The effect of speckle filtering on scale-dependent texture estimation of a forested scene

Spatial fluctuations in microwave backscatter may be an important piece of information in discriminating tree stands. However, the presence of speckle in synthetic aperture radar (SAR) image data is a barrier to the exploitation of image texture. We explored a new methodology that combines a recent adaptive speckle reduction algorithm by Lopes et al. [12] with a generic texture estimation scheme. We investigated the claim that this filter was capable of preserving backscatter texture. To understand if speckle reduction was destroying backscatter texture, we compared the strength of the relationship between forest inventory parameters and image texture as a function of spatial scale for both filtered and unfiltered images. We used Radarsat Fine mode image data: single look resolution is approximately 8.5 m, and pixel spacing is 3 m. Our study area was northern Vancouver Island, B.C., on the west coast of Canada. For the unfiltered data, we found that the ability of image texture to predict the forest parameters decreased as the texture scale increased from 3 to 13 m, suggesting greater information content in the small scale texture. For the filtered data, this relationship was much weaker at small scales and was not a function of distance. Our results suggest that the speckle filter was not retaining small scale texture, which is consistent with the theoretical hypotheses underlying its multiplicative noise model. We also show that there is significant information in small scale SAR image texture that may be used as an adjunct to other spatial information for discriminating tree stands in the temperate rain forest.