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Combining airborne CARABAS-II VHF SAR data and optical SPOT-4 satellite data for estimation of forest stem volume

The accuracy of forest stem volume estimation at stand level is investigated using a combination of airborne radar and optical satellite data. The hypothesis is that the accuracy will be improved for the combined stem volume estimates compared with that using single sensor data. The test site is located in the south of Sweden and consists mainly of coniferous forest. The stem volume for the selected stands was in the range of 15–585 m3·ha–1, with an average stem volume of 266 m3·ha–1 and an average size of 3.5 ha. Remotely sensed data have been collected with the airborne CARABAS-II very high frequency (VHF) synthetic aperture radar (SAR) sensor and the multispectral optical SPOT-4 satellite sensor. Regression analysis has been used to develop stem volume functions for each sensor and for the combination. The accuracy in terms of root mean square error (RMSE) was 49 m3·ha–1 (corresponding to a relative error of 18.5% of the average stem volume) for CARABAS-II, 63 m3·ha–1 (23.5%) for SPOT-4, and 42 m3·ha–1 (15.8%) for the combination. Thus, the improvement was 15% using only CARABAS-II data and 33% using only SPOT-4 data over the full range of stem volumes investigated. CARABAS-II gave the best results for high stem volumes, and SPOT-4 was more accurate for lower stem volumes, hence the combination of the two techniques provided significantly better results over the whole range of stem volumes. The results imply that the combination of low-frequency radar data and multispectral optical satellite data can be used for standwise stem volume estimation in forestry applications.