## 4 Results
### Soil Salinity Prediction and Accuracy Assessment
#### 4.4.2 Soil Salinity Prediction and Accuracy Assessment Using the OK Method  
The spatial distribution maps of soil salinity prediction of two modes using the OK method with defined parameters on semi-variogram modeling and standard searching \begin{table}
\begin{tabular}{c l c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Sub-District**} & \multicolumn{4}{c}{**ECe of HH in mS/m**} & \multicolumn{4}{c}{**ECe of VV in mS/m**} \\ \cline{3-8}  & & **Mean** & **SD** & **CV (\%)** & **Mean** & **SD** & **CV (\%)** \\ \hline
1 & Thanon Pho & 1147.60 & 157.90 & 13.76 & 1686.39 & 160.15 & 9.50 \\
2 & Makha & 988.82 & 314.31 & 31.79 & 1442.11 & 429.09 & 29.75 \\
3 & Sai O & 896.68 & 652.28 & 72.74 & 1292.91 & 571.27 & 44.18 \\
4 & Non Thai & 2653.78 & 2611.72 & 98.42 & 2360.45 & 1860.28 & 78.81 \\
5 & Dan Chak & 2411.66 & 2414.84 & 88.81 & 2670.37 & 1725.74 & 64.63 \\
6 & Banlang & 586.78 & 367.12 & 62.57 & 1082.93 & 634.24 & 58.57 \\
7 & Ban Wang & 3237.62 & 1253.54 & 38.72 & 3019.95 & 963.52 & 31.91 \\
8 & Khang Phlu & 5627.04 & 1615.48 & 28.71 & 4752.25 & 1078.33 & 22.69 \\
9 & Samrong & 2103.33 & 1664.26 & 79.13 & 2394.89 & 1390.01 & 58.04 \\
10 & Kampang & 2112.18 & 848.70 & 40.18 & 2696.82 & 764.34 & 28.34 \\ \hline
**Non Thai District** & **2276.31** & **2001.08** & **87.91** & **2395.08** & **1502.69** & **62.74** \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Statistical data of soil salinity prediction with ECe data using IDW of two modes by sub-district. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Measuring Mode** & **ME (mS/m)** & **RMSE (mS/m)** & **R\({}^{2}\)** & **PBIAS (\%)** & **Number of Samples** \\ \hline HH & 663.1821 & 1308.5117 & 0.8422 & \(-\)43.6737 & 121 \\ VV & \(-\)80.6340 & 803.9182 & 0.8354 & 3.2346 & 121 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Statistical data for accuracy assessment of the IDW method. neighborhood are displayed in Figure 10. Essential statistical data of soil salinity prediction using OK of two modes by sub-district are reported in Table 10.  
Figure 10: Spatial distribution of soil salinity prediction using OK of two modes: (**a**) HH and (**b**) VV. As shown in Figure 10, the variations in soil salinity distribution from two measuring levels of two modes at the sub-district level are slightly different. In Table 10, the top three sub-districts of ECe of the HH mode, Non Thai, Ban Wang, and Khang Phlu, show the highest mean values of soil salinity, with a value of about 2557 to 6122 mS/m. In contrast, three sub-districts, Banlang, Makha, and Sai O, display the lowest mean value of soil salinity, from about 558 to 1113 mS/m. The top three sub-districts of ECe of the VV mode, Dan Chak, Ban Wang, and Khang Phlu, show the highest mean soil salinity values of 2635 to 4973 mS/m. In contrast, three sub-districts, Banlang, Makha, and Sai O, display the lowest mean value of soil salinity, from about 1044 to 1583 mS/m. Like IDW, the CV value of the two modes displays the lowest at Thanon Pho, with a value of 13.91% and 6.90%, respectively, while the CV value of the two modes exhibits the highest at Non Thai, with a value of 88.57% and 63.34%, respectively. At the district level, the mean ECe value of topsoil (HH) is 2291.19 mS/m, with an SD of 1944.79 mS/m and a CV of 84.88%. Meanwhile, the mean ECe value of subsoil (VV) is 2401.41 mS/m, with an SD of 1338.30 mS/m and a CV of 55.73%. Like IDW, the variation of soil salinity at the topsoil level is higher than at the subsoil level. In addition, the result of the accuracy assessment of the OK method with the testing dataset was reported in Table 11. As a result, the ME and RMSE values of the HH mode are about 632.85 and 1247.72 mS/m, and those of the VV mode are about \(-\)145.42 and 807.13 mS/m. The R\({}^{2}\) value of the HH mode is 0.8526, and that of the VV mode is 0.8344. The PBIAS of the HH mode is about \(-\)41.68%, and that of the VV mode is about 5.83%. When comparing ME values between two measuring modes, the VV has a lower mean error.