Direct measurement of tree height provides different results on the assessment of LiDAR accuracy

In this study, airborne laser scanning-based and traditional field-based survey methods for tree heights estimation are assessed by using one hundred felled trees as reference dataset. The comparison between remote sensing and field-based methods were applied to four circular permanent plots located in the western Italian Alps and established within the Alpine Space project NewFor. Remote sensing (ALS), traditional field-based (IND) and direct measurement of felled trees (DIR) methods were compared by using summary statistics, linear regression models and variation partitioning. Our results show that tree height estimates by aerial laser scanner (ALS) approximated to real heights (DIR) of felled trees. Considering the species separately, Larix decidua was the species that showed the smaller mean absolute difference (0.95 m) between remote sensing (ALS) and direct field (DIR) data followed by Picea abies and Pinus sylvestris (1.13 m and 1.04 m respectively). Our results cannot be generalized to ALS surveys with low pulses density (< 5/m2) and with view angles far from zero (nadir). We observed that traditional field-based survey can be less accurate than laser scanner in estimating tree heights and this was particularly valid for tall trees with conical shape crowns.