Classification of crop and soil homogenous zones using multipolarization C-band SAR

Abstract

With the acquisition of polarimetric radar data from satellites such as RADARSAT-2 and ALOS PALSAR, this research project explored the potential of using multipolarization C-band synthetic aperture radar (SAR) data to map homogenous soil and crop characteristics within agricultural fields. Homogenous zones within crop fields were defined using coincidental apparent soil electrical conductivity (EC) measured from a soil EC mapping system, green leaf area index (LAI) derived from airborne hyperspectral CASI data, and yield converted to phytomass at harvest (Ph) derived from a yield monitor. Airborne polarimetric C-band SAR images (5.6 cm) at 4 m spatial resolution were obtained over a test site in Ottawa, Canada, on 13 June, 26 June, and 19 July 2001. Linear (HH, VV, HV) and circular (RR, LL, RL) polarization images were synthesized from these data. Once the homogenous zones were determined by EC, LAI, and Ph, the multipolarization SAR data were classified based on these zones using single polarizations and combinations of linear and circular polarizations. Within-field classification accuracies of SAR data were higher when they were based on EC and LAI homogenous zones relative to Ph delineations. The lower SAR classification accuracies based on Ph zones were likely attributed to biomass zones defined relative to harvest rather than coincident with SAR data acquisition. LAI zones were better classified using early season SAR data when greater between-zone variability was present. EC homogenous zones were better classified using SAR data later in the growing season when soil variability was manifested in the vegetation response. Within-field classification performance over the season was dependent on the crop type and the incidence angle. At larger incidence angles (55°), results were more dependent on crop type when compared with smaller incidence angles (35°). In general, a single polarization was capable of discriminating many of the within-field homogenous zones, including EC and LAI zones in corn and LAI and Ph zones in wheat. The linear (HV) and circular (LL or RR) cross-polarizations were the best single polarizations to classify these homogenous zones. Within-field classification of the SAR into EC homogenous zones in the wheat field required polarization combinations, particularly combinations that included HH and VV polarizations. Circular polarizations, either as single polarizations or as part of a combination, were necessary to classify SAR data into EC or LAI homogenous zones as the season progressed. These results suggest that multipolarization SAR data are useful in classifying within-field homogenous zones of corn and wheat, but that this application will require access to data from sensors that synthesize circular polarizations.