8 



HYDROGRAPHIC MANUAL 



shall be inked in blue, except that offshore 

 islets and rocks whose positions have been 

 definitely established shall be inked in black. 

 Photogrammetric compilations will often 

 show the approximate 'limits of shoals and 

 channels. These limiting lines shall be trans- 

 ferred to the boat sheet and indicated by 

 fine dash blue lines. 



1-23 Verification of alongshore details. — 



The location of rocks, limits of foul areas, 

 and other details in the area seaward from 

 the highwater line which have been trans- 

 ferred from the manuscripts must be verified 

 by the hydrographer (see 5-67). The sym- 

 bols which have been inked in blue shall be 

 inked black when verified. If the position of 

 an offshore rock is changed by the hydrog- 

 rapher, or if it is proved that there is no 

 rock at the position shown, a note shall be 

 made in the record and on the boat sheet. 

 Failure to reconcile and explain differences 

 between the manuscript and the hydro- 

 graphic survey results in unnecessary delays 

 and difficulties in verification and review 

 (see 6-89) . 



Each isolated bare rock or rock awash 

 must be located or the position verified and 

 its height determined by the hydrographer. 

 The important rocks of a group or rocky 

 area shall likewise be located and elevations 

 determined. 



1-24 Sounding lines. — A hydrographic 

 survey is accomplished by running a prede- 

 termined system of sounding lines in an 

 area. A system must be chosen that will 

 delineate the submarine relief in the most 

 thorough and economic manner (see 5-19 to 

 22). A series of evenly-spaced, parallel 

 sounding lines is the best method to accom- 

 plish this. In general, the sounding lines 

 should be normal to the depth curves, but 

 it is frequently more advantageous to adopt 

 some other system. For the development of 

 steep features, such as ridges or submarine 

 valleys, the system of lines should cross the 

 depth curves at an angle of approximately 

 45 degrees. A restricted channel should be 

 developed by a system of lines parallel to 

 the axis of the channel. A system of straight 



lines is customarily used; however, when 

 hydrography is controlled by Shoran or Ray- 

 dist, better control of the sounding vessel is 

 maintained by running lines parallel to the 

 distance arcs, especially in areas where 

 strong currents are encountered. 



1-25 Line spacing. — The spacing of 

 sounding lines required to properly develop 

 a given area depends upon the depth of the 

 water, the character of the bottom, the scale 

 of the survey, and the general nature of the 

 area (see 5-25 to 28). Harbors, channels, 

 anchorages, and shoal areas which may be 

 dangerous to navigation, should be surveyed 

 by a system of closely-spaced lines, for ex- 

 ample, 50 to 100-meter lines on a 1:10,000 

 scale. As the depth increases, the line spac- 

 ing is increased to a width of 5 miles in 

 depths greater than 1,000 fathoms. In all 

 cases, the line spacing should be reduced as 

 necessary to adequately delineate the bottom 

 configuration. The low-water line should be 

 delineated if possible ; however, sounding 

 lines are not required in extensive areas 

 which bare at low water. 



1-26 Crosslines. — The regular system of 

 sounding lines shall be supplemented by a 

 series of crosslines (see 5-23) for the pur- 

 pose of verifying the accuracy of the survey 

 and the control as follows : 



(a) All launch and small boat hydrography 

 shall be verified by crosslines to the extent 

 of 8 to 10 percent of the principal system of 

 sounding lines exclusive of development. 



(b) All ship hydrography in areas of fair- 

 ly regular bottom shall be verified by cross- 

 lines to the extent of 5 to 6 percent of the 

 regular system of lines exclusive of develop- 

 ment. In areas where the principal system 

 of sounding lines is generally parallel to the 

 depth curves, the crosslines shall be 8 to 10 

 percent of the regular system. 



(c) Crosslines need not be run in areas of 

 very irregular submarine relief because in 

 such cases they are of little value for check- 

 ing purposes. 



(d) Crosslines shall be run at an angle 

 of 45 to 90 degrees with the regular system. 



(e) In areas of regular bottom, a frame- 



