Colour printers that can print upto A0 size are now extensively used for obtaining copies of maps. They work with the same principle as the working of a laser printer. But printers and plotters are used to produce only a few copies of the maps. For making a large number of copies, generally the maps are published. For publishing the maps that have been prepared using GIS softwares, the maps have to be converted into a postscript format.
Postscript is a programming language used to make publication-ready output files.
In the press, the postscript files are changed into positives. Positives are transparencies that correspond to a particular percentage of a colour. For a colour map, four or more positives are made i.e. cyan, yellow, magenta and black and another shade of the required colour, or in other words, one positive for one colour. The contents of a positive are etched into plates made of aluminium. Thus, for preparing a colour map, four or more plates are required.