Dirk de Vries:
The 'Helvetia' wall-map by Gerhard Mercator
Cartographica Helvetica 5 (1992) 3–10
The acquisition of an enigmatic, loose map sheet titled Helvetia by the Leiden University Library nine years ago led to the discovery of the hitherto unknown wall-map of Switzerland by Mercator.
The author's curiosity was stirred because the work had been assigned to the wrong cartographer in an auction catalogue (Ortelius instead of Mercator) and because of some further peculiar details:
A closer look at the three well-known regional maps of Switzerland in Mercator's atlas, Wiflisburgergau, Aargau and Zürichgau, revealed that they had been drawn at the same scale and together with the missing fourth sheet resulted in a wall-map.
Each sheet bears the author's name, Per Gerardem Mercatorem, and a scale bar which, however, disappear under the adjacent sheet when the map is mounted. Very thin engraved lines on the maps Zürichgau and Aargau indicate how the sheets should be put together and overlap. Asterisks and small crosses in the margins of other maps serve the same purpose.
In the first two parts of his atlas, Gallia and Germania, Mercator applied the dual function of using the map sheets for the atlas as well as for the wall-map no fewer than 21 times. Most of the famous cosmographers and geographers of the 16th century published one or more wall-maps, but Mercator was a specialist in this branch. His intention was to publish maps compiled from many different sources which harmonize as much as possible in both projection and scale.