*3.2. Results of Reference Point Generation*

The accuracy of the algorithm is tested by comparing the coordinate points manually marked with those extracted by our algorithm. The error is calculated by the distance between two pixels. Referring to Figure 10, the error of Figure 10b is larger than errors of the other Figure 10a,c because the height of the tree trunks buried in the soil is irregular, and the error of manually marked points is lower than those obtained in our method. As shown in Table 2, there are five points in three sub-figures. The average error is 1.93 pixels.


**Table 2.** Error analysis of the trunk reference point.
