50 NAUTICAL ^HART MANUAL 



HYDROGRAPHY 



GENERAL STATEMENT 



Hydrographic features are charted only after a thorough study of the configuration of 

 the bottom as revealed by the original surveys. The variety of submarine relief must be pre- 

 sented in a clear and practical manner. Important objects must not be obscured by other 

 data. A dangerous rock, an islet, or the least depth on a shoal must be shown clearly and 

 prominently. Small islets and rocks should be exaggerated if necessary, to make them easily 

 discernible. 



Specific instructions to field parties call for basic hydrographic surveys to supersede all 

 previous surveys made by this Bureau in the areas covered. "When applying reviewed basic 

 siirveys to the charts, all prior soundings should be removed. However, critical soundings not 

 specifically disposed of by the reviews of the surveys should not be removed arbitrarily because 

 such soundings may have been charted subsequently to the date of the review. 



Hydrography should be applied progressively from the largest to tbe smallest scale charts. 

 Detail is eliminated and generalized in proportion to the scale reduction. 



A conflict between the delineation of the low-water line or the amount that rocks awash 

 uncover on contemporary hydrographic and topographic surveys should be adjusted by giving 

 greater weight to the information shown on the hydrographic survey. 



To aid in navigating by echo-sounding apparatus, depth curves have been given special 

 prominence on nautical charts in areas where modern hydrographic sm-veys have been made. 

 Deep-water submarine relief has thus become more important to the navigator, and the more 

 faithfully the chart depicts this relief, the closer the navigator can relate his depth readings 

 to the chart and the more certain he will be of his position. Significant submerged features 

 shall not be unnecessarily generalized. 



I 



Surveys 



UNVERIFIED HYDROGRAPHIC SURVEYS 



Unverified surveys should be applied to charts going to press when the chartered bottom 

 configuration of the areas covered by the surveys is inadequately delineated. Otherwise, only 

 the critical information should be applied. 



UNREVIEWED SURVEYS 



When an imreviewed hydrographic survey is used for charting, a statement to that effect 

 must be added to the special page provided in the Descriptive Report of the survey. 



