Astronomically determined localities, the core part of Ptolemy's GeographyThe paper deals with ancient methods of astronavigation and their potential use in finding geographical locations in Ptolemy’s Geographike Hyphegesis. Further, it describes the systematic errors in these methods and suggests how to correct them. The results include a new map which compares the locations of Ptolemy’s sites after removing the errors with their real locations. Subsequently, significant ancient locations according to Ptolemy’s list of noteworthy cities are identified on the map. In some cases, we presume that they were located on the map using astronavigation. The results of this Study imply that Ptolemy may have adopted a comparatively regular network of points from some older authors which was the basis of his extensive work.