## 1 Introduction  
For more than thirty years, civilian satellite sensors have been used for digital elevation model (DEM) extraction over large areas in a timely and cost-effective manner for a wide range of applications in engineering, land planning, and environmental management. Beginning with the year 1986, when SPOT--the first satellite providing stereo-images, with a panchromatic Ground Sampling Distance (GSD) of 10 m--was launched [1], the optical satellite industry has been experiencing continuous development. Today more and more space sensors are available that acquire not only stereo but also tri-stereo satellite imagery. The generation of high and very high resolution commercial space imaging systems for DEM generation started in September 1999, with the launch of IKONOS [2]. Among the Very High Resolution (VHR) optical satellites providing along- and across-track stereo,the following systems can be mentioned: Ziyuan-3 (2.1 m), KOMPSAT-2 (1 m), Gaofen-2 (0.8 m), TripleSat (0.8 m), EROS B (0.7 m), KOMPSAT-3 (0.7 m), Pleiades 1A/1B (0.7 m), SuperView 1-4 (0.5 m), GeoEye-1 (0.46 m), WorldView-1/2 (0.46 m) and WorldView 3 (0.31 m). The new generation of Earth observation satellites are characterized by an increased acquisition capacity and the possibility of collecting multiple images of the same area from different viewing angles during a single pass [3; 4]. This multi-view aspect is essential for extracting 3D information. In recent years, the potential of tri-stereo acquisition from high-resolution satellite images for surface modelling has become an interesting research topic that has been addressed in various publications. The capacity of the Pleiades system in performing 3D mapping was analyzed by [PERSON] et al. [5], where 17 images acquired from different points of view were used. They showed that by means of \"triple stereo\" configurations reliable digital surface models can be generated in urban areas. From their best image combination, a root mean square error (RMSE) of 0.49 m was obtained at 295 ground control points (GCPs). The radiometric and geometric characteristics of Pleiades imagery with a focus on digital surface modelling are analyzed by [PERSON] et al. [6]. The model derived from a \"triple stereo\" scene (nadir, forward and backward) showed median values close to zero and an RMSE in the range of 6-7 m, when compared with a reference light detection and ranging (LiDAR) DSM. An accurate 3D map with tri-stereo images can be obtained by optimizing the sensor models with GCPs, leading to accuracies in the range of 0.5 m in planimetry and of 1 m in height as demonstrated in [7]. Much of the previous research using WorldView-3 satellite images focused on their high resolution multi-spectral information, with applications in topographic mapping, land planning, land use, land cover classification, feature extraction, change detection, and so on [8; 9; 10; 11]. The 3D potential of WorldView-3 data is assessed by [PERSON] et al. [12], where the reconstructed DEM shows height differences of less than 0.5 m for 82.7% of 7256 ground LiDAR checkpoints located along road axes. A new algorithm for generating high quality digital surface models is proposed by [PERSON] et al. [13], where a dense image matching method is applied for multi-view satellite images from Pleiades and WorldView-3. The capability of satellite images regarding a rapid evaluation of urban environments is addressed in [PERSON] et al. [14]. They compared 3D data extracted from tri-stereo and dual stereo satellite images combined with pixel-based matching and Wallis filter to improve the accuracy of 3D models, especially in urban areas. The result showed that 3D models achieved by Pleiades tri-stereo outperformed the result obtained from a GeoEye pair, in terms of both accuracy and detail. This could mean that tri-stereo images can be successfully used for urban change analyses.