This work focuses on the accuracy estimation of canopy height models (CHMs) derived from image matching of Pleiades stereo imagery over forested mountain areas. To determine the height above ground and hence canopy height in forest areas, we use normalised digital surface models (nDSMs), computed as the differences between external high-resolution digital terrain models (DTMs) and digital surface models (DSMs) from Pleiades image matching. With the overall goal of testing the operational feasibility of Pleiades images for forest monitoring over mountain areas, two questions guide this work whose answers can help in identifying the optimal acquisition planning to derive CHMs. Specifically, we want to assess (1) the benefit of using tri-stereo images instead of stereo pairs, and (2) the impact of different viewing angles and topography. To answer the first question, we acquired new Pleiades data over a study site in Canton Ticino (Switzerland), and we compare the accuracies of CHMs from Pleiades tri-stereo and from each stereo pair combination. We perform the investigation on different viewing angles over a study area near Ljubljana (Slovenia), where three stereo pairs were acquired at one-day offsets. We focus the analyses on open stable and on tree covered areas. To evaluate the accuracy of Pleiades CHMs, we use CHMs from aerial image matching and airborne laser scanning as reference for the Ticino and Ljubljana study areas, respectively. For the two study areas, the statistics of the nDSMs in stable areas show median values close to the expected value of zero. The smallest standard deviation based on the median of absolute differences (sigma(MAD)) was 0.80 m for the forward-backward image pair in Ticino and 0.29 m in Ljubljana for the stereo images with the smallest absolute across-track angle (-5.3 degrees). The differences between the highest accuracy Pleiades CHMs and their reference CHMs show a median of 0.02 m in Ticino with a sigma(MAD) of 1.90 m and in Ljubljana a median of 0.32 m with a sigma(MAD) of 3.79 m. The discrepancies between these results are most likely attributed to differences in forest structure, particularly tree height, density, and forest gaps. Furthermore, it should be taken into account that temporal vegetational changes between the Pleiades and reference data acquisitions introduce additional, spurious CHM differences. Overall, for narrow forward-backward angle of convergence (12 degrees) and based on the used software and workflow to generate the nDSMs from Pleiades images, the results show that the differences between tri-stereo and stereo matching are rather small in terms of accuracy and completeness of the CHM/nDSMs. Therefore, a small angle of convergence does not constitute a major limiting factor. More relevant is the impact of a large across-track angle (19 degrees), which considerably reduces the quality of Pleiades CHMs/nDSMs.
