Comparison of fixed-size and variable-sized windows for the estimation of tree crown position

Tree crown recognition with high spatial resolution remotely sensed imagery provides useful information relating the number and distribution of trees in a forest. A common technique used to identify tree locations uses a local maximum (LM) filter with a static-sized (user-specified) moving window. LM techniques operate on the assumption that high local radiance values represent the centroid of a tree crown. The static nature of this technique is inconsistent with both natural canopy structure and digital images. A variable window size (VWS) LM technique operates under the assumption that there are multiple tree shapes and sizes within an image and that the LM filter should be adjusted to an appropriate size, based on the spatial structure found within the imagery. To compare the utility of the VWS LM technique versus that of static LM techniques, tree location accuracy was evaluated for static 3×3, 5×5, 7×7 filters, VWS, and VMS plus a false positive filter based on the Getis statistic. The study site incorporates two stands of Douglas fir (Pseudostuga menziesii); a 40 year old planted site and a >150 year naturally regenerating site. The imagery used was MEIS-II with 1 m ground resolution acquired in 1993 as part of the SEIDAM project. The plantation site has a uniform distribution of tree size and spacing, while the naturally regenerating stand is composed of irregularly sized and spaced trees. The spatially sensitive VWS technique out-performs the static technique when both plantation and naturally regenerating stands are examined. False-positive filters are introduced to screen for local radiance maxima which may not be representative of tree centroids