338 HYDROGRAPHIC MANUAL PaGE 238 



lines of buoys and were controlled by bearings taken at 10-minute intervals on the buoys visible. The 

 bearings were plotted and the dead reckoning was adjusted to the bearings. 



Neither of the above methods is now used and they should be considered obsolete. 



338. Astronomic Sights 



Hydrographic surveys beyond the limit of visibility of terrestrial objects and sur- 

 vey buoys or beyond the range of R.A.R. shall be controlled by dead reckoning and 

 astronomic observations. 



In general, where the sounding line is beyond the range of control stations for- 

 more than 8 hours, it shall be planned so as to be controlled by astronomic sights, 

 weather permitting. 



At least one and preferably several positions at both the beginning and end of a 

 sounding line ccntrolled by astronomic observations must be well fixed from the off- 

 shore control, which may be shore objects, survey buoys, or R.A.R. stations. The 

 beginning and end of the line should be fixed as required in 3376. 



Astronomic sights are used in hydrographic surveying in a manner similar to 

 their use in navigation, except that the sights are taken with greater precision and care, 

 and they are used more accurately to control the sounding line. It is not the intent to 

 explain fully in this Manual all details of observation, computation, and use of astro- 

 nomic sights as these are usually well known and are adequately explained in any good 

 epitome on navigation, such as Bowditch. The information in this Manual is limited 

 to refinements of observation and the best methods of using the observational data 

 to control hydrography. 



3381. The Use of Logs 



At least two well-rated logs should be in operation at all times that sounding lines 

 are controlled by astronomic sights. The log factors of these must be accurately known; 

 the method of determining these log factors is described in 4454. The use of the logs 

 is similar to their use in dead reckoning which is described in 3374. On long lines 

 controlled by astronomic sights logs should be read at 10-minute mtervals or oftener, 

 and each log reading recorded as a fix. (See also 4453.) 



The log reading at each astronomic sight must be determined to correlate the 

 latter with the depth measurements. When the astronomic sight consists of a series 

 of several (generally six) observations on one celestial body, the log may be read at the 

 midpoint of the series of observations. This is generally not as satisfactory as a 

 subsequent computation to determine the log reading at the time of the sight. Unless 

 the series consists of an odd number of observations, the midpoint will come between 

 two observations, and even when the series consists of an odd number, the mean time 

 of the series will not correspond exactly with the time of observation of the middle 

 sight. The ship's time which is used to record the log readings is kept so nearly correct 

 that for all practical purposes it may be assumed to represent Mean Time. After a 

 sight has been computed, the Greenwich Civil Time of observation may be used in 

 conjunction with the times of the log readings before and after a sight to determine 

 the log reading at the sight by interpolation. 



At morning and evening star sights the several lines of position are run forward 

 or backward to a selected central tune for which a position is determined. At these 

 observations the log reading for each sight must be computed as described above from 

 the series of regular 10-minute log readings extending over the time during which 

 all of the sights were taken. It is convenient to select a central time for the series 

 which is a regular 10-minute position. 



