754 HYDROGRAPHIC MANUAL PaGE 694 



754. The Low-Water Line 



The low-water line shown on the planetable sui-vey is usually not a surveyed line. 

 It has ordinarily been sketched by the topographer and is furnished as a guide to the 

 hydrographer during his hydrographic survey. Occasionally, however, where the 

 topographer is in a locality at low tide, he may actually delineate the low-water line 

 by rod readings. 



The low-water line is the ciu-ve of zero depth and its best delineation for hydro- 

 graphic purposes results from soundings obtained in its vicinity. Since such soundings 

 are subject to possible modification in the Wasliington Office, caused by changes in 

 the tide reducers or for other reasons, the low-water line shall be left in pencil on the 

 smooth sheet by the field party, to be subsequently inked in the Office when the survey 

 is verified. Where minus soundings are plotted, care must be taken to see that the 

 curve is correctly shown relative to them (see 7715). 



After the soundings have been penciled on the smooth sheet, the complete low- 

 water line shall be shown thereon in pencil in accordance with the following prefer- 

 ences, using the same symbols as are prescribed for its hiking (see fig. 169): 



(a) The zero depth curve as determined by the reduced soundings in areas where the hydro- 

 graphic survey has extended over the low-water line during periods of high water- — to be eventually 

 inked as a continuous yellow line. 



(6) Those parts of the low-water line that were rodded in by the topographer at low tide — to 

 be eventually inked as a black line of alternate dashes and dots. 



(c) Those parts of the low-water line which can be reasonablj^ well determined from notes in the 

 Sounding Records and sketches on the boat sheet made by the hydrographer at the time of the hydro- 

 graphic survey — to be eventually inked as a yellow dash line. 



(d) In cases not covered by any of the preceding three classes the low-M'ater line shall be taken 

 from the best available source, usually that sketched on the topographic survey — to be eventually 

 inked as a black dotted line. 



Occasionally it may be desirable to delineate the low-water line by symbols, such 

 as for rocky ledges or coral reefs. In such cases the offshore edge of the symbols shall 

 be considered the low-water line. 



The low-water line derived from an air photographic survey shall be considered 

 to be in class (d), unless there is a positive statement in the Descriptive Report that 

 parts of it were rodded in by planetable, when of course those parts shall be considered 

 in class (6). The latter, however, is rarely the case. 



755. Offshore Detail 



Topographic details offshore from the low-water line, such as rocks, reefs, ledges, 

 rocky areas, and shoal areas, except islands or islets of some extent, shall be left in 

 pencil on the smooth sheet until the soundings have been penciled. The hydrographic 

 survey frequently contains data which supplement, and in some cases modify, the 

 topogi-aphic sm-vey. Before such details are inked on the sheet, all of the data must 

 be considered and disposition made as to character, location, and elevation in accord- 

 ance with the rules prescribed in 782. (See also 7862.) 



It is unnecessary to transfer the elevations of bare rocks that do not fall within 

 the limits of the hydrography, such as the numerous rocks that are often found close 

 to the high-water line in certain areas. 



