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Airborne multispectral imagery for classification of intertidal habitat, Fraser River, British Columbia Aitken, Jennifer Anthea


In intertidal environments, complex vegetation associations, hydrodynamics, and substrate variabilities make accurate mapping and monitoring difficult. Airborne multispectral imaging systems offer a synoptic view with the advantage of programmable handset selection in the visible and near infrared (NIR), increased spatial resolution compared to satellite imaging systems and greater spectral resolution compared with aerial photographs. Eight flight lines of image data of an island marsh complex in the Main Arm of the Fraser River, British Columbia were collected on June 19, 1992 with a Compact Airborne Spectrographic Imager (CASI). An eleven channel handset was configured in the visible and NIR with a spatial resolution (pixel size) of 3.5 m². Spectral signatures of dominant vegetation communities (forest, willow, cattail, sedge, bulrush) and tidal flat, drift logs and necrotic biomass were generated from the image and used in a supervised classification to produce a habitat map. Close coexistence and similar spectral response complicated differentiation of sedge and cattail. Vertical growth of bulrush increased contribution of soil background and affected spectral response. The vegetation index (NIR-red/NIR+red) was found to be insufficient in estimating green biomass in areas containing a high percentage of necrotic biomass or silt covered vegetation. Results of the classification are presented in an error matrix. Vegetation communities and substrate classify 70%-82% correct, individual training areas 61% to 89% correct. Lower accuracies were found in mixed sedge communities (70%) and the dead biomass class (74%). Nonvegetated areas (tidal flat and drift logs) exhibit the most distinct spectral response, 81% and 82% correct. Two surveys of the marsh were conducted on foot in Ladner Marsh in spring and summer to field test the classified map. Fourteen sites were visited, of which three indicated some degree of misclassification, providing a field test accuracy of 78%. Spatial accuracy is determined by an overlay of GPS-referenced ground truth with the classified map. Spatial agreement between cattail, sedge and bulrush ground truth was 20.4 m., 14.4 m., and 53.5 m. respectively. Sedge had the greatest number of classified pixels and bulrush the least, suggesting the influence of population size in the agreement. Poor geometric rectification of imagery degraded agreement.

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