Combining CARABAS-II VHF SAR and Landsat TM satellite data for estimation of forest stem volume

The accuracy of forest stem volume estimation at stand level using a combination of airborne synthetic aperture radar (SAR) and optical satellite data is investigated in this paper. It is anticipated that the accuracy will be improved for the combined stem volume estimate compared to 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 m/sup 3/ ha/sup -1/ with an average stem volume of 266 m/sup 3/ ha/sup -1/ and an average size of 3.5 ha. Remotely sensed data have been collected with the airborne CARABAS-II VHF SAR sensor and the multi-spectral optical Landsat TM 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 was 49 m/sup 3/ ha/sup -1/ (corresponding to a relative error of 18.5% of the average stem volume) for CARABAS-II, 66 m/sup 3/ ha/sup -1/ (24.8%) for Landsat TM, and 38 m/sup 3/ ha/sup -1/ (14.3%) for the combination. Thus, compared to using only CARABAS-II data the improvement was found to be 23% and using only Landsat TM data 42%. For high stem volumes CARABAS-II gave the best result, while for lower stem volumes Landsat TM was more accurate. Hence, the combination of the two techniques provided significantly better results over the full range of stem volumes. The result implies that the combination of low-frequency SAR data and multi-spectral optical satellite data can be used for standwise stem volume estimation in forestry applications.