## 5 Conclusions  
This study addresses the potential and limitations of both Pleiades and WorldView-3 tri-stereo reconstructed DSMs for height estimation of single objects. Satellite image orientation and dense image matching were performed. The tri-stereo images of each sensor were acquired in less than one minute, resulting in similar radiometric characteristics, a fact that is beneficial for any image matching algorithm. For obtaining higher accuracies (at sub-pixel level in image space and sub-meter level in object space) the direct sensor orientation available as RPCs is improved by GCPs through a bias-compensated RPC block adjustment. Evaluated at 50 CPs in open areas, elevation accuracies of 0.85 m and 0.28 m (RMSE) were obtained for Pleiades and WorldView-3, respectively. Next, photogrammetric DSMs were generated from the Pleiades and WorldView-3 tri-stereo imagery using a dense image matching algorithm, forward intersections, and interpolation. The estimation of a single object's height depends on the accuracy of satellite image orientation, but also on the elevation accuracy of the derived DSMs and on the LiDAR reference DTM. We found that the vertical accuracy of the reference DTM w.r.t the RTK GCPs is 0.12 m, and the accuracies of the reconstructed photogrammetric models with respect to the DTM are 0.32 m for Pleiades and 0.20 m for WorldView-3 in open, smooth areas. The vertical accuracy of the DSMs from VHR satellite imagery is influenced by the acquisition geometry. The Pleiades scenes having a narrow convergence angle of 12\({}^{\circ}\) show a lower vertical performance compared to WorldView-3 scenes, which have a 23\({}^{\circ}\) convergence angle on the ground. In addition, the smoothness effect of the robust moving planes interpolation for deriving the DSMs is at sub-decimetre level. Under these conditions, the resulting height accuracy is reasonable and does not significantly influence our investigations, since the single objects measured are usually higher than 1.5 m. Based on the different GSDs and B/H ratios of the two satellite systems, it can be said that \(\sigma_{o}\) of block adjustment, RMSE of check point elevation error and height reconstruction success for individual objects are similar, with deviations of up to 20%. Figure 10: Height estimation of buildings: (**a**) for Pleiades; (**b**) for WorldView-3. In order to investigate the potential of Pleiades and WorldView-3 tri-stereo DSMs for single object height estimation, 50 vehicles, 100 trees, and 100 buildings were analyzed. Vehicle length/width, tree crown diameter and building length/width were identified as the main geometric parameters for classifying single objects into different groups. The results clearly show a gradual increase of the estimated heights with object dimensions. Starting from very small objects (whose heights are not reconstructed) the values for the height reconstruction percentage reached almost 100% for very large single objects (e.g., buildings with lengths \(>\)35 m in the case of WorldView-3 DSM). Hence, individual objects' reconstructed heights strongly depend on object type, size, and the sensor being used. As an example, if we consider the \(p=50\%\) threshold for an appropriate height reconstruction, then we obtain the following minimum size values for the single objects: 10 m (Pleiades) and 5 m (WorldView-3) for vehicle lengths; 5 m (Pleiades) and 2.5 m (WorldView-3) for tree crown diameters and 7.5 m \(\times\) 6 m (Pleiades) and 3.5 m \(\times\) 2.5 m (WorldView-3) for building length/width. The ratio of these dimensions fits well to the GSD ratio of 70 cm to 31 cm. Generally, for all single objects except deciduous trees, the results achieved with WorldView-3 data were better than those achieved by using the Pleiades data. Hence, the ability of WorldView-3-derived DSM to perform height estimation of single objects is higher compared with Pleiades DSM.