Page 271 hydrography 3532 



land topography, and reference should be made to any well-contoured topographic 

 sheets of the area. (See also 355.) 



3532. The Low-Water Line 



The low-water line is the depth curve of zero depth. Its position is best deter- 

 mined by the hydrographic survey. In areas where it is practicable and can be done 

 without endangering the personnel or the equipment, the low-water line and the adja- 

 cent offshore depth curves shall be completely and adequately defined by lines of hy- 

 drography run over the area at high tide which, when reduced to the sounding datum, 

 will define the low-water line. The survey shall be planned so that the sounding lines 

 closest inshore may be run at or near high tide and when the sea is calm. (See 3121 

 and 3122.) 



Where for any reason, it is impracticable to define the low-water line by soundings, 

 it should be delineated from the topographic survey (see 754). 



3533. 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 submarine relief it is considered 

 sound practice to draw all the curves that the scale of the boat sheet will permit. Such 

 a close spacing of the depth curves is obviously not required in areas of gently sloping 

 bottom with practically no irregularities, such as exist off the Gulf Coast. A good 

 general rule is that the depth curves should be drawn so far as practicable according 

 to the following intervals: 



At l-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. 



On steep slopes it wUl frequently be impracticable to draw the depth curves at 

 these intervals and a larger interval may be selected. (See also 7761.) 



354. Adequacy of the General System of Lines 



The spacing of the systematic sounding lines of the survey is not intended to be 

 close enough to pass directly over the shoalest part of any danger, but it is expected 

 that the spacing will be close enough to give at least an indication of every danger and 

 permit the accurate delineation of the depth curves, except for unexpected irregularities 

 of the bottom. 



The Chief of Party should not hesitate to decrease the sounding-line spacing pre- 

 scribed in the project instructions, if this appears desirable, in portions of the general 

 area, in areas where the nature of the adjacent topography indicates the possibility of 

 hidden dangers, where ships are likely to approach the land as in anchorages, and off 

 projecting points and promontories. It is especially important to do this in previously 

 unsurveyed regions where sufficient data may not have been available as a basis for 

 the spacing specified in the project instructions. 



Where the spacing is generally adequate, it should be split in areas of unusual 

 bottom irregularity, over extensive shoal areas, and where the sounding lines run have 

 diverged so far from the proposed ones as to give a spacing one-third greater than the 

 instructions call for. Splits should also be run where the depth curves parallel the 

 direction of the sounding lines and the spacing is inadequate to fix them accurately 

 in position. 



