Estimating tree crown dimensions using digital analysis of vertical photographs

The aim of this study was to develop a relatively rapid, simple and repeatable method to characterize the crown architecture of Queensland maple (Flindersia brayleyana F. Muell.) by vertical hemispherical photography. Calibration scales were developed to provide horizontal planes of measurement at heights from 15 to 40 m above the camera. Hemispherical images of tree crowns were merged with the specific calibration scale appropriate for the mid-height of the crown in order to adjust for image distortion during measurement. Merged images were analyzed by three procedures to yield measurements of crown diameter and projected area. The most precise measurement of projected crown dimensions was assumed to be provided by a complete analysis of a digital image of the crown, after removal of the trunk image. Crown diameters estimated by tape measurements on the ground were relatively imprecise (R-2 = 0.54), Crowns classified as symmetrical were described much more precisely (R-2 = 0.72) than those classified as asymmetrical (R-2 = 0.37). The precision of estimates of crown dimensions by tape did not alter significantly over the range of crown sizes sampled (2-10 m diameter in trees 15-40 m tall). This novel image analysis method can yield relatively precise and accurate measurements on the relatively shallow crowns of tall trees, often found in rainforests, regardless of the degree of asymmetry in crown shapes. However, the method is slower to apply than conventional tape techniques and additional equipment is required. The principal advantage of the photographic technique is the opportunity for precise measurements of changes in crown size and foliage projected area over time. (C) 2000 Published by Elsevier Science B.V. All rights reserved.