Page 239 hydrography 3382 



The log reading for each sight should be entered in the appropriate space on Form 

 719, Astronomic Sight for Hydrographic Control, and on Form 722, R.A.R. and Dead- 

 Reckoning Abstract. 



3382. Computations 



Form 719, Astronomic Sight for Hydrographic Control, has been arranged so that 

 any kind of astronomic sight may be computed on it. It is arranged for the use of the 

 Marc Saint Hilaire method using the cosine-haversine formula. Its use is not obliga- 

 tory if observers prefer other methods of computation which will give equally correct 

 results. For a series of morning or evening star sights which can be computed from 

 the same dead-reckoning position there is considerable advantage in using the Marc 

 Saint Hilaire method. For single observations this method is longer than some of the 

 newer methods which eliminate the necessity for the use of logarithms. 



Azimuths of observed celestial bodies can be scaled graphically to the nearest 

 one-fourth degree from Captain Weir's Azimuth Diagram. Considerable time is saved 

 by the use of the graphic method, especially if a number of azimuths are to be scaled 

 at the same time, and the results are as accurate as warranted. The declinations of a 

 number of stars used most frequently can be plotted on the diagram. It contains 

 complete instructions for its use. Azimuths can also be found from computed tables, 

 such as H. O. No. 214, Tables of Computed Altitudes and Azimuths. 



If other methods of computation are used the observed and dead-reckoning data 

 for which spaces are provided at the top of Form 719 should be arranged in the same 

 general way on the adopted form. All computations should be worked to the nearest 

 0.1 second of time and to at least the nearest 0.1 minute of arc. 



The original computations should be bound together with the sheets on which the 

 lines of position are plotted and with the dead-reckoning abstracts (Form 722) when 

 forwarded to the Washington Office with the other survey records. Smooth copies 

 need not be made of either the computations or the plotted lines of position. (See 8313.) 



3383. Lines of Position 



An observation on one celestial body gives a line of position passing through the 

 observer's position at right angles to the azimuth of the celestial body. One observa- 

 tion does not give a fix. 



Each Sim sight observed during the day should be plotted and used as a line of 

 position to which the dead reckoning is adjusted. At the time the line is being run, 

 one sun sight may be run up to another in order to determine the ship's probable posi- 

 tion at the moment, and whether or not a change in course is needed in order to follow 

 the proposed line. When the final adjustment of the sounding line is made on the 

 smooth sheet, sun sights shall not be run up, but must be used as separate lines of 

 position at the times of observation to which the line as a whole is adjusted. 



In adjusting the dead reckoning to separate lines of position the first step is to 

 make a preliminary adjustment of the line to those lines of position which most nearly 

 parallel it and those which are most nearly at right angles to it. The former serve to 

 place the line in azimuth and the lattet to determine a log factor for the interval. For 

 example, if the true direction of the line is east or west, morning and afternoon sun 

 sights taken when the sun is on the prime vertical will cross the line at right angles. 

 From the log distance between these and the corresponding scaled distance between 

 the lines of position a log factor for this interval can be derived. Without disturbing 

 it in azimuth, the line may then be moved north or south until it coincides with the 



