Geography consisted of eight volumes. The first discussed the problems of representing a spherical earth on a flat sheet of paper (remember, ancient Greek and Roman scholars knew the earth was round) and provided information about map projections. The second through seventh volumes of the work were a gazetteer of sorts, as a collection of eight thousand places around the world. This gazetteer was remarkable for Ptolemy invented latitude and longitude - he was the first to place a grid system on a map and use the same grid system for the entire planet. His collection of place names and their coordinates reveals the geographic knowledge of the Roman empire in the second century.
The final volume of Geography was Ptolemy's atlas - featuring maps that utilized his grid system and maps that placed north at the top of the map, a cartographic convention that Ptolemy created. Unfortunately, his gazetteer and maps contained a great number of errors due to the simple fact that Ptolemy was forced to rely upon the best estimates of merchant travelers (who were incapable of accurately measuring longitude at the time).
Like much knowledge of the ancient era, the awesome work of Ptolemy was lost for over a thousand years after it was first published. Finally, in the early fifteenth century his work was rediscovered and translated into Latin, the language of the educated populace. Geography gained rapid popularity and there were more than forty editions printed from the fifteenth through sixteenth centuries. For hundreds of years, unscrupulous cartographers of the middle ages printed a variety of atlases with the name Ptolemy on them, to provide credentials for their books.
Ptolemy erroneously assumed a short circumference of the earth, which ended up influencing Christopher Columbus that he could reach Asia by sailing west from Europe. Additionally, Ptolemy showed the Indian Ocean as a large inland sea, bordered on the south by Terra Incognita (unknown land). The idea of a large southern continent sparked countless expeditions.
Geography had a profound a profound effect on the geographical understanding of the world in the Renaissance and it was fortunate that its knowledge was rediscovered to help establish geographical concepts that we almost take for granted today.
(Note that the scholar Ptolemy is not the same as the Ptolemy who governed Egypt and lived from 372-283 BCE. Ptolemy was a common name.)