154 



HYDROGRAPHIC MANUAL 



In other areas the soundings shall be plotted 

 in whole units (see 6-55). 



5-64 Depth curves on boat sheets. — 



Depth curves are indispensable for inter- 

 preting and examining a hydrographic sur- 

 vey. There is no better gage of its complete- 

 ness, adequacy, and accuracy than the ability 

 to draw closely spaced depth curves with an 

 assurance that the submarine relief is ac- 

 curately depicted. The depth curves should 

 be drawn on the boat sheet by the hydrog- 

 rapher 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 re- 

 quire investigation (see 1-41). 



An adequate representation of the sub- 

 marine relief by depth curves is a problem 

 similar to the representation of land topog- 

 raphy by contours, except that the topog- 

 rapher has the opportunity to examine 

 visually the topography of the area whereas 

 the hydrographer has only the measured 

 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, inadequacies, 

 or possible errors in the hydrographic sur- 

 vey or the inking of the soundings, and 

 where they result from the data, the sound- 

 ings and positions should be carefully scruti- 

 nized. 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 ac- 

 tion 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 continental slopes 

 and the deeper waters off the Pacific and 

 Alaska Coasts. 



To draw closely spaced depth curves care- 



fully and accurately requires the inspection 

 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 (see 6-63). Where in- 

 terpretation is difficult, it is often helpful to 

 draw additional depth curves in pencil and 

 leave these curves uninked. 



In this respect some topographic experi- 

 ences is a great asset as is also the ability 

 to recognize predominating physiographic 

 shapes from preliminary sketched depth 

 curves. The ability to represent submarine 

 relief by means of depth curves is acquired 

 only by intensive training and practice and 

 by study of similar work which has been 

 done by an experienced hydrographer. 



Depth curves cannot cross or run abruptly 

 into each other. On approaching one another 

 they tend toward parallelism. In general, the 

 information from sounding lines should be 

 sufficient to permit the delineation of continu- 

 ous curves. Special care must be exercised to 

 avoid excessive spacing of the sounding lines 

 where their direction is parallel to the depth 

 curves. 



5-65 Depth-curve interval. — No single 

 requirement for the spacing of depth curves 

 can be prescribed to apply to all regions. In 

 an area of steep slopes and irregular sub- 

 marine relief it is considered sound practice 

 to draw all the curves that the scale of the 

 boat sheet will permit. Such close spacing 

 of depth curves is obviously not required in 

 areas of gently sloping bottom with prac- 

 tically no irregularities. A good general rule 

 is that depth curves should be drawn ac- 

 cording to the following intervals : 



At 1-fathom intervals to 10 fathoms. 



At 5-fathom intervals in depths between 

 10 and 50 fathoms. 



At 10-fathom intervals in depths between 

 50 and 100 fathoms. 



At 25-fathom intervals in depths greater 

 than 100 fathoms. 



The depth curves should be drawn first in 

 pencil. The standard depth curves listed in 

 Table 3 shall be inked in the colors specified. 



