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Remote Sens. 2017, 9(10), 999; doi:10.3390/rs9100999

Polarimetric ALOS PALSAR Time Series in Mapping Biomass of Boreal Forests

Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, P.O. Box 11000, FI-00076 AALTO, 02150 Espoo, Finland
VTT Technical Research Centre of Finland Ltd., Remote Sensing Team, PL 1000, FI-02044 VTT Espoo, Finland
Author to whom correspondence should be addressed.
Received: 4 July 2017 / Revised: 19 September 2017 / Accepted: 21 September 2017 / Published: 27 September 2017
(This article belongs to the Section Forest Remote Sensing)
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Here, we examined multitemporal behavior of fully polarimetric SAR (PolSAR) parameters at L-band in relation to the stem volume of boreal forests. The PolSAR parameters were evaluated in terms of their temporal consistency, inter-dependence and suitability for forest stem volume estimation across several seasonal conditions (frozen, thaw and unfrozen). The satellite SAR data were represented by a time series of PolSAR images acquired during several seasons in the years 2006 to 2009 by the ALOS PALSAR sensor. The study area was in central Finland, and represented a managed area in typical boreal mixed forest land. Utility of different PolSAR parameters, their temporal stability and cross-correlations were studied along with reference stand-level stem volume data from forest inventory. Further, two polarimetric parameters, cross-polarization backscatter and co-polarization coherence, were chosen for further investigation and stem volume retrieval. A relationship between forest stem volume and PolSAR parameters was established using the kNN regression approach. Ways of optimally combining PolSAR images were evaluated as well. For a single scene, best results were observed with polarimetric coherence (RMSE ≈ 38.8 m3/ha) for scene acquired in frozen conditions. An RMSE of 40.8 m3/ha (42.9%, R2 = 0.66) was achieved for cross-polarization backscatter in the best case. Cross-polarization backscatter was a better predictor than polarimetric coherence for few summer scenes. Multitemporal aggregation of selected PolSAR scenes improved estimates for both studied PolSAR parameters. Stronger improvement was observed for coherence with RMSE down to 34 m3/ha (35.8%, R2 = 0.77) compared to 38.8–51.6 m3/ha (40.8–54.3%) from separate scenes. Finally, the accuracy statistics reached RMSE of 32.2 m3/ha (34%, R2 = 0.79) when multitemporal HHVV coherence was combined with multitemporal HV-backscatter. View Full-Text
Keywords: synthetic aperture radar; SAR polarimetry; time series; stem volume; boreal forest; L-band; ALOS PALSAR synthetic aperture radar; SAR polarimetry; time series; stem volume; boreal forest; L-band; ALOS PALSAR

This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Antropov, O.; Rauste, Y.; Häme, T.; Praks, J. Polarimetric ALOS PALSAR Time Series in Mapping Biomass of Boreal Forests. Remote Sens. 2017, 9, 999.

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