Photogrammetric measurement of tree stems from vertical fisheye images

Abstract

Forest variables are typically surveyed using sample plots, from which parameters for large areas are estimated. The diameter at breast height (DBH) is one of the main variables collected in the field and can be used with other forest measures. This study presents an automatic technique for the mapping and measurement of individual tree stems using vertical terrestrial images collected with a fisheye camera. Distinguishable points from the stem surface are automatically extracted in the images, and their 3D ground coordinates are determined by bundle adjustment. The XY coordinates of each stem define an arc shape, and these points are used as observations in a circle fitting by least squares. The circle centre determines the tree position in a local reference system, and the estimated radius is used to calculate the DBH. Experiments were performed in a sample plot to assess the approach and compare it with a technique based on terrestrial laser scanning. In the validation with measurements collected on the stems using a measuring tape, the discrepancies had an average error of 1.46 cm with a standard deviation of 1.09 cm. These results were comparable with the manual measurements and with the values generated from laser point clouds.