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Fig. 1. Map of the study area.

Table 1. The main statistics of the field data; mean, median, standard deviation (std) and maximum (max) of the 2013 and 2017 field data sets.
2013 2017
Variable mean median std max mean median std max
Volume, m3 ha–1 126.3 121.4 81.5 380.8 126.1 113.9 88.6 475.5
Pine volume, m3 ha–1 76.4 66.2 66.3 306.6 74.9 68.0 69.1 366.4
Spruce volume, m3 ha–1 29.4 6.4 51.6 367.8 31.4 6.3 53.5 390.0
Deciduous volume, m3 ha–1 20.5 7.5 32.3 215.6 19.8 4.4 33.8 218.1
DBH, cm 16.3 16.5 6.8 36.2 16.9 17.7 7.6 50.8
Height, m 13.4 14.1 5.1 27.2 13.7 14.9 5.6 25.9
Basal area, m2 ha–1 17.3 17.4 8.7 45.0 16.4 16.3 9.3 50.6
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Fig. 2. Distribution of the species-specific growing stock volume classes in the 2013 and 2017 sample plot data sets.

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Fig. 3. Oblique view of the stereo-photogrammetric point clouds (points are colored with the spectral values of CIR image bands) based on the standard (top) and stereo-oriented (bottom) aerial data sets. Note the lower point density of the standard data visible in the foreground (there is a new clear-cut in the forefront of the point cloud derived from stereo-oriented image data).

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Fig. 4. Scatterplot showing the height at which 80% of canopy points are accumulated (H80) and stand mean height in standard and stereo-oriented data (linear trend line included).

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Fig. 5. The scatterplots presenting the measured values of forest variables as well as the values estimated with standard and stereo-oriented data sets.

Table 2. The accuracy of forest variables (relative RMSE, R2 and relative bias) estimated with standard and stereo-oriented data sets (aerial data only)
RMSE% R2 Bias%
Standard Stereo-oriented Standard Stereo-oriented Standard Stereo-oriented
Volume 26.50 27.44 0.85 0.86 0.10 –0.41
Pine volume 53.07 52.16 0.60 0.65 1.77 5.09
Spruce volume 136.03 112.78 0.39 0.50 –0.17 –10.87
Deciduous volume 104.94 107.75 0.39 0.43 –5.74 –4.62
DBH 21.93 23.16 0.71 0.75 –1.16 –0.08
Height 18.08 19.05 0.78 0.77 –0.99 –0.38
Basal area 23.98 25.46 0.76 0.79 0.56 0.08
Table 3. The accuracy of forest variables (relative RMSE, R2 and relative bias) estimated with aerial data sets combined with contemporary (Landsat/Sentinel) satellite imagery.
RMSE% R2 Bias%
Standard Stereo-oriented Standard Stereo-oriented Standard Stereo-oriented
Volume 25.86 26.95 0.84 0.84 0.29 –1.02
Pine volume 50.71 49.02 0.66 0.73 1.77 2.91
Spruce volume 124.96 98.81 0.50 0.66 –0.59 –6.27
Deciduous volume 101.56 102.74 0.59 0.65 –3.98 –7.59
DBH 22.27 22.18 0.59 0.72 –0.61 –0.05
Height 18.49 17.88 0.76 0.78 –0.95 –0.63
Basal area 23.65 25.64 0.78 0.79 1.39 –0.01
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Fig. 6. Distribution of estimated (est) total and species-specific stand volumes based on standard and stereo-oriented data sets and the respective field measurements.

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Fig. 7. Distribution of the stand mean height estimates based on standard and stereo-oriented data sets and the respective field measurements.