Thus when he found in any of his authorities that the distance between one port and another was 500 stadia, he assumed, in the first place, that this was accurate, and, in the second, that the distance between the two places was equal to a degree of latitude or longitude, as the case might be.
Accordingly he arrived at the result that the breadth of the habitable globe was, as he put it, twelve hours of longitude (corresponding to 180 deg.)--nearly one-third as much again as the real dimensions from Spain to China.

The consequence of this was that the distance from Spain to China _westward_ was correspondingly diminished by sixty degrees (or nearly 4000 miles), and it was this error that ultimately encouraged Columbus to attempt his epoch-making voyage.
Ptolemy's errors of calculation would not have been so extensive but that he adopted a method of measurement which made them accumulative.

If he had chosen Alexandria for the point of departure in measuring longitude, the errors he made when reckoning westward would have been counterbalanced by those reckoning eastward, and would not have resulted in any serious distortion of the truth; but instead of this, he adopted as his point of departure the Fortunatae Insulae, or Canary Islands, and every degree measured to the east of these was one-fifth too great, since he assumed that it was only fifty miles in length.

I may mention that so great has been the influence of Ptolemy on geography, that, up to the middle of the last century, Ferro, in the Canary Islands, was still retained as the zero-point of the meridians of longitude.
Another point in which Ptolemy's system strongly influenced modern opinion was his departure from the previous assumption that the world was surrounded by the ocean, derived from Homer.

Instead of Africa being thus cut through the middle by the ocean, Ptolemy assumed, possibly from vague traditional knowledge, that Africa extended an unknown length to the south, and joined on to an equally unknown continent far to the east, which, in the Latinised versions of his astronomical work, was termed "terra australis incognita," or "the unknown south land." As, by his error with regard to the breadth of the earth, Ptolemy led to Columbus; so, by his mistaken notions as to the "great south land," he prepared the way for the discoveries of Captain Cook.