Supporting Assessment of Forest Burned Areas by Aerial Photogrammetry: The Susa Valley (NW Italy) Fires of Autumn 2017

In October 2017, a large wildfire occurred in the Susa valley (Italian Western Alps) affecting wide areas of mixed forests (Pinus sylvetris L.; Fagus sylvatica L., Quercus pubescens Willd.) with a spot pattern. Few days after the event an aerial survey operated by an RGB camera Sony ILCE-7RM2-a7R II was done with the aim of testing a digital photogrammetry-based 3D rapid mapping of fire effects. Flight altitude was about 800 m above ground level (AGL) determining an average image GSD of about 0.2 m. Image block adjustment was performed in Agisoft PhotoScan vs 1.2.4 using 18 ground control points that were recognized over true color orthoimages (GSD = 0.4 m). Height values of GCPs were obtained from a 5 m grid size DTM. Both orthoimages and DTM were obtained for free from the Piemonte Region Cartographic Office (ICE dataset 2010). A point cloud, having an average density of 7 pt/m2 and covering 14 km2 was generated, filtered and regularized to generate the correspondent DSM (Digital Surface Model) with a grid size of 0.5 m. With reference to the above-mentioned ICE DTM, a Canopy Height Model (CHM) was generated by grid differencing with a grid size of 0.5 m. A true-orthoimage was also generated having a GSD of 0.5 m. The latter was used to map burned areas by a pixel based unsupervised classification approach operating with reference to the pseudo GNDVI image, previously computed from the native red and green bands (no radiometric calibration was applied aimed at converting back the raw digital numbers to reflectance). Results were compared with 2 official datasets that were generated after the event from satellite data, one produced by the Piemonte Region and the other one by the Copernicus Emergency System. In order to test differences between burned and not-burned areas, point density, point spacing and canopy heights were computed and compared looking for evidences of geometrical differences possibly characterizing burned areas in respect of the not burned ones. Results showed that no significant differences were found between point density and point spacing in burned and not burned area. There was a significant difference in CHM minimum values distribution between burned and not-burned areas while maximum values distribution does not change significantly, proving that fire change crown structure but tree height remain unchanged. These results suggest that aerial photogrammetry could detect fire effect on forest having higher accuracy respect to ordinary approaches used in forest disturbance ecology.