Page 733 the smooth sheet 7772 



atively even bottom, such as exists in the Gulf of Mexico, a difference of 2 or 3 feet 

 may be excessive, because of the amount the depth curve is displaced. On the other 

 hand, in areas of steep slopes a difference of several fathoms may be readily allowable 

 since the position of the depth curve may not be affected appreciably. 



If the control is strong, excessive differences should not be caused by uncertainty 

 of positions except in cases of pure blunder. Individual cases of excessive differences 

 may result from an error in protracting or from an error in the reduction or plotting 

 of the soundings, and the first step in the elimination of such errors is to verify the 

 positions and depths in the vicinity of the discrepancy. Where crosslines have been 

 run on a different date from the original system of lines, the discrepancies may be due 

 to differences in sea and weather conditions on the two dates; and these should be 

 examined. 



A statement shall be made in each Descriptive Report relative to the discrepancies 

 at crossings (see S42K). Generally, this may consist of a simple statement of the 

 percentage of crossings at which the differences are 1, 2, 3, etc., percent of the depth. 

 Because of the relative weakness of R.A.R. control in certain areas, a complete tabula- 

 tion of the depths at all crossings shall be made for each R.A.R. sheet (see 1122a). 



7772. Examination of Depth Curves 



The best evidence as to the adequacy and completeness of the survey is whether 

 the depth curves can be completely drawn, and whether their shapes and convolutions 

 are natural. A study of the depth curves may disclose discrepancies from several 

 sources. Where the depth curves do not represent normal types of bottom relief, 

 some source of trouble is to be sought. For example, a tide gage inadequately located 

 with reference to the hydrography will result in depth curves having a jagged unnatural 

 appearance if based on adjoining parallel lines run at different stages of the tide. 



A study of the depth curves, and a consequent study of the hydrography in the 

 vicinity, may also reveal discrepancies due to the use of different methods of sounding 

 or the use of different sounding instruments, and may result in actual corrections being 

 made to some of the lines of soundings and in rejections of others. 



78. ADDITIONAL DETAILS ON SMOOTH SHEET 

 781. Explanatory Notes 



A variety of miscellaneous explanatory notes is required on the smooth sheet before 

 it can be considered complete. Such notes should be made as short as practicable and 

 still give the desired information. (See fig. 171.) Those referring to hydrographic 

 features shall be in slanting letters, and those referring to land features shall be in 

 vertical letters. The lettering should be freehand, upper and lower case, the average 

 height of the capital letters being 1.8 mm, the lower-case letters being proportionately 

 smaller. Notes should ordinarily begin with a lower-case letter. The size of the 

 lettering should be consistent throughout each sheet, in order to present a pleasing 

 appearance. In congested areas it may be reduced in order to conserve space, but the 

 letters should never be so small as to require a magnifying glass to read them. 



A good deal of latitude may be taken in the placement of notes, as long as it is 

 perfectly clear to which feature a note applies. This may be accomplished in many 

 cases by the use of leaders (see 7914). 



