C- and L-band SAR interoperability: Filling the gaps in continuous forest cover mapping in Tasmania

Abstract

This paper addresses the shortfall in L-band SAR data availability for the purpose of extracting spatially explicit information on forest cover, and the capacity to fill the gap using shorter wavelength C-band data. Specifically, comparisons are made of forest/non-forest (F/NF) extent derived from independent classification of ALOS PALSAR and RADARSAT-2 data acquired over Tasmania. Focussing on a temperate forest landscape, it was demonstrated that C-band SAR data can be used interchangeably with L-band data to produce a comparable estimate of forest/non-forest cover. Only partial interoperability is achieved however, given the limitations of dual polarisation C-band SAR in discerning forests of different growth stage and biomass.Ambiguities in F/NF status were more prevalent in the C-band classification, despite inclusion of topographic (surface elevation and slope) and textural features in the training dataset, with the key observations as follows: (i) Reduced dynamic range and greater overlap amongst F/NF classes; (ii) Similarities in volume scattering from harvested/regrowth eucalyptus areas and background native forest; (iii) Confusion between young pine plantation and harvested/regrowth areas due to comparable roughness; (iv) Less variation in backscatter and poorer separation of intact and managed eucalyptus forest; and (v) Reduced capacity for discrimination of forest types. In almost every case, the use of L-band data was preferable. The one exception was the limited separation of young pine plantation and harvested/regrowth areas in both C- and L-band data. A similar level of performance was achieved in the discrimination of mature pine plantation, and between young eucalyptus plantation and harvested/regrowth.The findings were restricted to single-date classification of C- and L- band data. The potential to extend a time-series of L-band observations over forest using dense time-series (i.e., intra-annual) C-band observations acquired in dual or quad polarisation mode, warrants further investigation. Where a positive trade-off exists between the benefits and costs of integrating these data, multi-frequency (e.g., C- and L-band) and multi-sensor approaches (e.g., SAR and optical) are a viable way forward for operational forest monitoring and carbon accounting.