Measuring rangeland vegetation with high resolution airborne videography in the blue-near infrared spectral region
An airborne video system was used to investigate the visible and near-infrared (NIR) spectral properties of soil and vegetation features across a range of common arid landscape types. The four-camera system was equipped with filters of 25mm bandwidth centred on 450nm ('blue'), 550nm ('green'), 650nm ('red') and 770nm ('NIR'). The aim was to determine what vegetation properties could be detected by combining data from the blue part of the spectrum with the green, red and NIR range, thereby utilizing information contained in the first channel of Landsat Thematic Mapper (TM) (450-520nm). Adding information from the blue end of the spectrum did not assist in discriminating between green vegetation and dry vegetation or green vegetation and bare soil. This separation is best done with a red/NIR ratio. Neither was the blue band an improvement over the PD54 red-green perpendicular distance index in distinguishing between soil and vegetation, irrespective of phenological condition. The blue band can help separate soil from dry vegetation when combined with the sum of brightness values in the red and green bands in a perpendicular distance index. These properties of the spectral dataspace lead to a sequential classification procedure by which airborne videography data can be used to measure vegetation components which are much slower to assess with conventional ground-based methods. Videography has great potential for rapidly verifying or calibrating vegetation cover indices derivedfrom satellite data. Vegetation cover derived from classifying high resolution video data acquired from a heterogeneous floodplain area correlated well with vegetation indices computed from contemporary and co-registered TM data. The most effective indices for measuring vegetation cover with TM data are the PD54 index, brightness in the red band and a perpendicular index based on the sum of the red-green bands and the blue band. However, multiple regression indicates that the addition of a red/NIR ratio as an additional predictor of cover does not greatly improve the performance of these indices.
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