The Bodleian Map Room Blog looks at a specific kind of map of imaginary places, “designed to be a guide to cartographers by showing how to portray certain features, or for the map reader to show what the symbols mean.”

Here’s an example, ‘Colours and symbols used on fair sheets and fair tracing’, issued by the Hydrographic Office of the Navy in 1973. Fair here meaning a document after correction, ready to be used. But is it fair? With this ‘made-up’ map the Hydrographic Office have used real names but in random locations, so Campania, in reality a region of Italy, features, as does the English county of Rutland (though as a town here) and a made-up mountain, Montrosia. Best of all is mention of “Approaches to Valhalla”. The map shows different forms of landscape, both natural and man-made with the symbols used to show those features, and the names are added to give the whole an authentic feel.

It makes sense to do it this way: no single real map is likely to contain every single feature you’re trying to demonstrate.