Laser-based Field Measurements in Tree-level Forest Data Acquisition

Remote-sensing (RS) methods, such as airborne laser scanning (ALS), are capable of acquiring tree-level forest information with high accuracy. However, these methods need more detailed reference data from sample plots than standwise mean characteristics. Field-measured treewise characteristics must be matched with ALS data on the measured position of the tree. In addition to tree position and species, diameter-at-breast height (dbh) measurements are an essential part of these reference data, since dbh at the individual tree level cannot be measured directly from the ALS data. Here we compare the accuracy in treewise field measurements with different laserbased methods in practice. The characteristics under observation include dbh, tree height and stem position. The comparable instruments used include the Terrestrial laser scanner (TLS), laser-relascope and laser-camera. The data consist of 122 trees from sample plots in Saunalahti and Nuuksio. The standard errors in dbh measurements were 8.3 mm (4.5%), 8.5 mm (4.9%) and 14.3 mm (8.3%) with the TLS, laser-camera and laser-relascope, respectively. The standard errors in positional accuracy of the tree were 0.1 m with the TLS and 0.7 m with the laserrelascope. A bias of -0.6 m (-5.2%) and standard error of 1.0 m (10.1%) were present in tree height measurements with the laser-relascope. The results of the study can be utilized e.g. in developing new cost-efficient methods for collecting ground truth data for ALS-based inventories.