For maps that show several features in different colors sheets rubbed with blue, orange, brown, or green pencils may be used, one after another, for tracing each set of the features. Red should not be used, as it is not easily erased. This method insures distinctive lines for the separate features and prevents the confusion that might result from the use of one color only. Exact register of the features shown in the several colors used may be insured by fastening one edge of the drawing to be copied to the drawing paper by mucilage or thumb tacks. The colored sheets may then be slipped in and out without altering the position of the lines or symbols for one set of data with relation to those for the others.
In the final preparation of a base map to be engraved and printed in colors--for example, black, blue, and brown--tracings of the three colors appearing on the original base should generally be transferred, as described above, to one sheet of paper and thus worked up into a three-colored map. It is usually unnecessary and undesirable to draw each color on a separate sheet. The preparation of separate drawings may facilitate reproduction, but if they are made on tracing cloth the usual uneven shrinking or stretching of the cloth may produce misregister in the printing; therefore it is safer to make a single drawing, so that the photolithographer can make three negatives and separate the colors by painting out or "opaquing" the colors not wanted on each negative. A map drawn on a single sheet is also less bulky and can therefore be more conveniently handled and compared with proof.
If for any reason separate tracings for the different colors to be used on a map are considered desirable they should be made on linen cut from one roll and in the same direction according to the warp and woof.
In the celluloid method of transferring a map or parts of a map to paper upon which a complete new map is to be drawn the map or part of the map to be copied is photographed to the exact scale of the new drawing and reproduced in graphite on thin sheets of celluloid.
The celluloid sheet is then laid face down in the correct position on the drawing paper and firmly rubbed on the back with a steel burnisher, which makes a perfect offset of the map on the paper. After the parts desired are inked over the rest of the graphite print is easily erased with an ordinary rubber.