7322 



HYDROGRAPHIC MANUAL 



Page 668 



polyconic projection is a straight line, all other meridians and all the parallels being 

 curved lines, except for the special case of the Equator which is a straight line. The 

 parallels are arcs of nonconcentric circles with radii of decreasing length as the latitude 



increases, but whose centers lie in the exten- 

 sion ot the central meridian. The curved 

 meridians are concave toward the central 

 meridian in increasing amounts as the dis- 

 tance from the central meridian increases. 

 Practically, in the larger scales usual in 

 hydrographic siu'veying the meridians may 

 be considered straight lines within the limits 

 imposed by graphic methods of construction. 

 The longitude scale is everywhere correct, 

 but the latitude scale is strictly correct only 

 on the central meridian. 



The accuracy inherent in the polyconic 

 projection is a matter of some interest. 

 Some idea of the departure from true repre- 

 sentation is given by the following two 

 examples. On a 1:40,000 scale projection 

 on a sheet 31 by 53 inches in size the scale 

 distortion in the worst part does not exceed 

 9 parts in 1,000,000 and the angular error 

 does not exceed 1" of arc. On a 1:120,000 

 scale projection on a sheet 42 by 72 inches in 

 size the scale distortion in the worst part does not exceed 3 parts in 20,000 and the maxi- 

 mum angular error is 15''3 of arc. It is apparent, then, that the errors of azimuth and 

 distance on sheets usually used in plotting hydrographic surveys are of such small 

 proportions as not to be graphically measurable. 



Since the meridians of the polyconic projection are curved lines concave toward 

 the central meridian two adjacent maps or surveys cannot be joined together in an east- 

 west direction, because the curvatures of the marginal meridians on the two maps are in 

 different directions. 



Figure 144. — Polyconic projection of North America. 



7322. Verification of Scales and Straightedges 



Before beginning the construction of the projection, the meter bar and meter scales 

 and the straightedges should be verified unless definite information of their correctness 

 is at hand. It cannot be assumed that the scales are correct since in the past several 

 have been discovered with appreciable errors in the divisions. Perhaps the best test 

 that can be applied in the field is by comparison with a meter bar known to be correct. 

 In lieu of this a comparison of the various scales and meter bars against one another 

 will suffice (see 4821). 



Straightedges can also be tested by comparison with one another but a better test 

 is described in 4831. 



