# Detection of Forest Tree Losses in Cote d'Ivoire Using Drone Aerial Images
## 3 Results
### Orthomosaics, Digital Surface Models, and Digital Terrain Models  
The orthomosaics obtained at each of the study sites after photogrammetric processing (Figure 8A) were exported from the Photoscan software with a spatial resolution of 6 cm/pixel. The digital surface models (Figure 8B,C) were exported with a resolution of 12 cm/pixel. Ecologically, these orthomosaics and DSMs provide information on the state of vegetation (degraded or not, heavily cleared or not, cultivated or not, etc.). At Site 1, elevations varying from 145 m to 227 m in November 2018 and from 142 m to 229 m in April 2019 show the anthropization of the forest landscape. This degradation of the forest landscape can be seen on orthomosaics through both blue and green areas corresponding to forest clearing and cultivation. These areas are more accentuated in the western and central parts of Site 1.  
In Site 6, these clearings are more accentuated in the northeastern and southwestern areas. In Site 8, analysis shows that the forest is less degraded compared with other sites. Slight degradation is observed in the southeastern extremity of the site. Finally, Sites 9 and 11 are the most degraded sites with significant vegetation clearing.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & \multicolumn{3}{c}{**November 2018**} & \multicolumn{3}{c}{**April 2019**} \\ \cline{2-7}  & **Quality** & **A** & **E** & **Quality** & **A** & **E** \\ \hline Site 1 & Good & 100 & 0.580 & Good & 97 & 0.551 \\ Site 2 & Good & 100 & 0.718 & Poor & 100 & 0.739 \\ Site 3 & Poor & 100 & 0.738 & Good & 100 & 0.717 \\ Site 4 & Poor & 69 & 0.636 & Poor & 73 & 0.646 \\ Site 5 & Good & 95 & 0.689 & Poor & 59 & 0.653 \\ Site 6 & Good & 100 & 0.799 & Good & 100 & 0.625 \\ Site 7 & Good & 97 & 0.626 & Poor & 75 & 0.622 \\ Site 8 & Good & 100 & 0.648 & Good & 100 & 0.649 \\ Site 9 & Good & 98 & 0.655 & Good & 98 & 0.599 \\ Site 10 & Poor & 100 & 0.680 & Good & 100 & 0.649 \\ Site 11 & Good & 100 & 0.614 & Good & 100 & 0.615 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Evaluation of photogrammetric processing (A: percentage of aerial images correctly aligned; E: aerial image reprojection errors in pixels).  
Figure 8: Orthomosaics (**A**) and 3D (**B**) and 2D (**C**) DSMs resulting from photogrammetric processing of flights at Sites 1, 6, 8, 9, and 11 in November 2018 and April 2019.  
The DTM (Figure 9) allows for a better appreciation of the topography. It indicates a topography that varies from 142 m to 179 m in Site 1 (87 ha), representing a difference in level of 37 m. In Site 6 (94 ha), the topography varies from 136 m to 190 m, representing a difference in level of 54 m. On Site 8 (97 ha), the topography varies from 167 m to 222 m, representing a difference in level of 55 m. On Site 9 (102 ha), the topography varies from 131 m to 181 m, representing a difference in level of 50 m. Finally, Site 11, with a surface area of 121 ha, has a topography varying from 145 m to 199 m, representing a difference in level of 54 m.  
To ensure the quality of the digital terrain models, we compared the DTMs generated from the two different dates selected in the study. As an example, Figure 10 shows elevation profiles that show that the DTMs are comparable from one date to another. However, there is an error in z. Since we subsequently computed the digital elevation model from the DSM and DTM, the effect of this error is significantly mitigated [37].  
Figure 10: Comparison of altitudinal profiles corresponding to November 2018 (blue profile) and April 2019 (red profile) at Site 1 (**left**) and Site 9 (**right**).  
Figure 9: DTMs (contour lines are shown in pink) obtained at each of the study sites selected for the analyses.