## 5 Discussion  
Mountain ecosystem protection and mountain development hold great significance to global human community [2]. Remote sensing plays an irreplaceable and important role in monitoring mountain areas on a regular basis, especially for those spatially inaccessible areas with highly heterogeneous and complex land covers. However, topographic undulation modifies the configuration of STS geometry as well as solar irradiance reaching the surface, and makes topographic distortion and BRDF effect pronounced in remotely sensed images. This challenging problem hampers the quantitative application of remote sensing and proves to be rather serious in moderate and high-resolution imagery, like Landsat-5 TM images. More and more researches show that it is necessary to solve this problem without the Lambertian assumption [18, 19, 20]. However, Landsat-5 TM provides scarce multi-angular observations to enable the extraction of BRDF information, and other data sources of matching BRDF information for the land surface are not readily available. In literature [20], [PERSON] _et al._ (2012) applied 500 m resolution MODIS BRDF parameters to account for the BRDF effect in corresponding TM images, and achieved satisfactory results. Nevertheless, averaging BRDF parameters over the whole TM scene and using the same BRDF shape function for all the land cover types may need further discussion when non-homogeneous mountainous areas are considered. [PERSON] (2013) pointed out that MODIS BRDF parameters could only serve to provide a general, average correction for BRDF effects in Landsat TM imagery, and they do not carry any further information about BRDF effects on each Landsat pixel [54]. Thus, model presented in this paper, instead of using MODIS BRDF parameters for corresponding TM pixels, employed NDVI thresholds to divide land targets into eleven groups and each group used its own averaged BRDF parameters, and it had been experimented on Landsat TM images acquired under different sun elevation angles and shown to further improve the topographic correction effect. Nevertheless, some critical points still need further discussion. Figure 7: Correlation coefficients between reflectance and illumination condition for different aspect intervals before and after topographic correction; (**a**) is for the whole spring scene; and (**b**) for forest in spring scene. In this paper, correlation between image reflectance and IC was applied to quantitatively indicate the topographic correction effect. Since the imaging process of sensors aboard satellite is quasi-instantaneous, the direction of solar illumination depicted by solar zenith and azimuth angles during the image acquisition is unique. The aim of topographic correction is to model the process and compensate for the unevenly distributed sunlight, induced by instant and unidirectional solar illumination and terrain ruggedness, which leads to distortion of radiance and reflectance. Thus, image reflectance after successful correction is expected to show almost no correlation with the instantaneous illumination condition, as demonstrated in Figure 5. However, in the long term, the land covers, especially vegetation, do show some correlation with slope orientation, because of the aspect-related differences in abiotic conditions [55, 56, 57], like moisture and temperature. Theoretically, aspect-related correlation between reflectance and illumination condition still exists even after successful topographic correction. To prove that, aspect angles of terrain were divided into eight intervals, with resolution of 45 degrees. Then, taking spring TM scene for instance, correlation between NIR band reflectance and IC before and after correction was examined for each aspect interval. For the whole image (Figure 7a), correlation coefficient reached maximum of about 0.35 for Northwest and West aspect intervals before correction, while only 0.05 for South and East intervals.