## 4 Results and Discussion
### Summer Maize Recognition in Huanghuaihai Plain
#### 4.3.3 Validation of the Extracted Summer Maize Area in Huanghuaihai Plain  
The cultivated areas of summer maize in Beijing, Tianjin, Hebei, Henan, and Shandong extracted by MODIS EVI time series images were \(129.7\times 10^{3}\), \(175.4\times 10^{3}\), \(2995.4\times 10^{3}\), \(3322.7\times 10^{3}\), and \(3428.2\times 10^{3}\) ha, respectively. The planting area of summer maize was largest in Henan Province and the smallest in Beijing. The statistics and the extraction of summer maize were used to analyze the overestimation and underestimation of area extraction. If the comparison results were positive, it indicated that the extraction area was larger than the statistical area, which was overestimation. Otherwise, if the comparison was negative, it was underestimated. As can be seen from Table 5, the extraction of summer maize was overestimated in Hebei, Henan, and Shandong, and underestimated in Beijing and Tianjin. The extraction accuracy of Tianjin Province was the highest, reaching 98.04%, and Shandong Province was the lowest, 86.41%. The accuracy of the extraction results of five Provinecs was over 85% in the study. Figure 10: The distribution of the maize planting area in Huanghuaihai Plain in 2012. The statistical data of the maize area from 43 cities and 245 counties in Huanghuaihai Plain were employed to verify the extraction results. Comparing the maize acreage of two groups from MODIS-estimated and statistical data, the validation results showed that the R\({}^{2}\) reached 0.81 with an Root Mean Square Error (RMSE) of 143.8 ha at the city level and the R\({}^{2}\) reached 0.69 with an RMSE of 18.7 ha at the county level (Figure 11). Those demonstrated that there was a good correlation between the maize area estimated by MODIS satellite data and statistical data. Compared to the previous research that used the multi-temporal remote sensing images to map the summer maize area, the extraction accuracy was significantly improved in the current research [20; 21; 69]. Six county-level regions (Changyuan, Gaotang, Linzhang, Lushan, Yongnian, Xinji) were selected from the study area as the sampling validation areas. Based on Landsat ETM+ images with a spatial resolution of 30 m, the distribution regions of summer maize in the six regions in 2012 were obtained by the method used to estimate summer maize area in the reference area in Section 3.1. As shown in Figure 12, the summer maize distribution areas mapped by ETM+ images were roughly consistent with those drawn by MODIS images. The extraction results were compared to the summer maize distribution areas drawn based on the MODIS data. The two kinds of maize distribution areas were superposed and calculated to obtain the area of the public part. The public area is 29.4 ha in Changyuan, 37.7 ha in Gaotang, 36.6 ha in Linzhang, 24.7 ha in Lushan, 33.9 ha in Yongnian, and 43.2 ha in Xinji (Table 6). The proportions of the public part to the extraction results by Landsat images, the extraction results by MODIS images, and the statistical data were used to quantitatively analyze the relative accuracy of summer maize extracted with MODIS data. The proportion of the public part area to the extraction results based on MODIS and Landsat images was compared. The results showed that  
Figure 11: Correlation between the extracted area based on remote sensing and statistical area of summer maize in Huanghuaihai Plain in 2012.