Stem volume estimation in boreal forests using ERS-1/2 coherence and SPOT XS optical data

The use of spaceborne synthetic aperture radar (SAR) systems to estimate stem volume and biomass in boreal forests has shown some promising results, but with saturation of the radar backscatter at relatively low stem volumes and limited accuracy of stem volume estimation. These limitations have motivated evaluation of more advanced methods, such as interferometry. The results presented in this study show that ERS interferometry, under favourable conditions, may be used to estimate stem volume at stand level with saturation level and accuracy useful for operational forestry management planning in boreal forests. Five interferograms were analysed, covering a test site located in the central part of Sweden with stem volume in the range of 0-305 m³ ha^-1. The best interferogram showed a linear relationship between stem volume and coherence with a root mean square error (RMSE) of approximately 26 m³ ha^-1, corresponding to 20% of the average stem volume, throughout the range of stem volume. No saturation was observed up to the maximum stem volume. However, the sensitivity of coherence to stem volume varied considerably between the interferograms. Finally, four SPOT XS images were evaluated and compared with the stem volume estimations obtained from the interferograms, resulting in a relative RMSE of about 24% of the stem volume, for the best case. The estimation of stem volume using coherence data was found to be better than optical data for stem volumes exceeding about 110 m³ ha^-1. The statistical analysis was performed using linear regression models with cross-validation.