From the 1880s to the 1950s, the U. S. Geological Survey used engraved heavy copper plates in the process of printing many thousands of topographic and geographic quadrangle maps at several map scales.
The Printing Process – From Plates to Paper
A complicating aspect of this historical USGS printing process is that prints were not directly made from the plates. USGS transferred the image from the engraved plate to a special lithographic stone in order to make large numbers of prints, an approach that preserved the crispness of the engraving. Otherwise, the accuracy of the engraving would have been lost due to the repeated pressure required to transfer the image directly from the plate to each paper copy.
For printing purposes, the copper engravings were mirror images (left-to-right reversed). Etched by a USGS cartographic technician called a “map engraver,” the words and text characters are backwards. For maps, “east” appears on the left side of the plate instead of the right as it does on the printed map.
The engraving plates for a topographic map were color-separated in a set; that is, there was a plate for each color of ink to be printed. A typical topographic quadrangle map had a set of three plates: a black ink plate for cultural features, boundaries, and most of the text; a blue inked plate for hydrography; and a brown ink plate for topographic contours.