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Tree height (H) survey is a fundamental step in forest mensuration. The error affecting tree height measure, necessarily influences the correspondent tree estimates. The remotely survey of vegetation using PHODAR (PHOtogrammetric Detection And Ranging) or LiDAR (Light Detection And Ranging) techniques generates very high-density point clouds, that result into Canopy Height Models (CHMs) having GSD (Ground Sampling Distance) of few centimetres. This GSD value potentially allows to survey single crown apexes, which, from a forestry point of view, do not represent the actual tree height. Apex height value, in fact, does not represent the prevailing dendrometric height (PDH) but the maximum tree value. In this study we propose a new approach aimed at measuring dendrometric height by PHODAR derived CHM, taking care about this issue. The proposed method defines a correcting factor (found equal to 95% percentile of CHM values distribution within a given crown) for the tree height extraction from CHM based on the PDH concept. The method could be implemented to single crown approach in forest parameters extraction algorithms permitting more reliable results, especially in terms of tree volume and related estimations (

Tree height (H) survey is a fundamental measure in forest characterization; H is required to define a stand site index measuring the local fertility and give an estimate of standing trees volume. Especially tree stem volume has a key role in forest estimates being related to the amount of wooden commodity. Stem volume is ordinary calculated as (

where ^{CHM}) but, conversely, the maximum height value (H^{CHM}_{max} - ^{CHM}_{max} could not be used to estimate tree volume or site indices, as a not negligible overestimation would occur. In this work, PDH is suggested to be newly defined as that height value corresponding to the most frequently one occurring within tree upper canopy.

In this study, we propose a new definition of prevailing dendrometric height; consequently, a different approach, other than the Local Maxima one, has been considered to process CHM. A small and highly controllable CHM of 60 mixed broadleaf trees (^{©} tools (^{G}_{max }was measured by collimating tree top (maximum visible height), while PDH^{G} was estimated with reference to the main development of the crown (^{G}_{max }and PDH^{G} values were, finally, recorded into the attributes table of P. The test CHM was generated by grid differencing between a DSM generated by regularization of a PHODAR-derived point cloud surveyed by RPAS. A DJI Phantom4 RPAS equipped with a 12.4 Megapixel RGB camera was used for data acquisition. Flight and sensor’s technical features are reported in ^{®} receiver (3D positioning accuracy was ~ 3 cm). Leave-one-out procedure (_{z}^{DTM}= 0.60 cm (

The following positioning errors (MAE) were obtained for GCPs after image block adjustment adopting leave-one-out procedure: 0.267 m (σ_{x,y}), 0.229 m (σ_{z}) and 0.352 m (σ_{x,y,z}). A high-density point cloud was generated (about 36 million points, 250 pts m^{-2}), filtered, classified and regularized to generate the correspondent DSM with a cell size of 0.1 m. After regularization DSM height accuracy was tested with reference to the GCPs to exclude a degradation of the native accuracy. Results proved that height accuracy of DSM was the about same (σ_{z}^{DSM} = 0.236 m) as the above mentioned one (σ_{z} = 0.229 m). CHM accuracy (σ_{z}^{CHM}) was finally estimated by the Variance Propagation Law (VPL - _{z}^{DTM} and σ_{z}^{DSM}; σ_{z}^{CHM} resulted to be equal to 0.64 m. Such accuracy is higher than that obtainable using hypsometer and adopted in ordinary forest inventory (^{G} was computed. Sixty different PVs were found depending on the assessed crown. PV mean value (μ_{PV}) was assumed as reference threshold, for broadleaves, to filter out apexes while measuring PDH^{CHM }from CHMs. The computed μ_{PV} value was 95%; consequently, PDH^{CHM} was found equal to the value within DC which corresponds to the 95 percentile of H CFD (^{CHM}_{max} and PDH^{CHM} are compared, figuring out the effect on tree height measure given assuming PDH^{CHM} in place of H^{CHM}_{max}. The Kolmogorov-Smirnov test was run to test significance the similarity of the two distributions with a normal one. D-value was 0.075 (p = 0.871, Skewness: 0.046) and 0.069 (p = 0.957, Skewness: 0.020) for PDH^{CHM} and H^{CHM}_{max}, respectively, demonstrating that distributions did not significantly differ from a normal one. Since the two distributions were strictly correlated (^{CHM}_{Max} was significantly different from PDH^{CHM}. A ^{CHM} are significantly different from those ordinarily computed in forestry (H^{CHM}_{Max} -

Relative difference (ΔHs) between H^{CHM}_{max} and PDH^{CHM} was calculated for each tree and the correspondent cumulated frequency distribution generated. σ_{z}^{CHM }value was used as threshold to test if differences were significant or not (_{z}^{CHM }further demonstrating that tree height computed according to PDH^{CHM }is significantly different from that computed with reference to H^{CHM}_{max} in more than a half of cases.

PHODAR will represent a potential tool for more suitable and accurate forest inventories in the next future. Nevertheless, it introduces new ambiguities (^{th} percentile of this distribution can be assumed as PDH^{CHM}. Results showed that differences between PDH^{CHM} and H^{CHM}_{Max} (

We thank Dr. Martina Zucaro for the help in ground surveys, Dr. Claudio Rizzo for the RPAS flight, and La Mandria natural park technicians for the project support.

A new definition of prevailing dendrometric height (PDH) is proposed, corresponding to the most frequent height value occurring within upper tree crown.

(a) Study area is located in “La Mandria” regional park (Piemonte - NW Italy); (b) RPAS-derived orthomosaic and surveyed tree crowns (reference frame is WGS84 UTM 32N); (c) Frequency distribution function of ground measured tree height (H^{G}_{max} ).

An example of oak crown in the study area. (a) CHM and apexes positions; (b) 3D view of CHM showing the role of outliers played by apexes within the crown; (c) H CFD of DC considered; dotted line represent the threshold value (95% PV) used to filter out apexes (red dots).

H^{CHM} (left) and PDH^{CHM} (right) frequency distributions computed considering all surveyed trees in study area.

ΔHs cumulative distribution function and correspondent box-plot (min, Q1, median, Q3, max) computed with reference to all the trees in the study area. Red line shows CHM estimated accuracy (σ_{z}^{CHM}).

Parameters of DJI Phantom4 RGB camera and flight parameters adopted for the photogrammetric survey.

Flight/Sensors Parameters | Size | Units |
---|---|---|

Focal length | 8.6 | mm |

CMOS sensor size | 13.2 × 8.8 | mm |

Physical pixel | 2.2 | mm |

Forward image overlap | 93 | % |

Side image overlap | 83 | % |

Flight relative altitude (above ground level) | 90 | m |