Accuracy assessment of airborne photogrammetrically derived high- resolution digital elevation models in a high mountain environment Johann Müller a,⇑ , Isabelle Gärtner-Roer a , Patrick Thee b , Christian Ginzler b a University of Zurich, Department of Geography, Winterthurerstr. 190, 8057 Zürich, Switzerland b Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland article info Article history: Received 2 August 2013 Received in revised form 21 July 2014 Accepted 29 September 2014 Keywords: Photogrammetry Accuracy assessment of DEM High resolution airborne photogrammetry High resolution DEM High mountain environment abstract High-resolution digital elevation models (DEMs) generated by airborne remote sensing are frequently used to analyze landform structures (monotemporal) and geomorphological processes (multitemporal) in remote areas or areas of extreme terrain. In order to assess and quantify such structures and processes it is necessary to know the absolute accuracy of the available DEMs. This study assesses the absolute ver- tical accuracy of DEMs generated by the High Resolution Stereo Camera-Airborne (HRSC-A), the Leica Air- borne Digital Sensors 40/80 (ADS40 and ADS80) and the analogue camera system RC30. The study area is located in the Turtmann valley, Valais, Switzerland, a glacially and periglacially formed hanging valley stretching from 2400 m to 3300 m a.s.l. The photogrammetrically derived DEMs are evaluated against geodetic field measurements and an airborne laser scan (ALS). Traditional and robust global and local accuracy measurements are used to describe the vertical quality of the DEMs, which show a non Gaussian distribution of errors. The results show that all four sensor systems produce DEMs with similar accuracy despite their different setups and generations. The ADS40 and ADS80 (both with a ground sampling dis- tance of 0.50 m) generate the most accurate DEMs in complex high mountain areas with a RMSE of 0.8 m and NMAD of 0.6 m They also show the highest accuracy relating to flying height (0.14‰). The push- broom scanning system HRSC-A produces a RMSE of 1.03 m and a NMAD of 0.83 m (0.21‰ accuracy of the flying height and 10 times the ground sampling distance). The analogue camera system RC30 pro- duces DEMs with a vertical accuracy of 1.30 m RMSE and 0.83 m NMAD (0.17‰ accuracy of the flying height and two times the ground sampling distance). It is also shown that the performance of the DEMs strongly depends on the inclination of the terrain. The RMSE of areas up to an inclination <40° is better than 1 m. In more inclined areas the error and outlier occurrence increase for all DEMs. This study shows the level of detail to which airborne stereoscopically derived DEMs can reliably be used in high mountain environments. All four sensor systems perform similarly in flat terrain. Ó 2014 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS). Published by Elsevier B.V. All rights reserved. 1. Introduction High mountain environments have been identified by the Inter- governmental Panel on Climate Change (IPCC) as highly sensitive to climate change (Parry, 2007). Thus, these systems should be carefully monitored in order to observe environmental variability, quantify changes, assess natural hazards and understand mass- transport systems (Kääb, 2002). In particular, the IPCC has recog- nized glacial and periglacial systems as priority climate indicators and therefore, these indicators have received increasing attention from the scientific community (Houghton et al., 2001). Remote sensing is particularly well suited for repeated and rapid observation in remote and inaccessible areas of high moun- tains. Mono- and multitemporal digital elevation models derived from various remote sensing systems are repeatedly used to ana- lyze land surface features and processes (e.g. Debella-Gilo and Kääb, 2012; Gruen and Murai, 2002; Kääb et al., 1997 and Tarolli et al., 2009), numerical and hydrological modeling (e.g. Oppikofer et al., 2011; Storck et al., 1998), topographic and radiometric cor- rection of remote sensing imagery (e.g. Goyal et al., 1998) and the assessment of multiple terrain and geophysical parameters (e.g. Maune, 2007; Li et al., 2005). Typically, two sensor systems are used to generate high resolu- tion DEMs in mountain areas: stereophotogrammetrical sensors that acquire optical stereoscopic imagery (Lane, 2000) and LiDAR http://dx.doi.org/10.1016/j.isprsjprs.2014.09.015 0924-2716/Ó 2014 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS). Published by Elsevier B.V. All rights reserved. ⇑ Corresponding author. ISPRS Journal of Photogrammetry and Remote Sensing 98 (2014) 58–69 Contents lists available at ScienceDirect ISPRS Journal of Photogrammetry and Remote Sensing journal homepage: www.elsevier.com/locate/isprsjprs