One of al-Khwarizmi’s greatest works, Kitāb ṣūrat al-Arḍ (translated as “The Book of the Description of the Earth” or “The Image of the Earth”, that is, geography), was written in 833. This book is a revised and expanded version of Ptolemy’s Geography and, after a general introduction, consists of a list of the coordinates of 2,402 cities and other geographical objects.
Only one copy of the book Kitāb ṣūrat al-Arḍ has survived and is today kept at the University of Strasbourg. Its Latin translation is kept in the National Museum of Spain in Madrid. This work also contains the oldest map of the Nile. However, neither the Arabic copy nor the Latin translation includes a world map, but Hubert Daunicht reconstructed the missing map on the basis of the list of coordinates.
One of al-Khwarizmi’s main achievements was correcting Ptolemy’s gross overestimation of the length of the Mediterranean Sea (from the Canary Islands to the eastern shores of the Mediterranean). Whereas Ptolemy overestimated it at 63 degrees of longitude, al-Khwarizmi estimated it almost correctly at nearly 50 degrees of longitude. He also described the Atlantic and Indian Oceans as open bodies of water rather than closed seas as Ptolemy had done.
Al-Khwarizmi placed the prime meridian of the Old World on the eastern shore of the Mediterranean Sea, 10–13 degrees east of Alexandria (the prime meridian previously established by Ptolemy) and 70 degrees west of Baghdad. Most medieval Muslim geographers continued to use al-Khwarizmi’s prime meridian.