The image BRDF of forests and woodlands is a statistical function which operates at the scale of an average patch of cover rather than at the scale of crowns. Studies of the image Hotspot effect using aerial photography and high spatial resolution scanner data shows very high variance at this detailed scale. In addition

the directional effects of the sun and observer positions interact significantly to create an angular anisotropic variation which persists up to aggregated scales. This has been called the BRVF or Bidirectional Reflectance Variance Function. There has been a recent growing interest in the directional variance or variogram structure of high resolution images as a means to interpret structure and such data have become regularly flown. This work is an extension of the use of image variance to interpret structure as pioneered by Strahler and Li and explored by various authors over the last 15 years. Directional variograms and BRVF functions for forests at the crown scale can be computed using approximations to the overlap functions driving these second order statistics and compared with numerically integrated simulations. It has been shown that the scaling of BRVF and the anisotropy introduces into the variogram can be accurately modelled.