## Methods
### TanDEM-X DEM Generation  
The DEM from TanDEM-X data is preferred when the data is acquired in a bistatic mode. Therefore, most of the TanDEM-X DEMs generated in this study belong to the period 2011-2014 as the TanDEM-X mission was operated in pursuit monostatic mode from October 2014 to February 2015. The TanDEM-X mission was set back to bistatic mode afterwards but with large spatial baselines (e.g., 700 m in some cases). With such large baselines radar interferometer loses its sensitivity to map elevation. Therefore, we could only process two bistatic scenes of 2015-2016 which had comparably short baselines (100-150 m).  
We applied an interferometric processing approach in order to generate TanDEM-X DEMs [26]. The unwrapped phase (\(\Phi\)) of the complex interferogram is related to the absolute height (\(h\)), while neglecting the phase related to motion, temporal decorrelation and atmospheric disturbances as can be assumed for bistatic interferometry [20], as  
\[\Phi=\frac{2\pi B_{per}h}{\lambda Rsin\theta}, \tag{2}\]  
where (\(B_{per}\)) is the perpendicular baseline, (\(\lambda\)) is the wavelength (3.1 cm for X-band), (\(R\)) is the slant range distance between sensor and the earth's surface and (\(\theta\)) is the incidence angle. The simulation of two interferograms with 100 m height difference was chosen to precisely estimate the phase-to-height sensitivity and to account for its spatial variability depending on the sensor geometry. First we simulated the phase (\(\Phi_{0.1}\)) with 0.1 m height in radar geometry and subtracted it from the complex interferometric phase to generate differential interferogram (\(\Phi_{d}\)) as  
\[\Phi_{d}=\Phi-(\Phi_{0.1}). \tag{3}\]  
The phase noise associated with the differential interferogram was removed by a Goldstein filter with a filtering coefficient (\(\alpha_{filter}\)) of 0.35 (\(\alpha_{filter}\) \(\epsilon\) [0,1]). The areas of low coherence (\(<\)0.3), which mainly originate from layover, foreshortening, shadow [27] and water bodies, were masked out before unwrapping the differential phase with a Minimum Cost Flow (MCF) algorithm. The starting point of the phase unwrapping, referred to as the phase reference point, was taken on a stable and flat surface. The differential phase of this point was manually kept at 0.0 radians before phase unwrapping. We estimated the phase-to-height sensitivity (\(\frac{\delta\Phi}{\delta h}\) in radians m\({}^{-1}\)) by differencing the simulated phase for 100.1 m (\(\Phi_{100.1}\)) and 0.1 m (\(\Phi_{0.1}\)) in radar geometry, as calculated in Equation (4)  
\[\frac{\Phi_{100.1}}{100}-\frac{\Phi_{0.1}}{100}=\frac{\delta\Phi}{\delta h}( rad/m). \tag{4}\]  
The unwrapped phase of the TDX differential interferogram was converted into elevation using the phase-to-height sensitivity. The phase to height conversion resulted in 0.0 m elevation of the phase reference point. This conversion also estimated the elevation of every pixel in the TanDEM-X scene relative to the phase reference point. The absolute elevation of this point (\(h_{ref}\)) was extracted from the reference DEM of 1957 and added to the relative DEMs. The resulting absolute elevation maps were geocoded from radar geometry using the DEM of 1957 and then in an iterative step with its own TDX elevations. The DEMs were generated with a spatial resolution of 10 m \(\times\) 10 m.