3143 HYDROGRAPHIC MANUAL PaGE 200 



The development of an isolated shoal is frequently best accomplished by running 

 a series of lines radiating from a buoy anchored near the center of the shoal. Where 

 the coast forms a background to the buoy used as a front range mark, part of the 

 radiating lines can be run on ranges, the remaining being run by compass course. (See 



also 3453.) 



3143. Systems of Circular Arcs 



The development of an isolated shoal area can be very effectively controlled by 

 the use of distance angles. Two control stations should be selected, so located that 

 the loci of the angles between them cross the shoal area in flat arcs. Preparatory to 

 the survey a series of such loci can be drawn on the boat sheet spaced as desired (see 

 37). At one end of each arc the value of the angle between the two stations is noted. 

 With this angle set on a sextant the sounding vessel may be readily maneuvered onto 

 the line and maintained there by periodic observation of the angle, the course being 

 changed slightly to maintain the angle on. Distance angles are particularly advan- 

 tageous when the sounding vessel must proceed at slow speed and either the wind or 

 current is strong and variable. Only by the use of this method, or ranges, can a 

 system of closely spaced lines be run without having the sounding vessel get too far 

 off line. Where the principal system of Hnes has been controlled by distance angles, 

 any required split line can be accurately run by using the mean of the two distance 

 angles used for the lines on each side of the split. 



315. Spacing Sounding Lines 



The proper spacing of sounding lines depends on the scale of the survey, the 

 depth of the water, the proximity to shore, the character of the submarine relief, and 

 the importance of the region. The general spacing should give a methodical repre- 

 sentation of the depths and generalized depth curves in the area, and be sufficiently 

 close to give indications, at least, of all banks and dangers therein. Subsequent 

 splitting of the general system of lines and additional development must be adequate 

 to locate all dangers and shoals, to determine the least depths on them, and to enable 

 the hydrographer to draw all depth curves with assurance. 



The maximum spacing of lines for the various zones or depths of the project is 

 ordinarily prescribed in the project instructions. It is to be noted that this is the 

 maximum spacing, and it is the responsibility of the Chief of Party to reduce this 

 spacing as necessary to obtain an adequate development in any specific part of the 

 project area. If the Chief of Party is of the opinion that the general spacing specified 

 in the project instructions should be changed for the entire area or for large parts of it, 

 he shall make suitable recommendation to the Washington Office and ask for additional 

 instructions. 



It is obvious that, although sounding lines can be run with refined methods at as 

 close intervals as desired, there is a practicable limit to the number which can be plotted 

 at any given scale. In general, four or five sounding lines to an inch on the sheet can be 

 plotted and the soundings inked without difficulty, and in small areas at least twice 

 as many to an inch can be shown legibly if sufficient care and patience are exercised 

 in the inking. The scales of the surveys should be selected with this in mind so that 

 the closest spacing of lines expected to be required in an area can be conveniently 

 shown at the scale chosen. Table 6 gives the distances between lines (in meters) 

 which should ordinarily be allowed for various scales, and the minimum distances 

 between lines which can sometimes be tolerated for the same scales. 



