With the compass were used the sea charts, which were simply maps on a rather larger and more exact scale than the land maps of the period.

There were no soundings or currents marked on the old charts, which were drawn on a plane projection; and they can have been of little--practical use to navigators except in the case of coasts which were elaborately charted on a large scale.

The chart of Columbus, in so far as it was concerned with the ocean westward of the Azores, can of course have contained nothing except the conjectured islands or lands which he hoped to find; possibly the land seen by the shipwrecked pilot may have been marked on it, and his failure to find that land may have been the reason why, as we shall see, he changed his course to the southward on the 7th of October.

It must be remembered that Columbus's conception of the world was that of the Portuguese Mappemonde of 1490, a sketch of which is here reproduced.
This conception of the world excluded the Pacific Ocean and the continent of North and South America, and made it reasonable to suppose that any one who sailed westward long enough from Spain would ultimately reach Cathay and the Indies.

Behaim's globe, which was completed in the year 1492, represented the farthest point that geographical knowledge had reached previous to the discoveries of Columbus, and on it is shown the island of Cipango or Japan.
By far the most important element in the navigation of Columbus, in so far as estimating his position was concerned, was what is known as "dead-reckoning" that is to say, the computation of the distance travelled by the ship through the water.