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HYDROGRAPHIC MANUAL 



hydrography on the smooth sheet. The over- 

 lays shall be page-size, complete with pro- 

 jection, inked soundings and position num- 

 bers, and shall be inserted in the descriptive 

 report. A note "see overlay" shall be made 

 on the smooth sheet. Overlays should not be 

 used when essential hydrography can be 

 clearly shown on the smooth sheet. 



6-45 Combination boat and smooth sheet. 



— When the combination boat and smooth 

 sheet method of plotting is used (see 1-10 

 and 5-5) , there is necessarily a limiting time 

 factor. The positions shall be plotted as accu- 

 rately as circumstances permit. The position 

 should not be pricked through to the smooth 

 sheet unless the field plotter has a firm posi- 

 tion. Doubtful positions should be plotted 

 later when there is sufficient time to evaluate 

 the position data. On electronically-controlled 

 surveys the positions should not be pricked 

 through unless the distance corrections are 

 known and applied. Final calibration of dis- 

 tance measuring equipment should not result 

 in a significant shift of positions. The error 

 in a hydrographic position plotted by this 

 method should not exceed 1 to 2 mm depend- 

 ing on whether the sounding is a 1- or 4-digit 

 number and the general character of the bot- 

 tom. Greater tolerance can be permitted in 

 areas of smooth bottom or in great depths 

 than in areas of irregular bottom. Critical 

 detail, such as the least depth on a sub- 

 merged danger, shall be replotted to its cor- 

 rect position when final calibrations differ 

 by a plottable amount. 



Soundings and other hydrographic detail 

 plotted under this field procedure shall con- 

 form to the standards of accuracy and cartog- 

 raphy set forth elsewhere in this chapter. 

 It is advisable to defer plotting of soundings 

 on the smooth sheet until the survey is other- 

 wise complete. 



6-46 PosKion plotting. — Hydrographic po- 

 sitions are usually plotted in the order in 

 which they were observed and recorded. 

 This systematic procedure will help to pre- 

 vent accidental omission of recorded data 

 which should be plotted. If the plotting is 

 done while field work is still in progress, it 



may be preferable to plot the positions in 

 critical areas first in order to give imme- 

 diate evidence whether additional field work 

 is required in the area. 



The plotter should be particularly careful 

 to watch for uneven distortion of the smooth 

 sheet when using a 3-armed protractor to 

 plot cuts to signals or other details at some 

 distance away. In plotting Shoran distances 

 directly from floating stations, compensa- 

 tion for distortion in the smooth sheet should 

 be made. This can usually be done by mark- 

 ing circle intervals in a strip of plastic to 

 correspond with circles drawn on the sheet 

 from a fixed station. 



6-47 Minimized protracting. — Two smooth 

 plotting procedures are authorized which are 

 designed to reduce the number of fixed posi- 

 tions plotted by protractors. These proce- 

 dures and the limitations imposed on each 

 are described in the following paragraphs. 

 Both procedures are applicable only for sur- 

 veys of areas having a uniform depth or 

 gently and evenly sloping bottom. 



When the hydrography is controlled by 

 sextant fixes based on final positions of con- 

 trol stations and the hydrography has been 

 accurately and legibly plotted on the boat 

 sheet, a film positive copy of the boat sheet 

 may be used to transfer positions to the 

 smooth sheet (see 5-63). The first and last 

 positions and infrequent intermediate posi- 

 tions on a line shall be lightly pricked 

 through the film positive to the smooth 

 sheet and shall be checked by protracting. 

 If the transferred positions differ signifi- 

 cantly from the protracted positions (see 

 6-45), the transfer method shall be aban- 

 doned for the line or area or for the entire 

 sheet when several tests reveal similar differ- 

 ences. If the check proves consistent accu- 

 racy of the boat sheet plotting, the remain- 

 ing positions can be transferred without 

 check (see 5-50). Adjustments must be made 

 for any differences in the two projections. 

 Transfers may also be accomplished by use 

 of paper tracings. 



Detached positions, positions of limited- 

 area development lines, and positions con- 



