764 



HYDROGRAPHIC MANUAL 



Page 708 



764. Plotting Astronomic Positions 



star Sights 



Beyond the limit of the usual control, sounding lines are controlled by dead reckon- 

 ing and astronomic sights. The only data for location of the sounding lines and the 

 positions are the astronomic sights and the carefully recorded dead reckoning. It is 



usual, when practicable, to observe a series of star sights at 

 dawn and dusk to give positions at these two times. At 

 other times, single astronomic sights, usually sun sights, are 

 taken and these must be used in conjunction with the dead 

 reckoning. (See also 337 and 338.) 



In plotting a smooth sheet controlled by astronomic 

 sights, the positions are usually computed and determined 

 on separate records and it is only necessary to plot the most 

 probable positions as determined by the hydrographer. 

 These positions may be considered as fixed positions, a note 

 being placed on the smooth sheet explaining their determi- 

 nation, such as "star sights" (see 3387). Each intermediate 

 single sight must be plotted as a line of position, accompanied 

 by a legend indicating the astronomic body observed and 

 the initials of the observer (see 3383). Where all of the 

 astronomic sights between two positions fixed by star sights, 

 have been plotted on the smooth sheet, the dead reckoning 

 between the star sights must be carefully plotted, either on 

 the smooth sheet itself or on a piece of tracing paper, and 

 adjusted to the two end positions and the intermediate 

 sights, taking all circumstances into consideration in order 

 to arrive at the most probable location of the entire line. 

 This type of work can be plotted only by one thoroughly 

 familiar with ship hydrography, navigation, and the probable 

 errors that enter into dead reckoning and astronomic sights. (See 3384.) 



ERROR OF CLOSURE 



Star Sigtits 



Figure 161.— Dead reckoning ad 

 justed to astronomic sights. 



765. Plotting the Dead Reckoning 



Dead reckoning may be plotted in various ways depending on the accuracy of the 

 hydrographic survey and the purpose for which the dead-reckoning plot is to be used. 

 (See also 337.) 



Where dead reckoning is used alone or in conjunction with astronomic control^ 

 since comparatively few changes in course are made, it is generally satisfactory to plot 

 it lightly in pencil directly on the smooth sheet and make an adjustment similar to a 

 traverse adjustment. Where dead reckoning is used in conjunction with R.A.R. control 

 it is almost always preferable to plot it on tracing paper so that it can be adjusted to 

 the other control data. For such purpose a good grade of comparatively thick tracing 

 paper should be used; Keuffel and Esser Ionic No. 197 -H is recommended. This might 

 at first appear too thick and opaque but actually it is sufficiently transparent and 

 otherwise very satisfactory. 



Several steps in the plotting are similar, regardless of the purpose for which it is 

 to be used. Wliere the dead-reckoning line is long and it is plotted directly on the 

 smooth sheet, the direction of each course should be determined by reference to the 



