## 4 Results and Analysis
### Consistency Assessment with WELD Products
#### 4.2.2 Radiometric Consistency Assessment  
To test the radiometric consistency between HJ and WELD, scatterplots of all four bands were drawn for each month. Considering the limitations on space, Figure 7 shows the scatter plots for each middle month of the four seasons. The standard seasonal definition for the Northern Hemisphere adopted by the climate modeling community was used where spring is defined as the months from March to May. The agreement measures for each month are shown in Table 2. Generally, the range of TOA reflectance increased with the growth of vegetation and decreased after the exuberant growing season period. Correlations between TOA reflectance of HJ-1A/B composites and the WELD TOA reflectance were strong in most of the months, with the mean R\({}^{2}\) ranging from 0.4239-0.6934. The mean slope of linear models ranges from 0.9793-1.1433, with mean intercepts ranging from \(-\)0.0159-0.0018. It can be seen from Figure 7 that the visible bands generally exhibited lower consistency than the NIR bands, attributed to the fact that atmospheric effects were inversely proportional to the wavelength. Correlations between HJ and WELD TOA reflectance were apparently linear in eight months (January to April and September to December) in Table 2. The ranges of R\({}^{2}\) in these months for each band were 0.4105-0.7040 (blue band), 0.5373-0.7951 (green band), 0.6724-0.8254 (red band) and 0.6933-0.8385 (NIR band), respectively, with mean R\({}^{2}\) ranging from 0.6695-0.7716. Radiometric consistencies were higher in the near infrared band and lower in the visible wavelengths in these months. For NIR band, the intercepts of linear models were near zero (mean value was \(-\)0.0098 and ranged from \(-\)0.0201-0.0027) and the slopes ranged from 1.019-1.281. Radiometric consistency for visible bands between HJ and WELD were similar to those of the NIR band. However, the spread of the points were wider, and the reflectances of most of the blue bands of HJ were lower than that of WELD. The slopes for the blue band ranged from 0.7742-1.1714, and intercepts ranged from \(-\)0.0185-0.0044, presumably due to more potential clear observations of HJ than WELD. Figure 6: The visual consistency evaluation (from left to right): (**column 1**) browse images of HJ-1A/B composites (true color); (**column 2**) HJ corresponding DOY flag images; (**column 3**) browse images of Landsat Web-Enabled Landsat Data (WELD) composites (true color); (**column 4**) WELD corresponding DOY flag images. The black color in February and October in WELD represents no data pixels. “BOM” means beginning of month, and EOM means end of month. Correlations between HJ and WELD were much lower from May to August. Figure 7 showed that the all scatter plots in July deviated from 1:1 line, which indicated there were distinct changes between the two composites. The ranges of R\({}^{2}\) from May to August for each band were 0.0614-0.2663 (blue band), 0.0931-0.3191 (green band), 0.0593-0.3759 (red band) and 0.2739-0.6977 (NIR band), respectively, with mean R\({}^{2}\) ranging from 0.1638-0.5369. To further verify the radiometric consistency of the four months with lower R\({}^{2}\) in Table 2 (_i.e._, May to August), 400 \(\times\) 400 pixels subset regions cropped from Figure 6 were shown in Figure 8. Obviously, the composite results of HJ were visually smooth and the color of different land surfaces seemed normal. In contrast, due to there being only two different days available for compositing, still 20.57% and 13.34% of gap pixels stored in WELD in May and June, respectively, were invalid, which was mainly caused by the SLC-off of ETM+ images [47]. Although no invalid pixels appeared in the July and August images, the transition region boundaries were not smooth and many residual cloud patches still remained in the results. The poor atmosphere conditions in the cloudy months are responsible for the discrete distribution of scatter points.