242 HYDROGRAPHIC MANUAL PaGE 104 



Stations located by hydrographic methods shall be symbohzed on the boat and 

 smooth sheets by circles in blue ink, 3 mm in diameter. 



If a hydrographic station, during or after use as control for the hydrographic 

 survey, is relocated by geodetic or topographic methods it shall be symbohzed on the 

 smooth sheet according to the latter more accurate method of location, and so listed 

 in the Sounding Record (see 2154). Its symbol on the boat sheet need not be changed, 

 but an appropriate note should be made in the Sounding Record or notebook where 

 the cuts obtained by the hydrographic party were recorded that these data have been 

 superseded. 



242. Frequency of Stations 



A sufficient number of hydrographic signals is requu'ed so that the hydrographer 

 may determine the position of his vessel at any point in the area of the survey with 

 sufficient accuracy for purposes of charting on the largest-scale published chart and with 

 sufficient accuracy to control the development of any shoals m the area as closely as 

 may be required. 



The number of hydrographic stations which may be required depends entirely on 

 the deficiencies in the topographic surveys. If the topographer has provided the 

 control for the inshore hydrography from the viewpoint of the hydrographer and the 

 hydrographic survey follows immediately, the hydrographer should have to locate 

 few, if any, stations. 



243. Methods of Location 



The sextant is the instrument commonly used by a hydrographic party to deter- 

 mine positions and there are three general methods in which this mstrument is used 

 to locate hydrographic stations. 



Combinations of the methods may be used, of course, when this can be done to 

 advantage and will result in greater accuracy than would be attained by the use of one 

 method alone. 



Many of the methods described in section 23, Topography, may also be used 

 by hydrographers, the stations being classified according to the methods used. Refer 

 particularly to the methods listed in 238 and 2393. 



Shore hydrographic stations shall be located by other methods only m emergencies 

 and as a last resort, when it is impossible to utilize one of the prescribed methods. Sta- 

 tions so located must be kept at a mmimum and an effort should be made to locate them 

 subsequently by some more accurate method. For each statjon so located the method 

 used shall be described and the circumstances explained. 



2431. Three-Point Fix at Station 



The observer occupies the new station and, with a sextant, measures angles be- 

 tween control stations that will provide a strong three-point fix at the new station. 

 Two angles measured between three control stations appropriately located with refer- 

 ence to the new station (see 333) will provide the data for the fix. A check angle should 

 always be measured to a fourth station. A navigating sextant should be used for 

 this purpose and wherever practicable the angles should be observed to triangula- 

 tion stations. 



The position of the new station can then be plotted on the boat and smooth sheets 

 by means of a three-arm protractor, or may be computed by the three-point problem 



