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HYDROGRAPHIC MANUAL 



Page 670 



tions are necessary to ensure the inclusion of the required control points as, for example, 

 where they fall dangerously close to opposite edges of the sheet. 



Although the projection can be made by continued reference to the projection 

 tables, it is most desirable to make a rough sketch of the projection on a sheet of paper 

 (see 322), noting on it all of the distances to be plotted with the meter bar and beam 

 compass. Depending on the scale of the projection being made and the scale of the 

 meter bar being used, these distances will of course be reduced for use directly on the 

 meter bar. For instance, if the projection is to be 1:40,000 and the scale of the meter 

 bar is 1 : 20,000 all distances taken from the tables must be halved. All data from the 

 tables must be taken out for the latitude in which they are to be used. 



Referring to figure 146, the distances wi?ni, 7mn2, rnws, and rnnu, are taken from the table headed 

 "Meridional arcs," under "Continuous sums of minutes." The distances mx, mxi, mx2, mxz, mxi, and 

 tnxi, for each parallel are taken from the table headed "Arcs of the parallel in meters" for scales of 



Figure 146.— Construction of a polyconic projection 



1:40,000 or larger. For scales smaller than 1:40,000 they must be taken from the values of "X" 

 under the table headed "Coordinates of curvature," interpolating for latitudes and longitudes inter- 

 mediate between those given in the tables. 



The construction is carried out by drawing the central meridian and the straight construction 

 line ab perpendicular to it. The distances jnmi, mm2, etc., are laid off along the central meridian using 

 the beam compass and meter bar. The straight construction lines cd, ef, gh, and ij are drawn parallel 



