## 5 Conclusions
This is mainly because of the very high resolution (0.31 m) that leads (through photogrammetric processing) to very dense 3D point clouds, describing the real ground surface and the objects on it more accurately. The acquisition times also play a significant role in the photogrammetric procedure. For stable objects such as buildings, the different periods of acquisition do not affect the 3D reconstruction, but for deciduous trees, the automatic reconstructed heights are highly underestimated in the leaf-off season. Their leafless appearance in the images brings difficulties to the image matching process when finding correct correspondences for tree branches. From this observation, we can conclude that the image appearance and texture of trees (and other single objects) are also very important for the 3D reconstruction and height estimation. For all objects investigated, the resulting histograms (Figures 5, 6 and 9) show interesting trends, opening new investigation options for refinement such as adding the influence of object orientation, color, and texture in the feature reconstruction ability of heights, but these are outside the scope of the present research. The poor performance on small individual objects is mainly caused by the continuity constraint of dense matching algorithms. A topic for future research would be to investigate if a combination of different matching strategies can deliver better results on single object heights. In our investigation, the current findings are valid for Pleiades and WorldView-3 DSMs derived from the tri-stereo images with their specific acquisition geometry. Nevertheless, the acquisition geometry could be changed, but this is only to a very small extend under the control of the user. A higher B/H ratio and implicit convergence angle to the ground will increase the geometric intersection quality, leading to a higher vertical accuracy of the photogrammetric DSM. This will have a positive effect on the single object height signature in the DSM. From the analyses and investigations performed, the results suggest that the acquisition geometry clearly has an effect on accuracy, but the object height estimation based on automatically derived DSMs primarily depends on object size. Objects of a few pixel in size are hardly mapped in the DSM, whereas, with some dependency on object type, objects of 12 pixels in size are typically mapped with approximately 50% of their real height. To reach 90% of the real height, a minimum object size between 15 and 30 pixel is necessary. Conceptualization, A.-M.L., J.O.-S. and N.P. ; Formal analysis, A.L. ; Investigation, A.-M.L. ; Methodology, A.-M.L., J.O.-S. and N.P. ; software, J.O.-S.; Project administration, J.O.-S.; Software, J.O.-S.; Supervision, N.P. and J.O.-S.; Validation, A.-M.L., J.O.-S. and N.P. ; Visualization, A.-M.L. ; Resources, J.O.-S. and N.P. ; Data curation, A.-M.L., J.O.-S. and N.P. ; Writing--original draft preparation, A.-M.L. ; Writing--review and editing, J.O.-S. and N.P. ; funding acquisition, N.P. All authors have read and agreed to the published version of the manuscript. **Funding:** The work was partly funded by the Austrian Research Promotion Agency (FFG), Vienna, Austria within the research project \"3D Reconstruction and Classification from Very High Resolution Satellite Imagery (ReKlaSat 3D)\" (grant agreement No. 859792). Open Access Funding by TU Wien. **Acknowledgments:** The authors acknowledge Open Access Funding by TU Wien. We would like to thank the Institut fur Militarisches Geowesen ([PERSON] and [PERSON]) for support in satellite image acquisition and the company Vermessung Schmid ZT GmbH ([PERSON]) for providing the RTK ground control point coordinates used in this study. **Conflicts of Interest:** The authors declare no conflict of interest.