The bistatic acquisition mode of the TanDEM-X radar satellite mission provides a new reliable source for generating high-resolution (~10m) digital elevation models (DEMs) of active volcanoes. For high relief-mountainous areas with steep slopes - like stratovolcanoes - geometric distortions considerably affect the accuracy of the DEMs. The TanDEM-X system allow for the first time access to high-resolution DEMs for both orbits (ascending and descending) which, by using an adaptive approach to combine multiple acquisitions from different viewing angles, considerably increase the accuracy of the DEM over difficult terrain like active volcanoes. By integrating these DEMs into standard numerical flow modeling, we are able to enhance the performance of standard terrain-dependent volcanic flow models and reduce the uncertainty that inevitably arises in application of topographic models of varying resolutions. Our study pushes the limits on where and how SAR data can be used to derive accurate DEMs under extreme conditions.