In this paper we propose methods for evaluating the geometric accuracy of three-dimensional city models. The approach based on the concept of an error matrix, statistical analysis of height differences and a buffer-overlay-statistic leads to accuracy parameters for an automated city modelling workflow from aerial images footnote The modelling workflow comes from Microsoft Photogrammetrywhich is a research and development unit of Microsoft. It emerged from the merger of Microsoft and Vexcel Imaging GmbH.. We study the theoretical properties of our approach and we show that in practice the concept of the paper behaves very well on real test data from the 3D-model of the inner city of Graz. The work concludes with a summary and an outlook to future work.