## 3 Results and Discussion
### Comparison with MODIS Albedo Product  
To further demonstrate the ability of the proposed algorithm using HJ data in albedo estimation, a comparison was made between the albedo maps from HJ data and 500-m MODIS albedo product (MCD43A). Due to the difference in acquisition time of the two satellite series, the white-sky albedo was chosen for comparison, which is theoretically independent of solar zenith angle and thus acquisition time. The black-sky albedo included in MODIS product was generated with solar zenith angle at noon local time; thus, in order to compare the black-sky albedo from HJ data with MODIS product, the black-sky albedo was calculated from MODIS BRDF parameters with the HJ overpass solar zenith angle. Because MODIS albedo products are generated based on a temporal composition approach, their albedo values represent the dominant surface conditions within the 16-day composite window under clear-sky conditions. Thus, the comparison was made for data obtained in later summer to reduce the impact of surface changes as well as covering a variety of land cover types including forest, cropland, grassland, and urban areas (Figure 8). Manual georegistration was carried out on the scene using the Level 1T Landsat images (standard terrain correction) downloaded from USGS. A second-order polynomial transformation function successfully reduced the geolocation error from kilometers in the original Level 2 HJ data to within \(\pm 30\) m compared with Landsat data. The MODIS albedo products were reprojected to UTM projection \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Land Cover * & **Mean** & **Standard Deviation** & **Bias** & **RMSE** & \(N\) \\ \hline DBF-ENF-EBF & 0.138 & 0.045 & 0.004 & 0.026 & 27 \\ GRA-CRO & 0.177 & 0.036 & \(-\)0.018 & 0.052 & 37 \\ OSH-WSV & 0.196 & 0.062 & 0.022 & 0.031 & 13 \\ \hline \hline \end{tabular}  
* ENF: Evergreen needleleaf forest; EBF: Evergreen broadleaf forest; DBF: Deciduous broadleaf forest; OSH: Open shrublands; WSV: Woody savannas; GRA: Grasslands; CRO: Croplands.  
\end{table} 
Table 3: Albedo estimation accuracy for different land cover types at Ameriflux sites. Figure 7: Relationship of relative estimation error in shortwave albedo and sun-sensor-target geometries. to match the georegistered HJ data. HJ albedo was then spatially aggregated to MODIS resolution using the mean HJ albedo value within a MODIS pixel. The white-sky albedo maps from the two datasets (Figure 8a,b) show similar patterns of albedo spatial distribution in general, while much more detail of spatial variation can be found in the HJ albedo map. Figure 8c shows that the albedo derived from HJ data agrees well with the high-quality MODIS product, with a negligible bias of 0.004 and an RMSE of 0.014 (about 10.6% in relative value). The comparison of black-sky albedos (Figure 8d) shows improved results over that of white-sky albedos with a bias of 0.002, RMSE of 0.011, and R\({}^{2}\) of 0.650, which agrees well with the statistics shown in Figure 4 and suggests that with HJ's limited angular sampling the black-sky albedo is more accurate than white-sky albedo. Accompanied by the low signal-to-noise ratio (SNR), some significant striping noise is visible in part of the HJ data, which contributed to the errors found in the comparison. In addition, the adjacency  
Figure 8: White-sky albedo maps from (**a**) HJ data and (**b**) MODIS 500 m albedo product; Scatter plots (density plot: low density in blue and high density in red) between HJ albedo and MODIS product for (**c**) white-sky albedo and (**d**) black-sky albedo in Mississippi, USA on 23 September 2014. Some water bodies are masked in white in the MODIS product. effects not considered in the spatial aggregation of HJ albedo to MODIS scale also led to some uncertainties in the comparison.