134 



HYDROGRAPHIC MANUAL 



the formula d = Z — cot <x , in which X is 

 2 



a scale factor introduced so that the re- 

 sults can be measured directly on a 1:10,000 

 scale meter bar. For any scale the scale 

 10,000 



factor X ^= 



Thus if the scale is 



1 :20,000, X=i/2. See Section 7-31 and Table 

 18 for natural half-cotangents. 



After the centers have been computed and 

 plotted, a system of arcs is drawn with a 

 beam compass using a distinctive color of 

 ink for each set of circles. The circles are 

 numbered in the same color ink to indicate 

 the angle each represents. The maximum 

 distance between concentric circles should 

 not exceed three inches. A check on the 

 plotting is obtained if a circle passes through 

 the center of a circle of half that angle. 



A little time is required to compute the 

 data and plot the arcs, but the ease of plot- 

 ting positions will soon establish the advan- 

 tages of the system. All hydrographers are 

 encouraged to make wider use of this method 

 where appropriate. All computations should 

 be retained for use when plotting the smooth 

 sheet. 



5-14 Transfer of topographic details. — A 



boat sheet for any inshore survey shall con- 

 tain the high water line, and all available 

 information concerning alongshore and off- 

 shore rocks, aids to navigation, channels, 

 approximate limits of shoal areas, and posi- 

 tions of reported dangers to navigation. The 

 two principle sources of such information 

 are the published charts and topographic 

 manuscripts. The hydrographic party will 

 be furnished photogrammetric manuscripts 

 or copies of prior planetable topographic 

 surveys. Classification of photogrammetric 

 manuscripts and their uses are explained in 

 Chapter 4. 



After the positions of known signals have 

 been plotted on the boat sheet, the high 

 water line and all details seaward thereof 

 shall be transferred to it. Reverse blueline 

 tracings on vinylite of all photogrammetric 

 manuscripts will be furnished. Shoreline 

 and other details are transferred to the boat 



Figure 53. — Principle of plotting angles without a 

 protractor. 



sheet by burnishing the tracing after the 

 projection intersections have been properly 

 matched. Film positives of planetable sur- 

 veys are furnished when photogrammetric 

 maps are not available. Details may be 

 transferred by applying "dri-rite" ink on the 

 reverse side, matching projections, and trac- 

 ing the shoreline with a stylus or pencil. 

 Shoreline is not required on boat sheets for 

 offshore surveys, especially where scale 

 changes are necessary to accomplish the 

 transfer. 



After the transfer of data from the manu- 

 scripts or topographic sheets has been veri- 

 fied, the station symbols should be inked (see 

 5-10) and the shoreline inked in black. 

 Offshore rocks and limits of kelp or foul 

 areas transferred from photogrammetric 

 manuscripts should be inked in blue on the 

 boat sheet at the time of transfer and inked 

 in black after verification of position and 

 character by the hydrographic survey (see 

 5-67). 



5-15 Transfer of data from prior sur- 

 veys. — All dangers to navigation, including 

 least depths on shoals, shall be transferred 

 to the boat sheet from copies of prior hy- 

 drographic surveys, if any have been made. 



