## 5 Discussion
### Improving the Correlation between Impervious Surface Area and Land Surface Temperature Measures  
The differences of pixel-wise and object-based ISA relationship with LST illustrate that the detailed urban morphology information addressed through objects is likely to have improved accuracy and stability for urban thermal environment studies. Although the resulting ISA fractions based on pixel-wise LSMA method are weakly and positively correlated with the LST distribution in Guangzhou, it is suggested that the UHI is inconsistent with the distribution of impervious areas with high fraction \begin{table}
\begin{tabular}{c c} \hline
**Type** & **Categories** \\ \hline High Temperature & \(T_{s}>(T_{a}+Sd)\) \\ Normal Temperature & \((T_{a}-Sd)\leq T_{s}\leq(T_{a}+Sd)\) \\ Low Temperature & \(T_{s}<(T_{a}-Sd)\) \\ \hline \end{tabular}
\end{table} 
Table 2: LST classification analysis of Guangzhou from 2009 to 2011. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Years** & **\% ISA** & **\% H-LST** & **\% N-LST** & **\% L-LST** & **LST** \\ \hline
2009 & 49.6\% & 16.24\% & 67\% & 6.76\% & 37.12 \\
2010 & 51.3\% & 12.88\% & 72.72\% & 14.5\% & 36.97 \\
2011 & 55.7\% & 15.04\% & 66.48\% & 18.47\% & 37.24 \\ \hline \end{tabular}
\end{table} 
Table 3: Analysis of relative ISA and LST from 2009 to 2011.  
values. By contract, the UHI areas (LST higher than 37 \({}^{\circ}\)C) were mainly (>43%) located in areas with high ISA fraction values. The distribution of ISA based on the object-based SVM-LSMA method is consistent with the statement that impervious surface is a dominant influencing factor in the urban thermal environment. Our results also confirmed that the impact of urbanization on UHI may be mitigated by optimizing landscape arrangement at object level. The LST and ISA data derived from remote sensing imagery yielded different UHI spatial patterns. Since the ISA value also contains information of the land surface albedo, it is difficult to explore fully the relationship between LST and surface albedo derived from medium resolution imagery in this research. The robustness of ISA and LST measurements in this study was mainly influenced by the availability of remote sensing data. Using more optical or thermal data of higher resolution would improve further the results. In the future, seasonal LST results should be included to explore further the potential quantitative relationship with ISA. In addition, it will also be worth examining whether there is global warming effect on the LST of Guangzhou. Improving the Correlation between Impervious Surface Area, Population Density, Global Human Settlement and Night-Time Light Images  
In this research, we assumed that the identification of imperviousness fraction can be correlated with the disaggregated population density maps, night-time light images and global human settlement products based on linear regression model. The correlation between the estimated ISA and the percentage imperviousness seems to be crucial to understanding the capability of widely existing 100 m population density maps, 250 global human settlement density maps and 1 km night-time light maps in quantifying both spatial extent and density variations of ISA. Nevertheless, it is also worth paying attention to the sensitivity of the disaggregation procedure in the future. In our study, although the correlations between the object-based ISA results and the population and human settlement density related maps were weak, it still sheds lights on understanding the underlying types and patterns of urban land cover, and subsequently, social studies on cause-effect relationships. The insignificant correlations between pixel-wise ISA results and the reference data imply that the selection of appropriate spatial units of ISA is very important for delineating the impervious surface pattern and for understanding urbanization processes. Nevertheless, the robustness of the reference imperviousness maps is still unclear. This should be investigated in future research.