# Detection of Forest Windstorm Damages with Multitemporal SAR Data--A Case Study: Finland
## 1 Introduction
### Windstorm Damage Studies with SAR  
Several windstorm studies with SAR are shortly reviewed in this section, as well as studies using airborne SAR instruments. It is expected that the number of SAR-based studies will increase with the increasing data availability. [PERSON] [18] investigated the sensitivity of SAR backscatter to forest windstorm damage gaps using multi-polarization C, L and P band data acquired by the NASA/JPL AIRSAR in August 1991. The study showed that changes in backscatter due to the presence of windstorm damage gaps were evident in each polarization channel used, especially with C band HH polarization in a coniferous plantation. It is suggested that backscatter is sensitive not only to the presence but also to the shape and geometry of the windthrow gaps. [PERSON] et al. [19] used ESA's C-band ERS-1/ERS-2 interferometric image pairs and found them to be effective in differentiating between damaged and undamaged forests when the damaged areas were larger than or equal to 2-3 ha. The damage happened in Jura mountain in France in December 1999. Ready software by ESA made fast data processing possible. [PERSON] et al. [3] studied the potential of CARABAS-II long wavelength SAR imagery for high spatial resolution mapping of windstorm damage forests. The results of this research show that the backscattering amplitude, at a given stem volume, is considerably higher for windstorm damage thrown forests than for unaffected forests. In addition, the backscattering from fully harvested storm-damaged areas was, as expected, significantly lower than from unaffected stands. These findings imply that VHF SAR imagery has potential for mapping windthrown forests. Another study by [PERSON] et al. [4] investigated simulated wind-thrown forest mapping (controlled experiment with filling of trees) using multitemporal ALOS PALSAR (L-band), RADARSAT-2 (C-band) and TerraSAR-X (X-band) imagery. The detection methodology was based on bitemporal change detection and visual interpretation of scenes acquired before and after a simulated windthrow event. Stripmap ALOS PALSAR images were found less suitable for a damage area detection, likely due to a coarse spatial resolution. The windthrown forests were well visible when the RADARSAT-2 and TerraSAR-X HH polarization images were used. [PERSON] et al. [20] used space- and airborne SAR data to map windthrown forests in southern Sweden. Analysis of the Space- and Airborne C-band SAR images including Envisat and Radarsat showed that they are unable to detect forest storm damage. The CARABAS VHF-band SAR, on the other hand, showed that these data can detect most storm-damaged forests as well as power lines, and sometimes better than the aerial photographs. [PERSON] et al. [21] used TerraSAR-X data and focused first on the border line extraction of forest areas to enables a fast estimation of windthrown areas, whereby the pre-event forest border is derived from multi-spectral data. Second, clean-up operations were monitored in the affected forest areas by applying a change detection operator. They presented a method to extract the border of forest areas by fusing multi-aspect SAR images. They found that this extraction of multi-temporal changes and displacements of the forest border enables a rapid damage estimation, which is very useful to plan first clean-up operations. In addition, their intensity-based change detection showed good results to highlight small areas especially with hard to analyze data, even for human operators. [PERSON] et al. [6] showed that, when trees are felled, the backscattered signal from TerraSAR-X (X-band) increases by about 1.5 dB, while for ALOS PALSAR (L-band) a decrease with the same amount is observed. Radar images with fine spatial resolution also showed shadowing effects that should be possible to use for identification of storm felled forest. [PERSON] et al. [22] applied L-band space-borne SAR data to windthrow and insect outbreak detection in temperate forests.