable if the hydrog- 

 rapher happens to sound in that vicinity when the sea is imusually calm. On boat 

 sheets prepared from photographic surveys (see 7333), the limits of shoals and channels 

 are indicated by fine dash lines. These are useful in planning the system of sounding 

 lines and development of the area. 



