PROJECTIONS AND GRIDS 101 



COMPLETED PROJECTIONS 



On compilations cut apart in an east-west direction, the cartographer must indicate the 

 distance (in millimeters) across the cut between parallels of latitude. 



Projection intersections (ordinarily in blue) and other auxiliary information used by the 

 compiler must be retained for use by the verifier. 



PROJECTION RULING MACHINE 



There is available in the Bureau a machine to rule accurately, and to a certain extent 

 mechanically, polyconic and other projections of slight curvature, as well as Mercator pro- 

 jections and grids. This machine can rule on paper, plastic, and other mediums. The size 

 of a projection that can be drawn on the ruling machine is limited by a rectangle 41 by 59 

 inches whose sides are parallel to the projection lines. The sheet itself, however, can extend 

 somewhat beyond these limits, but ordinarily should not exceed 42 by 60 inches. In special 

 cases a sheet 46 by 65 inches can be ruled. 



It is a responsibility of the cartographer to check the projection before starting to compile 

 a chart. 



COORDINATE PLOTTER 



The coordinate plotter can be used to rule Mercator projections and grids. It has a maxi- 

 mum working area of 43 by 45 inches. The length can be extended by supporting the outer 

 end of the sheet. Dials read to 0.1 of a millimeter and can be estimated to 0.01 of a millimeter. 

 Prepare computations to 0.001 of a millimeter. 



CORPS OF ENGINEERS RECTANGULAR COORDINATES (LOCAL) 



The local grid systems used by the Corps of Engineers are rectangular and have a pro- 

 gressive deviation from the meridians and parallels. The meridian through the origin of the 

 system is ordinarily parallel to the north-south grid line. However, caution must be exercised 

 in using local grids, because in some cases the north-south grid line is not parallel to the meridian 

 at the zero point of the system. The point to be used as the origin for the following computa- 

 tions is the point at which the grid line and meridians are parallel. 



Geographic positions of Corps of Engineers coordinate grid intersections have been com- 

 puted for many harbors of the United States. From these positions, the coordinate grids have 

 been drawn on copies of the harbor charts. When Corps of Engineers blueprints showing 

 plane coordinate control are to be applied to a Drawing, the geographic position of the grid 

 intersection should be scaled from the above-mentioned chart copies and transferred to the 

 Drawings to furnish control for the blueprints. . 



