743 HYDROGRAPHIC MANUAL PaGE 688 



(b) by graphic plotting on an aluminum sheet. In both methods the geographic posi- 

 tions of the buoys, and consequently their dms. and dps. are obtained; hence positions 

 of the buoys are plotted on the smooth sheet in the same manner that shore stations are 

 plotted by dms. and dps. (see 7411). Graphic methods usually furnish the required 

 accuracy of position for buoy-control surveys, provided accurate methods are used and 

 care is exercised in the plotting; but it is frequently desirable and easier to compute the 

 positions. 



a. By computation. — Positions of buoys are determined by computation generally 

 when it is not practicable to plot the observed data on the smooth sheet, as when the 

 sheet is distorted or the scale is too small or the stations observed on are too far distant. 



b. By graphic plotting. — Graphic plotting should generally be resorted to when the 

 data can be plotted on an aluminum sheet, or where the computations are too involved 

 as in the case of cuts obtained from a vessel whose position is determined by a three- 

 point fix. (See 2552 for detailed description of latter method.) Graphic plotting 

 should also be used where buoys are located by taut-wire distances or by subaqueous 

 sound ranging or by reference to submarine relief. 



Where a number of buoys have been located and plotted by three-point fixes, it 

 may be desirable to compute the positions of selected ones to verify the accuracy of the 

 graphic plotting. 



In gi-apliic plotting, an accurately adjusted metal protractor should always be 

 used and the work should be done with the utmost care and precision. Usually the 

 buoys are in such locations that the angles of the three-point fix are relatively small, 

 which tends to magnify plotting inaccuracies. 



743. Station Symbols 



Each station used for the control of the hydrography shall be identified on the 

 smooth sheet by its appropriate symbol and name, both inked in the color specified (see 

 fig. 169) . The actual station point is a fine needle hole, the edges of which are blackened 

 by rotating a sharp hard pencil point in the hole to facilitate its use in plotting the 

 hydrography. Ink should never be used for this purpose (disregard fig. 169 in this 

 respect) . The symbols and colors shall be according to the following scheme : 



(a) Triangulation and traverse stations shall be identified by red equilateral triangles, 4.5 mm 

 on a side (fig. 153), symmetrically placed around the station point with the base of the triangle normal 

 to the meridian and the apex north. 



(6) Topographic stations shall be identified by red circles 3 mm in diameter, symmetrically 

 located around the station point. Stations on air photographic surveys which have been located 

 by the radial plot for the use of the hydrographic party, are classed as topographic stations. Stations 

 to supplement those located by the radial plot, and which are located by the hydrographic party by ap- 

 proved methods, shall also be classified as topographic stations (see 2393). 



(c) Hydrographic stations (stations located by sextant or other means, see section 24) shall be 

 identified by blue circles 3 mm in diameter symmetrically located around the station point. The 

 positions of ordinary survey buoys are to be identified by the symbol for hydrographic stations. 



(d) Stations which have been spotted by the hydrographic party from the details of the photo- 

 graphs or the air photographic survey (see 2394), shall be identified by green circles 3 mm in diameter, 

 symmetrically placed around the station point. 



(e) R.A.R. stations shall be identified by a double concentric circle, the inner 3 mm in diameter 

 in blue ink, and the outer 5 mm in diameter in a distinctive color corresponding to the distance arcs 

 drawn from it (see 7637). 



The symbols for topographic and hydrographic stations are easily made and 

 centered by the use of the drop-bow pen (see 4841). There is some difficulty, however, 



