Page 699 the smooth sheet 762 



Thin celluloid may also be used as a protective cover. This material is so trans- 

 parent that small circular holes at the locations of the signals are unnecessary. The 

 principal objection to celluloid as a protective medium is the difficulty in making tem- 

 porary notes on it and in identifying the successive positions as they are plotted. 



After being correctly placed in position, the cover should be secured by weights or 

 thumbtacks. Since it is necessary to lift the cover frequently, it will be found con- 

 venient to secure it along the edge of the sheet farthest from the plotter with a sufficient 

 number of tacks to hold it in position, using only one or two tacks elsewhere. 



It is not necessary to lift the protective coyer every time a position is plotted. 

 Occasional positions, say every fifth one, may be numbered temporarily on the cover 

 itself. It is customary to plot at least all of the positions on one page of the Sounding 

 Record, or all of the positions on a line, before raising the cover to number the positions 

 on the smooth sheet and to connect them (see 7682). The smooth sheet should always 

 be covered when not in use. 



R.A.R. smooth sheets are difficult to keep clean since the plotting of positions must 

 be done directly on the surface of the sheet. A convenient method of protection is to 

 use two obsolete charts or pieces of wrapping paper folded in the form of envelopes to 

 fit loosely over each end of the smooth sheet, each envelope being held together by 

 Scotch tape. These are supplemented by two pieces of paper folded lengthwise over 

 the sides of the smooth sheet, but with the ends open. The paper should completely 

 cover the smooth sheet when each pair is in its innermost position. The envelopes 

 should fit loosely enough so that they may be slid away from the smooth sheet as desired 

 in order to expose the area being worked on. Several loose sheets of paper or a hand 

 towel should be laid on top of the exposed surface to prevent its being touched by the 

 hands or arms any more than is absolutely necessary, 



762. Protracting Three-Point Fix Positions 



Three-point fix positions are plotted graphically in hydrographic surveying by the 

 use of a protractor. This is usually a three-arm protractor made of either metal or 

 celluloid having a fixed center arm and two movable arms with which the two observed 

 angles can be set. The left angle of the fix is set with the left movable arm and the 

 right angle with the right movable arm. The protractor is then manipulated into a posi- 

 tion on the smooth sheet so that the three arms pass precisely through the respective 

 centers of the plotted positions of the stations between wliich the angles were measured. 

 The intersection of the three arms, wliich is the center of the graduated circle, then 

 marks the position of the vessel or sounding. (See 4534.) 



7621. With Metal Protractor 



Most three-point fix positions are plotted on the smooth sheet with the metal three- 

 arm protractor described in 4531. The construction of the metal protractor is such 

 that it is inconvenient or impossible to use it in plotting positions which are at short 

 distances from the stations. Neither can it be used with the rapidity or ease with which 

 a celluloid protractor can be used. 



Extensions to the arms of a metal protractor are provided for use where the stations 

 observed on are so distant that the regular arms do not reach them. When these 

 extensions are used, extra precaution should be taken in the plotting. 



