Estimation of tree stem attributes using terrestrial photogrammetry

The objective of this work was to create a method to measure stem attributes of standing trees on field plots in the forest using terrestrial photogrammetry. The primary attributes of interest are the position and the diameter at breast height (DBH). The developed method creates point clouds from image from three or more calibrated cameras attached to a calibrated rig. SIFT features in multiple images in combination with epipolar line filtering are used to make high quality matching in images with large amounts of similar features and many occlusions. After projection of the point cloud to a simulated ground plane, RANSAC filtering is applied, followed by circle fitting to the remaining points. To evaluate the algorithm, a camera rig of five Canon digital system cameras with a baseline of 123 cm and up to 40 cm offset in height was constructed. The rig was used in a field campaign at the Remningstorp forest test area in southern Sweden. Ground truth was collected manually by surveying and manual measurements. Initial results show estimated tree stem diameters within 10% of the ground truth. This suggest that terrestrial photogrammetry is a viable method to measure tree stem diameters on circular field plots