# Improving Accuracy of Impervious Surface Extraction Based on a Threshold Hierarchical Method (THM)
## 5 Discussion  
The THM was designed to address the problem of impervious surfaces being underestimated in the LSMA for areas with a large number of impervious surfaces. The study focused on understanding the capabilities of the THM for ISA extraction and the optimal threshold. It has been proved that the THM helps achieve a higher ISA extraction accuracy when an entire image is considered; the optimal threshold was 0.5 when the THM was employed for a GF-2 image. The accuracy of the THM, considering different thresholds, demonstrated a \"U\" trend, as the THM was employed for two images with different resolutions. The spectral information on the medium-resolution remote sensing data was abundant; however, it was too coarse for extracting impervious surfaces. Moreover, the spatial information on the high-resolution remote sensing image was rich, but its scale was often on a pixel level. Limitations exist for both medium-resolution and high-resolution images in extracting impervious surfaces. The threshold is a key aspect in determining the range of application of these two images. Considering the optimal threshold, an optimal balance was achieved by applying the LSMA and OOM at pixel and subpixel levels, respectively, helping achieve suitable accuracy levels for extracting impervious surfaces. The THM helped extract impervious surfaces, considering an entire region. Four zones (urban-rural fringe zones, areas with abundant bare soil, central business district (CBD) areas, and riverside) were considered to test the capabilities of the THM (Figure 10); its accuracy was determined using the optimal threshold and a value of 0.5 was adopted. The comparison of the RMSE of the four zones is shown as Table 3. A few shortcomings were observed in applying the THM to some zones, especially the CBD zone. The RMSE through the THM and LSMA for the CBD zone was found to be 0.322 and 0.2597, respectively. The THM was unsuitable for the CBD zone primarily due to the existence of the shadows of tall buildings, which have similar spectral properties as water bodies, roads, and soil. The size and intensity of building shadows vary with solar illumination angle and miscellaneous building attributes. In CBD zones, the abundance of ISA through the LSMA was usually >0.5 and the THM was employed; however, the shadows in the high-resolution image, such as those from the GF-2, were always mistaken for water, and the accuracy decreased if these shadows were not removed before the OOM. Figure 10: Four validation zones: (**a**) urban–rural fringe zones, (**b**) areas with abundant bare soil, (**c**) central business district (CBD) areas, and (**d**) riverside. Furthermore, in order to compare the capabilities of the THM for different high-resolution imagery and understand the variations in the optimal threshold, two other high-resolution images, viz., a 16 m resolution GF-1 image and a 1 m resolution panchromatic GF-2 image, were adopted and analyzed through the THM. The RMSE of these high-resolution images at different thresholds are as Table 4.  
The RMSE at different thresholds demonstrated a \"U\" trend in both the 1 m resolution panchromatic GF-2 image and the 16 m resolution GF-1 image, same as that for the GF-2 image. The optimal threshold of the two high-resolution images was 0.3 and 0.6, respectively. Considering the three high-resolution images, i.e., panchromatic GF-2, GF-2, and GF-1, it was obvious that the optimal threshold through the THM increased as the resolution improved. According to [PERSON] [35], the global ISA of \(45.26\times 10^{4}\) km\({}^{2}\) was mainly distributed in central and southern North America, eastern Asia, and Europe, as well as coastal regions around the world. Since the cities in Europe and North America exposed a dramatic mosaic of impervious surface and green space composites in urban construction, the distribution of impervious surfaces in these local districts had its own characteristics, which may differ from that of China.