Author
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DYMOND, JOHN - LANDCARE RESEARCH |
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Qi, Jiaguo |
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Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/19/1997 Publication Date: N/A Citation: N/A Interpretive Summary: Images of farmland are often acquired with aircraft-mounted sensors that have wide viewing angles. The brightness of these remotely sensed images is usually not uniform across the image because of the position of the sun and viewing angles of the sensor. The brightness ought to be corrected for in order to derive information on which firm conclusion can be made. Models have been developed for this purpose. However, most models for correcting this brightness are either too complicated to be used on an operational basis, or they are too empirical to be used with a high degree of confidence. We have developed a semi-empirical, simple model that is quite accurate. The model was tested with data acquired over pasture and pine forest. The results indicate that a simple model can result in an accuracy for correcting effects of the sun on remote sensing images comparable to that of more complicated models. This also suggests that correction of effects from the sun's position or from viewing angles can be made on an operational basis, therefore providing more accurate information from aircraft-based remote sensors. Any remote sensing application and the interpretation are especially useful for farmland management by growers and commercial entities. Technical Abstract: Monitoring vegetation conditions by remote sensing is made difficult by the variation of canopy brightness (radiance) with view direction and sun position. In this paper, we develop a physical model of canopy radiance that models this variation as a product of three functions, each being expressed as a simple analytical formula. The first function, the S function, represents the proportion of canopy seen as sunlit, excluding the hotspot. The second function, the hotspot function, corrects the S function for the higher probability of seeing sunlit leaves near the anti-solar point. The third function represents the average radiance of sunlit leaves. The model was fitted to densely- sampled, multi-view radiance measurements of pine forest (Pinus radiata) and pasture (Lolium spp) taken at a large (= 60 degree) sun zenith angle. Radiance predictions for multiple views were then made for a small sun zenith angle (= 20 degrees). Comparison of predicted with measured radiances showed that the model out-performed the linear Roujean et al. model (J. Geo. Res., 97:20, 455-20, 468) and the non- linear Verstraete et al. model (J. Geo. Res., 95:11,755-11,765). The improvement is due to two factors: the hotspot function has a sun zenith angle dependence; and the S function allows the average projected leaf area to vary with off-nadir view angle. |
