The invention relates to providing atmospheric correction data in a GNSS network-RTK system for correcting GNSS data, wherein a base triangulation that encloses at least part of the reference stations of the GNSS network-RTK system is subdivided into child triangles by means of a recursive division of parent triangles into four child triangles, synthetic data are determined for each of the child triangles based on a triangulation algorithm applied to basic data of the reference stations such that the synthetic data represent a gridded representation of the basic data, and access to correction data is provided, wherein the correction data comprise at least part of the synthetic data arranged in a quad-tree hierarchy.