As Waldseemüller wrote: "All that has been said by way of introduction to the Cosmography will be sufficient, if we merely advise you that in designing the sheets of our world-map we have not followed Ptolemy in every respect, particularly as regards the new lands, where on the marine charts we observe that the equator is placed otherwise than Ptolemy represented it" (Waldseemüller 1907, 78).

What charts did Waldseemüller have for guidance? This question cannot be answered with certainty. No "marine charts" (maps called "portolan charts" which feature coastlines, harbors, reefs and shoals important for mariners) showing the Americas as Waldseemüller does have survived. These portolan charts would have been Portuguese or Spanish in origin, and these were supposed be kept secret, but copies were made of them.

The chart of Nicolo Caveri (Canerio), made about 1505 in Genoa, could have been used for the depiction of the Americas. The Caveri is a large manuscript map on vellum, made in ten sections. The center of the map is placed in Africa. From the center two circles are drawn with divisions and connecting lines that form the grid of the map (see portolan chart commentary). However, the Red Sea on this chart is oriented almost east-to-west, while the Waldseemüller map shows it with a strong north-to-south orientation. Also, India shown on the Caveri map has a nearly correct shape, while it is truncated on the Waldseemüller map.