The broad reception of Vitruvius in architectural history has especially accounted for the fact that fields of knowledge essential for the understanding of ancient processes of design and planning remain hitherto unconsidered. Although Vitruvius discusses various methods for designing ideal type and modularised architecture the question of mathematical and technical basics for creating a real building is still open, i.e. the practical transformation on the actual building site with all its needs such as architectural surveying and logistics. An as yet widely unsolved problem is which knowledge enabled antique and late antique architects and engineers to provide the rationally comprehensive frame needed to make the theoretical constructions calculatable and plannable buildings.The study of the Hagia Sophia (532-537) and its architects Anthemius of Tralles and Isidore of Miletus leads us to an important source which can fill this gap of knowledge effectively and which proved to be an indispensable basis for understanding ancient architecture in its whole. Late antique sources and primarily the structure of the building itself document that the exceptional achievements of design and planning must be associated with the writings of Heron of Alexandria. From the 1st century AD to the Byzantine period in his name handbooks for engineers of various disciplines were distributed which provided obligatory instruments of calculation with systematically compiled tasks for all groupsof profession engaged in building. Particularly Heron’s scientific discipline of surveying and his treaty on vaults demonstrated to be revised by Isodore, can be assessed as a basis for planning and building. Only if knowing these sources the processes necessary for transforming an ideal plan into a real still existing construction can be reconstructed. TheHagia Sophia therefore is a unique example in which written sources and architectural remains can be analysed and complement each other in a singular way; at the same time it establishes an entirely new model of interpretation for ancient planning praxis.