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- Remote sensing of forest environments concepts and case studies [2003]
Moreover, the measurements were conducted in a relatively flat area to avoid uncertainty errors. In this study, the number of scanning directions and the resolution of the laser pulse were extremely important because TLS measurement was assumed and used as the actual estimates by which the ALS-derived factors of the individual trees could be approximated to the actual value. Therefore, the instrument factors were set as high as possible. Thereafter, the four acquired datasets were integrated into one dataset based on the absolute positions of four sphere balls which were installed on stationary objects before the acquisition of the laser returns Figure 2.
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As the along-track distances were longer than those of across-track, the surface models may potentially have contained false strips in the flying direction, caused by the precipitous difference between neighboring cells when the interpolation is performed to generate the surface model [ 5 ]. To estimate individual tree parameters, individual trees initially have to be detected and their crown boundaries delineated. The watershed segmentation method was employed because it is a powerful partitioning tool for gray-scale images such as CHM [ 13 ].
In image processing, watershed segmentation is an algorithm based on the topology of the image by flooding process which is performed on the gradient image [ 27 ]. However, watershed segmentation methods tend to suffer from overestimation and underestimation due to the fluctuant height variation within their topography or hidden treetops of the lower trees under the crowns of the higher trees [ 5 ]. In this study, the extended maxima transformation of the image processing method was applied to decrease the incidence of spurious treetops caused by the CHM computed with unfiltered LiDAR returns, whereas Popescu et al.
The CHM was segmented for individual crown delineations using the watershed segmentation method after processing the extended maxima transformation. Details of the delineation of individual tree crowns are described in Kwak et al. To estimate TH and CBH, the process of segmentation was iterated until a one-to-one relationship was achieved between field measurement and detected individual trees.
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Thereafter, individual TH values were determined to be the highest value of the CHM within the segmented boundaries [ 29 ]. Moreover, the k -means algorithm was employed for determining and estimating CBH because it was the fastest consistently working method at the individual tree level for the classification of ALS data [ 16 , 29 ]. For determining the CBH, ALS data reflected within individual trees after segmentation were classified into three clusters ground, understory and crown cluster using k -means clustering [ 30 ], since Korean Pine stands included few bi-storied, applicable to the three above-mentioned clusters Figure 3.
The k -means statistics applied here is an algorithm used to classify or group attributes or features into k number of groups, and uses an iterative algorithm to minimize the sum of the distances SOD, Equation 1 from each object n to its cluster centroid i , over all clusters [ 31 ]. Classification of ALS data according to individual trees segmented using the Kwak et al. The crown area CA was estimated easily from the boundaries delineated by the extended maxima transformation and watershed segmentation.
The height of the individual trees was estimated by extracting the highest point data among the entire point data of the individual trees which were integrated with the four directional TLS dataset.
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Once the point P x i , y i , z i was selected, the coordinates could be calculated in the TLS device coordinate system according to Equation 2 and Figure 4. The CA values were measured with the projected areas of the CHMs of individual trees that were calculated as shown in Figure 5. The dislocated individual trees from ALS were compulsively matched with the closest individual trees derived from TLS.
To assess the accuracy of the developed regression function, 15 Korean Pines were subjected to TLS measurements. The parameters estimated by the regressed functions were statistically compared with the TLS measurement data using the coefficient of determination R 2 and root mean square error RMSE. Finally, the developed regression functions were used to correct the ALS-estimated parameters.
Thereafter, the error rate was used to determine the extent to which the ALS-caused errors would be improved by the application of the regression models Equation 4. The underestimation of ALS was attributed to the possibly low probability of the laser pulses emitted from ALS being reflected on real tree tops [ 5 , 17 ]. In general, it is more difficult for laser pulses to be reflected on precise tree tops due to the conical crown shape of coniferous trees [ 34 ].
The mean of the observed and predicted TH was estimated at Reduction of TH-estimate errors through the regression model. The k -means clustering was used to classify the LiDAR returns into three clusters: That was attributed to the inability to detect the real CBH under the overlapping crown edges between neighboring trees [ 30 , 35 , 36 ]. However, the error rate did not show any decrement by the regressed function Figure 7 c due to the insufficiency of unbiased data for the verification. Furthermore, the measurement error in the field was also strongly attributable to the overlapping trees.
Reduction of CBH-estimate errors through the regression model. Therefore, TLS-emitted laser pulses can describe the entire crown shape, including an actual CBH, which is the real boundary of the individual tree crown. On the other hand, ALS records the reflected pulses vertically with a low point density from the crowns of individual trees to the ground. Reduction of CA-estimate errors through the regression model. The difference of the mean CGV values was also attributed to the uncertainty due to the overlapping effect and the underestimation of the ALS-derived TH.
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Remote sensing of forest environments concepts and case studies [2003]
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