Page 127 control and signal building 2536 



an azinuitli line on the sheet. It is essential that the survey be well controlled in 

 azimuth as it will be held in azimuth when it is fitted to the prior survey. 



The sounding- lines of the survey to locate the buoy should duplicate the sounding 

 lines of the prior survey insofar as practicable. This cannot be done exactly because 

 the datum is not known at the time. The lines should follow the same pattern, how- 

 ever, and the area should be surveyed in approximately the same detail so that the 

 features may be delineated on both surveys with about the same accuracy. Fre- 

 quently, additional sounding lines will be required to develop certain features appearing 

 on the prior survey that have been missed by the systematic lines of the new survey. 



For greater accuracy, the soundings taken to locate the station should be plotted 

 on a scale larger than that of the future survey for which it is to be used, so that it is 

 frequently necessary to enlarge the prior survey. The required part of the smooth 

 sheet of the prior surve}^ will usually be enlarged photographically to the desired scale 

 at the Washington Office upon request. The soundings should be transferred accu- 

 rately from itjto an accurate projection, preferably on an aluminum-mounted sheet, 

 and closely spaced depth contours should be carefully drawn at the selected location. 

 A knowledge of submarine relief is required (see 353 and 7762) and the contours should 

 be delineated by the most experienced member of the party. The interval to be used 

 between depth contours depends on the gradient of the bottom relief and the distinctive- 

 ness of the feature. It should be close enough to leave no doubt as to the exact form 

 of the feature but not so close that confusion will result when the two surveys are 

 superimposed. 



The soundings of the new survey should be carefully plotted with reference to the 

 station to be located at the enlarged scale chosen. They may be plotted on tracing 

 paper but celluloid is preferable because of its greater transparency and dimensional 

 stability. The positions and soundings are plotted with the utmost accuracy by the 

 usual method for this type of survey. The soundings should be reduced to the plane 

 of reference of the prior survey. After they have been plotted, depth contours are 

 drawn on this survey, comparable in all respects to those drawn on the enlarged copy 

 of the prior survey, using the same care. 



If the bottom feature selected is adequate for the purpose, little difficulty will be 

 experienced in correlating the two surveys, when superimposed. The two surveys 

 must be maintained in relative azimuth while they are adjusted until the depth curves 

 and soundings are in the best attainable agreement. Perfect agreement is seldom 

 possible from the very nature of the case. It is frequently necessary to effect a com- 

 promise adjustment that satisfies, as nearly as possible, all detail represented. To 

 attain this requires painstaking and thoughtful consideration of all the factors involved 

 if the position is to be sufficiently accurate. Wlien the best adjustment has been 

 attained, the position of the station is pricked through the transparency to the under- 

 lying sheet and the position scaled in the usual manner. 



The accuracy of the method depends primarily on the selection of suitable sub- 

 marine relief, and secondarily on accurate determination of the positions and soundings, 

 and comparable reduction of the soundings of both surveys. The smooth plot and the 

 selection and delineation of depth contours of both surveys must likewise be of compa- 

 rable accuracy. If all phases of the operation have been performed with care, the 

 relative position of the station should be adequately accurate for use at the usual scale 

 of offshore surveys. 



