Abstract
This paper presents a practical application of a technique that uses a vertical optical flow with a fisheye camera to generate dense point clouds from a single planimetric station. Accurate data can be extracted to enable the measurement of tree trunks or branches. The images that are collected with this technique can be oriented in photogrammetric software (using fisheye models) and used to generate dense point clouds, provided that some constraints on the camera positions are adopted. A set of images was captured in a forest plot in the experiments. Weighted geometric constraints were imposed in the photogrammetric software to calculate the image orientation, perform dense image matching, and accurately generate a 3D point cloud. The tree trunks in the scenes were reconstructed and mapped in a local reference system. The accuracy assessment was based on differences between measured and estimated trunk diameters at different heights. Trunk sections from an image-based point cloud were also compared to the corresponding sections that were extracted from a dense terrestrial laser scanning (TLS) point cloud. Cylindrical fitting of the trunk sections allowed the assessment of the accuracies of the trunk geometric shapes in both clouds. The average difference between the cylinders that were fitted to the photogrammetric cloud and those to the TLS cloud was less than 1 cm, which indicates the potential of the proposed technique. The point densities that were obtained with vertical optical scanning were 1/3 less than those that were obtained with TLS. However, the point density can be improved by using higher resolution cameras.
Highlights
Tree attributes can be obtained using different terrestrial techniques for 3D data acquisition
The accuracy assessment was based on the discrepancy tape to obtain the reference values
This paper presented a technique named vertical optical scanning and detailed experimental results of a field test, which demonstrate its applicability in a forest plot for collecting tree attributes and mapping
Summary
Tree attributes can be obtained using different terrestrial techniques for 3D data acquisition. Laser scanning (LS), which is referred to as Light Detection and Ranging (Lidar), has been widely used for commercial purposes in forest inventories It can directly acquire dense and accurate 3D point clouds to provide a detailed and accurate description of the objects of interest. Studies have used TLS for forest data collection, including the estimation of tree height, plot- and tree-level volumes, forest canopy, trunk density, and diameter at breast height (DBH), as noted by Liang et al [1]. Such data are important for monitoring of tree growth, timber volume estimation, biomass estimation, and forest inventory, for example
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