5. HYDROGRAPHY 



155 



Supplemental curves listed in Table 4 may 

 be inked at the discretion of the hydrog- 

 rapher. If still more curves seem necessary 

 to display the bottom configuration they may 

 be shown with brown ink (see 1-41). 



5-66 Inspection of the boat sheet. — After 

 the proposed system of lines has been run in 

 an area and the soundings inked on the boat 

 sheet, the results should be carefully studied 

 for indications of submerged features which 

 should be more closely examined. The fatho- 

 gram should be in view while this study is 

 being made, as side echoes are important in- 

 dications of shoaler depths. The study of the 

 boat sheet will reveal where additional lines 

 must be run in order to comply with line 

 spacing requirements and for more adequate 

 delineation of depth curves. Critical depths 

 which have been transferred to the boat sheet 

 from the chart or previous surveys must be 

 compared with the new hydrography (see 

 5-15). Additional soundings may be re- 

 quired to prove or disprove the existence of 

 the feature. 



It is good practice to run splits and make 

 detailed examinations of shoal indications as 

 soon as possible after it has been determined 

 that they are necessary. It is especially im- 

 portant that the surveys should be complete 

 in any area as the end of the season is 

 approached. 



5-67 Verification of alongshore and off- 

 shore details. — The hydrographic party is 

 responsible for the correctness of the topo- 

 graphic detail falling within the limits of 

 the hydrographic survey, including the low- 

 water line and waterfront details if the sur- 

 vey is carried close thereto. See Section 915, 

 Topographic Manual, Part II. Whether the 

 along-shore and offshore details originate 

 with planetable or photogrammetric sur- 

 veys, the information shall be verified while 

 hydrography is in progress. Offshore rocks 

 and other detail transferred from photogram- 

 metric manuscripts and shown in blue (see 

 5-14) should be inked in black as each fea- 

 ture is verified as to position and character. 



It is important that sufficient information 

 be obtained to permit correction of the 



manuscripts where necessary and to permit 

 the smooth plotter to reconcile any differ- 

 ences between positions of features shown 

 on manuscripts and the boat sheet. If ade- 

 quate information is not furnished, it is 

 frequently difficult in later processing to de- 

 termine whether there is one rock with a 

 different position on the boat sheet and the 

 manuscript, or whether the positions are for 

 different rocks (see 6-89). Manuscripts com- 

 piled without the benefit of field inspection 

 may show offshore rocks which do not exist. 

 The existance or nonexistance of such fea- 

 tures must be proved and clearly recorded. 

 Corrections of all topographic detail shall be 

 shown on the boat sheet in red with ap- 

 propriate notes explaining methods used to 

 determine the corrections. 



Each isolated bare rock and rock awash 

 in the project area must be located and its 

 height determined by the hydrographer, ex- 

 cept that, when the rocks on or near the 

 high water line have been located by a to- 

 pographer or on photographs during field 

 inspection, the hydrographer must verify the 

 data. The important rocks of a group or 

 rocky area should likewise be located and 

 elevations determined. 



Where it is practicable to land on such 

 features the location should be determined 

 by a strong three-point fix and check angle 

 taken at the rock. Otherwise the rock may 

 be located by a minimum of three cuts from 

 stations ashore or afloat giving a strong in- 

 tersection at the rock, or by sextant positions 

 taken at the boat with the rock in range 

 with control stations bearing in several 

 directions. 



The height of the rock should be deter- 

 mined as accurately as practicable and the 

 time of observation noted. If a landing can 

 be made and the sea is calm, the height of 

 a rock can be measured on a staff whose 

 lower end is held at the water's edge, the 

 height being noted on the staff by lining it 

 in with the horizon with eye at the top of 

 the rock. If it is not practicable to land on 

 the rock, the height must be estimated as 

 accurately as possible from a position nearby. 



Where a rock has been adequately located 



