this interval is 500 feet, although the map is contoured on a 100-foot interval. 

 The difference in this type of shading and that employed in shadow-graphic maps 

 is that the latter suggests an illuminated model, whereas the former is shaded 

 strictly on the basis of contour values. 



Reproduction of Maps 



Most maps and other geologic drawings must be reproduced by one means 

 or another. Better prints can be expected if the draftsman is aware of the 

 peculiarities and limitations of the more common processes of reproduction. 

 An exceptionally well-drawn map may lose much of its effectiveness in reproduc- 

 tion if certain principles in drafting are neglected. 



The so-called photocopy processes depend upon the transmission of light 

 through the medium upon which the map is drawn. The lines and figures drawn 

 on the map prevent the light from passing through and exposing those portions 

 of the sensitized printing paper. It is, therefore, desirable for the lines to be 

 opaque and the medium transparent or highly translucent, and the quality of 

 prints will depend largely on the degree to which these conditions are met. 

 Obviously the best reproductions will be obtained from drawings made in black 

 drawing ink on transparent material, such as cellulose acetate. Lines drawn with 

 colored, waterproof inks may look well on the original map, but they are liable 

 to be indistinct or discontinuous on the print because these inks are not opaque 

 to the intense lights of the printing machines. If colors must be used on originals 

 to be reproduced by photo processes, the water-soluble types are more satis- 

 factory. When extremely thin lines are necessary on acetate or similar, thick 

 materials, it is better if they are inked on the back side where they will be in 

 direct contact with the printing paper. Thin lines on the upper surface are 

 likely to burn out in printing because of diffraction along the edges of the lines. 



Since the necessary exposure time is longer with more opaque media, such as 

 thick vellum or tracing paper, the opportunity for light leakage along thin lines 

 is greater. 



Photocopies can be made from penciled originals; but it is essential to 

 maintain considerable contrast between the opacity of the tracing material and 

 the lines. For this reason thin tracing paper with a toothy texture combined 

 with pencils which make very black lines gives the best results. It is somewhat 

 difficult to make reproducible maps with penciled lines on acetate and tracing 

 linen. 



Good photostats can be made from copy that is too weak to reproduce by 

 light-transmission processes. Better reproductions are obtained from originals 

 having high line and background contrasts. Generally speaking, prints by 

 Ozalid and Van Dyke methods are better than photostats for hand coloring by 

 methods described earlier. Photostat prints are more receptive to colored pencils 



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