741 HYDROGRAPHIC MANUAL PaGE 684 



741. Plotting the Shore Control 



Shore control may be divided into three classes according to the method and accu- 

 racy of determination (see 21 1) . First, is the control located by triangulation ; second, that 

 located by topography; and third, the remainder, which is located by the hydrographic 

 party, usually by sextant angles. Of these three types of control the latter, the hydro- 

 graphic stations, are invariably located for the purpose of controlling the hydrographic 

 survey. The topographic stations are almost always located for this purpose, espe- 

 cially if the topographic survey immediately precedes the hydrographic survey. The 

 triangulation stations, however, have often been located for the purpose of control- 

 ling the topography or as part of an arc of triangulation, and may or may not be valu- 

 able in controlling the hydrography. 



There are several methods in use for plotting the shore control on the smooth sheet, 

 depending upon the form in which the data are available. These methods are de- 

 scribed in the following items. 



Cuts, angles, and distances may often be combined in various ways to furnish 

 positions of control stations. These are described in chapter 2. Such determinations 

 are usually best plotted with the three-arm protractor in the manner described in 4534, 

 if computations are impracticable. 



7411. Plotting by Dms. and Dps. 



The data for plotting triangulation stations are usually furnished on Form 28B, 

 Geographic Positions. The latitude and longitude in degrees, minutes, and seconds 

 are given for each station, as well as the distances and azimuths between stations. 

 The equivalents in meters of the seconds of latitude and longitude are also given for 

 stations along the coast and these are known as the dms. (meridional differences) 

 and dps. (parallel differences), respectively, of the stations. Thus, if the position of a 

 station is given as latitude 54°44'34':i89 N. (1,057.2 meters), longitude 130°56'42':362 

 W. (756.5 meters), its dm. is 1,057.2 meters north of the 44-minute parallel, and its 

 dp. is 756.5 meters west of the 56-minute meridian. 



To plot a triangulation station, the quadrilateral on the projection within which 

 the station falls is first identified. With a pair of dividers and a metric scale (see 4821), 

 plot its dm. near each meridian line and mark with a fine prick point. Connect the two 

 points thus plotted with a fine pencil line, using a chisel-edged 6H pencil. Plot the 

 dp. of the station along this line and mark with a fine prick point. This will be the 

 position of the station provided there is no distortion in the paper. 



As a check of the plotting, and to compensate for any possible distortion, the 

 station must also be plotted from the north parallel and the west meridian of the 

 quadrilateral. These distances are the back dm. and back dp. for the station, and if 

 not already available, they may be obtained by subtracting the dtn. and dp. values 

 from the value of 1 minute of latitude and longitude, respectively, as given in the 

 polyconic projection tables for the latitude of the station. If the sheet contains an 

 appreciable amount of distortion the plotted distances will result in two pairs of points, 

 each pair closely adjacent. The true position of the station will be at a distance from 

 the points proportional to their respective distances from the projection lines. (See 

 fig. 152.) 



The position of the station should be marked with a fine prick point and identified 

 temporarily by a small circle and the name of the station, both in pencil. 



