2535 HYDROGEAPHIC MANUAL PaGE 124 



distant from each arc by an amount proportional to its distance from each fixed posi- 

 tion. The velocity of sound used should be verified before the proportional method 

 of determining the position is resorted to; any distance arc suspected of being in error 

 should be rejected. The true position of the station is always inside the rough figure 

 formed by the nonintersecting arcs, and if the station is farther from the fixed positions 

 than the arcs are, this is an indication that too low a velocity of sound was used, and 

 if nearer, then too high a velocity was used. (Fig. 26.) 



VELOCITY TOO HIGH VELOCITY TOO LOW 



Figure 26.— R.A.R. station located by subaqueous distances from three-point fix positions. 



2535. Astronomic Observations 



Isolated shoals far from shore stations must be surveyed and they often require 

 extensive development but are located where it is impossible to extend shore control 

 to them. Taut-wire traverse, for instance, from the shore control to a shoal area 

 surrounded by extensive areas of deep water would be impracticable. An isolated 

 shoal area in the open ocean far from land is an example. Local control to coordinate 

 the survey is more important than a precise determination of the latitude and longitude 

 of the area. In such areas, where it is impracticable to locate control by other more 

 accurate methods, it may be located by astronomic sights. This method has a rather 

 large probable error, and should be resorted to only when this lesser accuracy can be 

 tolerated. 



In order that the position of a station located by astronomic observations may be 

 as accurate as possible, the methods and observations must necessarily be of a higher 

 standard of accuracy than is required in ordinary navigation. The accuracy which 

 can be attained will vary considerably, depending on the conditions at the time the 

 observations are made. Accurate observing under the best conditions should deter- 

 mine the position of a station with a probable error not greater than one-fourth nautical 

 mile. Poor observmg conditions will increase the probable error considerably. Under 

 ideal conditions sufficient accuracy will be attained by sets of six observations on four 

 well-distributed celestial bodies taken independently by two or more experienced 

 observers at evening and morning periods of observation. Wlien observing conditions 

 are poor or the observers are inexperienced, the observations should be extended over 

 several days in order to obtain the desired accuracy. To obtain this accuracy, a certain 

 definite procedure must be followed in the selection of stars, observing schedule, and 

 elimination of errors. (See also 3384 and 3387.) 



The best niethod of locating a station by astronomic observations, where the depth 

 of water permits, is to anchor the ship at the position where the buoy will be anchored 

 later and obtain all observations while at anchor. The ship's heading should be recorded 

 to determine the direction of current at each series of observations. The scope of the 

 anchor chain should be noted for use with the direction of the cm-rent in transferring 



