Writings about projections on maps.Ptolemy used a grid to set places on his maps.Ptolemy thought the Indian Ocean was enclosed by land.

Ptolemy emphasizes the importance of spatial relationships in maps.

He writes that some mapmakers "give the greatest part of their map to Europe in latitude and longitude because the sites and places to be inserted therein are more numerous. They leave very little space for Asia in longitude, and for Africa in latitude, on account of an erroneous consideration of the relative size of the countries, and therefore misplace the Indian sea to the north of Taprobana, and in the same map give intimation of an empty space toward the east, and have nothing to describe toward the north, and extend the western ocean to the eastern shore" (Ptolemy 1932, 165). One error in the Ptolemy maps is that Taprobana (Sri Lanka) is shown about fifteen times larger than its actual size.

Ptolemy notes another problem that plagued the mapmakers of his time when he warns that "Recently the making of new copies from earlier copies has had the result of increasing the faults that were originally small into great discrepancies" (Ptolemy 1932, 38). This same problem persisted until the invention of printing in the mid-15th century made it possible to duplicate maps exactly. Once a map was engraved it could be copied as often as needed, provided that the wood block or the copperplate from which the map was printed was in condition to be used. Printing would spread the geographical thought of Ptolemy far and wide beginning in the fifteenth century.

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