Page 215 hydrography 332 



each astronomic sight which is to be used as a separate line of position. 



Wliere an area has to be developed by a system of lines more closely spaced than is 

 customary at the scale of the survey, a proportional reduction in the interval between 

 positions must be made. 



The maximum interval between positions should be reduced when there is any 

 likelihood of its use resulting in misplaced intermediate soundings or errors in the 

 positions of the depth curves. 



Although there is no objection to the use of unrecorded trial positions to check 

 the course or distance while sounding, valuable control data are lost if thoy are not 

 recorded. The fact that a trial position is required is evidence of uncertainty between 

 fixed positions. "In such case the trial position should be observed at the time of a 

 sounding, and recorded. (See 3452.) 



332. Methods of Control 



The methods usually used to control hydrographic surveys depend on the distance 

 from land and the depth of the water. In general, the accuracy with which any sound- 

 ing can be located in latitude and longitude .also varies proportionally with the dis- 

 tance from the land, until astronomic sights are resorted to. 



Wliere the survey vessel is close to the shore, its position, or the positions of the 

 soundings, may be determined most accurately from observations at control stations 

 on shore. This method is slow, its reliability depends on the correlation of observa- 

 tions taken at widely separated pomts, and the accuracy attained is usually unwar- 

 ranted at the scale of most inshore coastal hydrographic surveys. Its use may be 

 warranted, however, in harbors, in dredged channels, in the vicinity of docks and piers, 

 and where subsurface construction is to be undertaken in the area. 



3321. By Three-Point Fixes 



The usual method oi fixing hydrographic surveys within sight of land is known as 

 the three-point fix method, described in detail in 333. This method is almost univer- 

 sally followed for position finding. It consists in measuring simultaneously on board 

 the survey vessel two sextant angles between objects or signals whose geographic 

 positions are known. Where moderate depths extend a considerable distance from the 

 shore this method may still be used to determine the position of the survey vessel 

 beyond sight of land with reference to anchored control buoys, whose geographic 

 positions have been determined with reference to the shore control stations (see 

 section 25). 



3322. By Radio Acoustic Ranging 



Beyond the limit of visibility of shore objects and wliere the use of buoys for three- 

 point fix control is impracticable or unwarranted, a number of methods of control have 

 been used in the past — all considerably less accurate and consequently less satisfactory 

 than fixed position determinations. The limit of visibility may be imposed by the 

 curvature of the earth, but generally it is dependent on the atmospheric conditions in 

 the area. 



Radio Acoustic Ranging (R.A.R.) is the method employed beyond the range of 

 the thi"ee-point fix method. It is described in detail in chapter 6. By this method the 

 position of the survey ship is determined by the indirect measurement of distances 

 from two or more previously located control stations. The method may be thought 



