The “Koralm tunnel basic network” had to be prepared for the design, construction, and construction supervision of the ÖBB railway project “Koralmbahn Graz-Klagenfurt” with the 32.8 km long Koralm tunnel as its core piece. This network is based on the “primary network” which is integrated into a network of GPS permanent stations and was or will be complemented as required. Since the project area covers two meridian zones of the national system, it was necessary to create a projection in a separate meridian zone for planar calculations. The paper describes the preparation, surveying and calculation of this network. In addition, it includes procedure instructions for the use of the network by planners and construction companies. Since the network has to be adapted to each construction phase, it has to be continuously developed. This will be described too. All data are managed by means of an ArcGIS server.