Our paper reports about temporal changes of the Earth's topography found in first samples of TanDEM-X data when compared to SRTM Digital Elevation Models collected ten years earlier. We give an accuracy prediction and explain the methods to avoid systematic errors from biases and phase unwrapping ambiguities. Our paper provides examples from different Earth surface domains such as glaciers, volcanoes, mining activity and urban areas. We demonstrate and conclude that the higher the resolution of a DEM gets, the more frequent updates are required to keep the DEM valid with time.