6. THE SMOOTH SHEET 



215 



trolling critical hydrographic data shall 

 never be transferred in this manner. 



Where the transfer method is considered 

 a proper one, the chief of party shall request 

 the film positive from the Washington Office 

 when the boat sheet is forwarded for pre- 

 liminary application of new information to 

 the chart. The proposed transfer will be 

 evaluated and the film copy furnished if the 

 proposal is approved. 



The second procedure for minimizing pro- 

 tracting of positions is to omit protracting 

 some positions as specified below : 



Where the sounding lines or segments of 

 lines are apparently straight for three or 

 more positions, or if the lines follow distance 

 arcs closely, the protracting of not more 

 than two consecutive positions on the line 

 may be omitted. The first three and the 

 last three positions on each line shall be 

 protracted. Positions at abrupt changes in 

 course shall be protracted. The positions of 

 all critical detail and all detached positions 

 shall be protracted. 



Positions not protracted shall be located 

 along the line in proportion to time intervals 

 and shall be numbered in the usual manner. 

 Positions so located shall be checked in the 

 record book with a small letter "n" in lieu 

 of the usual check mark. 



Positions determined by electronic dis- 

 tance measuring devices can be plotted so 

 rapidly by Odessey protractors that little 

 time is saved by these methods. 



6-48 Use of protractors. — Three types of 

 protractors described in Sections 3-16 to 22 

 are used in plotting hydrographic positions. 

 Three-point fixes are plotted by use of three- 

 arm metal or plastic protractors. Metal pro- 

 tractors should be tested and adjusted, if 

 necessary, before plotting is started and peri- 

 odically while in use (see 3-18). Plastic pro- 

 tractors cannot be adjusted, but should be 

 tested daily while in use. Index errors can be 

 determined and applied to observed angles 

 when plotting positions. Plastic protractors 

 may warp, particularly near the ends of the 

 arms, and the usable part of each arm should 

 be determined and marked or the fiducial 

 lines on the warped portion should be re- 



scribed. Plastic protractors having an in- 

 dex error greater than 3 minutes in either 

 arm from causes other than warping of the 

 fiducial lines should not be used for smooth 

 plotting (see 3-21). There is no objection to 

 the use of plastic protractors for plotting 

 at full range of the arms, provided that the 

 positions are correctly plotted. 



Metal protractors are more precisely con- 

 structed than the plastic models and shall 

 be used to plot all detached positions for 

 location of signals, rocks, buoys, least depths 

 on shoals or obstructions, and to plot weak 

 fixes. 



Three-armed protractors shall be moved 

 into position by grasping the circle and the 

 fixed arm; the movable arms shall never be 

 used for this purpose. Plotters must be espe- 

 cially careful when using a metal protractor 

 with extension arms attached. 



For plotting a position close to a control 

 station, it is occasionally necessary to use a 

 small unarmed protractor made by marking 

 the angles on a printed transparent compass 

 circle. 



When electronic ranging systems are used 

 to control the hydrography, Odessey (see 

 3-22) protractors are used to plot the posi- 

 tions. The concentric circles on this tjrpe 

 of protractor permit centering it at a point 

 representing an increment from the nearest 

 distance circle drawn on the sheet. 



6-49 Abnormal Positions. — Abnormal po- 

 sitions are usually the result of error, mis- 

 understanding, expediency, or necessity in- 

 curred by the nature of hydrographic 

 surveying. 



When only one angle is recorded or ac- 

 cepted the position is plotted on the locus 

 of the single angle at a point adjusted for 

 time and course from adjacent positions. 

 Acceptance of this position depends on steady 

 speed and course of the survey ship and also 

 on a strong line of position for the locus of 

 the single angle. If the position is indefinite, 

 the soundings should be plotted according to 

 time, angle, or other hydrography. If severial 

 consecutive positions are not firm it may be 

 necessary to reject that portion of the line. 

 This action shall be recorded in the sounding 



