The Fixing of RTK-GNSS satellite positioning depends on the satellites' position, number of visible satellites, signal quality and positioning algorithm. The satellites' position can be calculated from the navigation messages, while the number of visible satellites and signal quality can be predicted using simulations taking into consideration the multipath from surrounding buildings, antenna and receiver performance. In this paper, the signal quality is predicted using simulations with 3D buildings and compared with measured data. Furthermore, simulated and measured RTK-GNSS positioning from signal quality are compared to evaluate the possibility of using 3D buildings to predict the Fixing status of RTK-GNSS Positioning. The evaluation gives reasonable comparable results.