254 Moore and Ryder: Ground-Penetrating Radar to Locate Tree Roots Figure 13. A top-down view of the 12 scans conducted on the root system of Pistacia chinensis (Chinese pistachio) in situ for the third experiment and the predicted root locations. The x represents roots and the square symbols represent other harder objects, such as stones or utility pipes. Table 3. Number of roots detected by GPR and the predicted depth of roots for the paired root detection experiment for each scan at depths of 200, 400, and 800 mm. Depth of roots (mm) 200 Scan number Mean depth Range (mm) 1 2 3 400 Total 4 5 6 800 Total 7 8 9 Total 146 145 136 -143 -282 -263 -242 -262 -378 -315 -355 -348 (mm) (164,115) (173,120) (157,116) (115,173) (-360,-240) (-295,-230) (-275,-200) (-360,-200) (-410,-335) (-370,-280) (-400,-300) (-410,-280) Number detected 18 19 16 - 13 20 14 - 18 20 16 - between the number of roots that the GPR predicted and the number of roots counted, especially in the trenches 2.5 m from the trunk (Table 5), where 52 roots were predicted by the GPR but only one root was counted. Conse- quently, there were significant differences (P < 0.05) between the observed and predicted roots numbers. A sample of the 12 scans con- ducted for this experiment is provided in Figure 12, and a summary of the locations of all predicted roots is presented in Figure 13. ©2015 International Society of Arboriculture DISCUSSION Statistical analysis showed that at 200 mm depth, the vertical location of roots was more accurately depicted by the GPR scans than the horizontal location with two out of three scans and the over- all average at 200 mm depth was not significantly different from the known root locations, which is consistent with other studies (Guo et al. 2013). Pre- dicted root locations (Figure 6; Figure 9) and the true root locations were in close proximity, with roots located within 130–200 mm of their actual location. For 200 mm depth, scans provide valu- able information about root position for arborists. Nadyezdhina and Cermàk (2003) noted an error margin using GPR of about 50 mm, with a detection rate of approximately 80%. The average horizontal and vertical error across the three scans was 54 mm and 21 mm, respectively, showing that the depth reading was more accurate than root location along the scan line, probably because it was difficult to follow exactly the same transect line in successive passes. All roots were identified at 200 mm depth. As the depth of inspection increased to 400 mm, so did the level of error in accurately locating the roots (Figure 7; Figure 10). However, for most arboricultural field situations, this level of accu- racy would be both acceptable and useful. Similar
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