Page 307 hydrography 39 



of the field work. In all such cases the daily journal of the hydrographer making the 

 survey is invaluable. 



Such a journal should, of course, be neatly made from rough memoranda accumu- 

 lated during the day. As the survey progresses, earlier entries should be reviewed 

 from time to time and any which are finally inapplicable should be carefully ruled out 

 or the entries restated as necessary. Each entry should be initialed by the person who 

 made it and each journal accompanying an unplotted or incompletely plotted hydro- 

 graphic survey or partial sm-vey to be completed by another party should be authen- 

 ticated by the Chief of Party. 



After the complete Descriptive Report has been prepared, journals or notebooks 

 which have served their purpose can be destroyed. 



During a hydrographic survey each hydrogi'apher shoidd keep a journal in which 

 notes and memoranda are entered for use in compiling the general coast pilot report 

 at the end of the season (see 159). 



39. SPECIAL TYPES OF SURVEYS 

 391. Wire-Drag Surveys 



Sounding, in rocky regions, even where the sounding lines are closely spaced, is 

 insufficient in itself to ensure that all existing obstructions, pinnacle rocks, boulders, 

 and ledges have been found and that the least depths over them have been determined. 



Before such areas can be considered to be thoroughly surveyed they should be 

 wire dragged. In many cases, owing to the high cost of wire-drag surveys, thorough 

 coverage and development by sounding has to be considered sufficient, at least tempo- 

 rarily. It must be borne in mind, however, that the only positive way to find the 

 highest point of, and the least depth over, a rock or pinnacle whose top cannot be seen^ 

 is by means of the wire drag. 



For the final examination of such areas, the Coast and Geodetic Survey has devel- 

 oped an apparatus known as the wire drag, consisting of a horizontal wire which is 

 towed through the water by a vessel at each end, and which will strike or hang on any 

 obstruction rising above the depth at which it is set. The wire is maintained in a 

 horizontal position at any desired depth below the surface by means of weights sus- 

 pended from floating buoys by cables and by submerged floats attached to the wire 

 at regidar intervals. The depth of the wire below the surface can be varied as desired 

 by the upright cables between the weights and the surface buoys, as the cables are 

 wound on reels mounted on the tops of the buoys. 



The wire sweep is a modification of the wire drag for use in regions where the 

 general depths are considerably deeper than the depth to be verified and where few, if 

 any, obstructions are to be expected. The buoys are much farther apart than in the 

 drag, and no provision is made for varying the depth of the wire while dragging or for 

 preventing the sag of the wire between buoys. 



Wire-drag surveys are ordinarily executed by parties especially organized for the 

 duty and the vessels used have often been built for this kind of work. But the survey 

 ships and the larger auxiliary vessels are equipped with a short wire drag, 2,000 to 

 3,000 feet long, utilizing standard equipment. On the survey ships the buoys are 

 stowed in racks on the upper deck and are kept ready for use at all times; the remainder 

 of the equipment may be stowed in the most convenient place but it must be kept 

 clean, dry, and free from rust. The wire drags for the auxiliary vessels are usually 

 stored on shore until needed. 



