353 HYDROGRAPHIC MANUAL PaGE 270 



SO, a new least depth on each of those so affected must be obtained from that agency or 

 a hydrographic examination made to determine it. Rocks and obstructions supposed 

 to have been removed by blasting should be wire-dragged, if possible, to ensure that 

 they have been removed in their entirety, unless they have been properly dragged 

 after removal by the agency responsible for the removal. 



353. Depth Curves 



Depth curves are indispensable for interpreting and examining a hydrographic 

 survey. There is no better gage of its completeness, adequacy, and accuracy than the 

 ability to draw closely spaced depth curves with an assurance that the submarine relief 

 is accurately depicted. The depth curves should be drawn on the boat sheet by the 

 hydrographer as the work progresses, and a careful interpretation of the data will disclose 

 where the lines have not been spaced closely enough, where additional development is 

 required, and where errors have been made which require investigation. 



An adequate representation of the submarine relief by depth curves is a problem 

 similar to the representation of land topography by contours, except that the topog- 

 rapher has the opportunity to examine visually the topography of the area whereas 

 the hydrographer has only the measm^ed depths as his guide. The hydrographer should 

 make a study of the characteristic bottom forms, as such forms usually repeat them- 

 selves in the same region and in similar regions. 



Abnormal or improbable depth curves are strong evidence of inaccuracies, inade- 

 quacies, or possible errors in the hydrographic siu"vey or the inking of the soundings, 

 and where they result from the data, the soundings and positions should be carefully 

 scrutinized. On extensive coastal shelves, such as exist on the Atlantic and Gulf 

 Coasts of the United States, the depth curves are generally smooth and regular because 

 the bottom forms are the results of wave or tidal current action on the loose materials 

 generally found on the bottom. On the continental slopes, however, in depths greater 

 than about 100 fathoms, the bottom forms are generally similar to those found on land. 

 In general, an interval of 25 fathoms between depth curves is adequate for the conti- 

 nental slopes and the deeper waters off the Pacific and Alaska Coasts. (See 3533.) 



To draw closely spaced depth curves carefully and accurately requires the inspec- 

 tion and consideration of each sounding not only once but often several times, whereas 

 in sketching widely spaced depth curves many of the intermediate soundings may not 

 be considered at all and important indications may be overlooked. 



In this respect some topogi'aphic experience is a great asset as is also the abUity 

 to recognize predominating physiographic shapes from preliminary sketched depth 

 curves. The abUity to represent submarine relief by means of depth curves is acquhed 

 only by intensive training and practice and by study of similar work which has been 

 done by an experienced hydrographer. 



Depth curves ordinarily cannot cross or run abruptly into each other. On ap- 

 proaching one another they tend toward parallelism. In general, the information 

 from sounding lines should be sufficient to permit the delineation of continuous curves. 

 Special care must be exercised to avoid excessive spacing of the sounding lines where 

 their direction is parallel to the depth curves. 



3531. Similarity of Submarine Relief to Adjacent Land 



A study of the nature of the land adjacent to the project area is often helpful in 

 developing the depth curves. The submarine relief is likely to be similar to the adjacent 



