The height profile used in the Orbis Romanus Atlas is based on the 30 arc second resolution data set GTOPO30 published by the US Geological Survey.
The hydrographic system used in the Orbis Romanus Atlas is based on the CIA WorldMap database (see e.g. ftp://ftp.uu.net/graphics/maps/WorldMap.tar.Z-split/ though this location might change since the database is not officially published.)
Additional position information was taken from the Index of the Times Atlas of the World, Comprehensive 5th edt., Times Books, London, 1975.
The projection used in the Atlas is a Conical Lambert projection with fixed projection of the latitudes 40 deg and 50 deg North. The same projection is used in the Tabula Imperii Byzantini (see F.Kelnhofer, Die topographische Bezugsgrundlage der TIB, Wien 1976)
The first edition of the Orbis Romanus Atlas is based on published sheets of the Tabula Imperii Romani (TIR) published under the auspices of the Union Academique International since 1932. See R.A. Talbert, Mapping the Classical World, J.Roman Arch.,5,p5-38,1992, for a comprehensive account of the history of the TIR and other classical mapping projects.
Later editions will include material from multiple archeological sources and hopefully from user contributions.
The first edition of the Atlas maps are generated by a set of programs written in the Interactive Data Language (IDL) which is a commercial graphics package produced by RSI Inc. The IDL code of these programs will be published soon. It can easily be translated into any other programming language with high level graphics support.