In the early period of European exploration, captains had rudimentary navigation techniques and a fairly detailed set of Mediterranean portolanos (charts) showing the bodies of water, landmasses, and ports. These charts were not drawn to a grid system of degrees, but were based on compass findings and estimated distances. From this point of view, cartography was very similar to early astronomy in that the product was paradoxically the result of both scientific and metaphysical endeavours. Some astronomers were attracted to the study of stars because of their interest in astrology. The same is true of cartographers whose main purpose was not to discover what actually existed but, rather, to rationalise the world around preconceived notions of religion and philosophy.

Nevertheless, it is possible to make a few general statements about the practice of cartography. If it is possible to identify one great influence, it would undoubtedly be that of Claudius Ptolemaeus, or Ptolemy, (130 C.E.) and the Arab geographers who followed. In 151 C.E., Ptolemy published a work on map-making called Geographia. The world, according to him, stretched from Iceland and the Canary Islands in the west, to Ceylon in the east, with a mass of unknown lands south of North Africa and beyond India. Moreover, Ptolemy believed that Africa connected with an undiscovered southern landmass. If this were true, it would have been impossible to reach Asia by rounding southern Africa by ship. Nevertheless, for centuries, Ptolemy's ideas remained the basis for cartographers and geographers alike, with a few important modifications and contributions from later Arabic scholars. The significant change to Ptolemy's conception of the world came in the tenth century when, among other changes, Massoudy, an Arabic scholar, suggested that a channel existed between southern Africa and the unknown land mass around the southern extremities of the world. In the twelfth century, Massoudy's work was continued by Edrisi, an Arab geographer in the service of King Roger of Sicily. Edirsi's main contribution was to record the travels of his contemporaries in both the Christian and Muslim worlds and to include those discoveries in the maps he created.

From this intellectual heritage sprang the twin tendencies of medieval European geographers and cartographers to create maps that were, generally speaking, either fanciful or factual. On the one hand, there was a tendency to construct a symmetrically perfect design of God's creation that also incorporated the metaphysical beliefs of Europe. For example, the Psalter Map of the thirteenth century depicted a world with Christ at the top and dragons crushed beneath him at the bottom. Furthermore, in the Psalter Map, Jerusalem was the centre of the world, and legends and myths were inscribed in places instead of geographical data, thus effectively blending fact and fantasy.

On the other hand, there were attempts to accurately represent the information and knowledge gathered by explorers. Perhaps this is best illustrated by the early fourteenth century Venetian map of Marino Sanuto which attempted to use Italian ideas about the Atlantic coastline of Africa as the basis for its projections. Sanuto's map also showed a sea route to the south of Africa, but this was a conjectured, rather than proven, route. The most important contribution of Prince Henry to the theoretical side of exploration was his willingness to focus attention on the more "realistic" maps and the first-hand accounts of the travellers, rather than on the symmetrical and theological aspects demonstrated in the Psalter Map.

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