769 HYDROGRAPHIC MANUAL PaGE 714 



769. Plotting Supplemental Data 



It is frequently impossible or impracticable to determine the positions of rocks 

 awash, dangerous wrecks, floating aids to navigation, etc., by three-point fixes, or 

 other means described above in section 76. In areas where the general hydrography is 

 controlled by three-point sextant fixes, such objects are usually cut in (see 7415). In 

 this method of location, cuts may be taken from known shore positions, but more fre- 

 quently they are obtained during the hydrographic survey by a third observer, who 

 takes a sextant angle to the object simultaneously as the angles for position are being 

 observed. These extra, or third, angles are recorded in the Sounding Record with the 

 position at which they were taken, and the object cut in is described. Three or more 

 cuts from different positions, which will give an adequate angle of intersection, are 

 usually taken to each object. 



As the smooth plotting proceeds, these cuts must also be plotted. The boat sheet 

 should be used as a guide for the approximate locations of the objects cut in. 



Such cuts rarely result in a perfect intersection and a judicious interpretation of 

 the data is necessary to determine the most probable position of the object. The 

 distance of the object from the vessel, the inherent strength of the position fix, and 

 the size of the angle of the cut must aU be taken into consideration. So far as un- 

 certainties in the position fix are concerned, the smaller the angle of the cut the less 

 will be the effect on it due to slight inaccuracies in the fix, especially when the object 

 cut in is at the same approximate distance as the stations used for the fix. 



If the object being located is a floating aid to navigation, the direction of the 

 current at each observation should be considered. 



Where many cuts have been taken to different objects in an area, it is frequently 

 extremely difficult to be certain which cuts were taken to identical objects, and a 

 careful study of all the facts and data is necessary to avoid the possibility of confusion 

 of cuts. 



77. SOUNDINGS AND DEPTH CURVES 



The soundings are the most important part of a hydrographic survey, and the 

 greatest care must be taken in putting them on the smooth sheet. They must be clear 

 and legible and, in general, a magnifying glass should not be required to read them. 

 The smooth plotter must always bear in mind that an illegible sounding erroneously 

 transferred to a published chart may possibly result in loss of life and property. The 

 fact that the work is verified before publication does not relieve the plotter from exer- 

 cising care in the first instance. 



All soundings are penciled on the smooth sheet in the field. Under no circum- 

 stances are they to be inked by the field party. This is accomplished only after veri- 

 fication in the Washington Office. 



771. Depth Units 



Only one depth unit (fathoms or feet) shall be used on each hydrographic survey 

 sheet. The depth unit to be used will depend on the general region (in which ocean 

 the survey is located), on the specific locality, and on the unit used on existing charts. 



7711. Depth Unit in the Atlantic Ocean 



The depth imit of surveys in the Atlantic Ocean and bodies of water tributary 

 thereto shall be integral feet (and occasionally to the nearest half-foot), except — 



