Page 687 the smooth sheet 7415 



7415. Plotting by Cuts 



Cuts are sextant angles, usually taken from a survey vessel, between an object 

 whose position is known and an object whose position is to be determined. The posi- 

 tion of the observer is located by two sextant angles. Two or more such cuts taken 

 from different positions determine the position of the unknown object. Cuts may 

 also be taken from previously located shore stations. Cuts are recorded in the various 

 Sounding Records and indexed on page 2 of volume No. 1 (see 248). 



To plot a control station from sextant cuts, the position of the observer is first carefally plotted, 

 if not at a previously determined station, using a metal protractor whose adjustment has been pre- 

 viously checked (see 762). The third angle or cut is then set on the protractor and with the center 

 placed directly over the observer's position and one arm passing through the known station, the 

 angle to the new station is plotted by a short line drawn along the edge of the protractor arm with 

 a hard chisel-edged pencil (see 724). This must be temporarily identified by the name of the new sta- 

 tion in pencil. All other cuts to this same station are plotted in a similar manner from the respective lo- 

 cations from which they were taken. If the observing has been carefully done and based on control 

 precisely located, and the sheet is free from distortion, all of the cuts should intersect at one point, 

 which is the location of the station. It rarely happens that this is the case, and a position must be 

 adopted which will most nearly fall on all of the plotted cuts, taking into consideration their respective 

 probable accuracies. The penciled cuts must not be erased from the smooth sheet until after the 

 verification of the survey in the Washington Office. 



7416. Stations Located by Three-Point Fixes 



Control stations are sometimes located by three-point fixes at the station. In 

 such cases their positions may be plotted on the smooth sheet with the three-arm 

 protractor (see 4534) verifying the plotting with the check angle, if taken. If there is 

 distortion in the sheet or if stations observed are considerable distances away, the 

 position should be computed on Form 655 (see 2431) and plotted on the smooth sheet 

 by the dm. and dp. method. 



742. Plotting the Floating Control 



The floating control which may be used to control a hydrographic smwey usually 

 consists of survey buoys or sono-radio buoys, and occasionally small marker buoys. 

 There are various methods of determining the positions of such buoys depending upon 

 the availability of shore signals, the distance offshore, the character of the area sur- 

 veyed, etc. These methods can be grouped generally as foUows: (a) sextant locations 

 either by three-point fixes at the buoys or by cuts from the vessel; (6) directions from 

 shore stations; (c) sun azimuths from the vessel to shore stations; (d) sun azimuths 

 and distances; (e) astronomic observations; (/) by reference to submarine relief ; and 

 (g) various combinations of these. 



A complete discussion of these methods as well as the methods to be used in 

 determining the geographic positions of the buoys is contained in section 25. In 

 determining the geographic positions for smooth-sheet plotting, the plotter should 

 follow the method recommended for the particular case involved. The following item 

 deals only with certain general aspects of the problem. 



7421. Determining Positions of Floating Control 



The positions of the floating control are ordinarily known prior to the construction 

 of the smooth sheet, since they must be known in order to execute the field work. 

 There are two general methods of obtainmg these positions, (a) by computation and 



