## 5 Discussion
After topographic correction by Li model and the proposed improved model, correlations decreased except for East, Southeast and South intervals. This implies reflectance still correlates with IC to various extents for different aspect intervals, though topographic correction has successfully removed the impacts by unidirectional illumination for the whole image scene. The same were examined and presented for forest pixels solely in Figure 7b. Correlation also reached maximum (0.4) in Northwest interval, and remained about 0.25 for other intervals before correction. They decreased to about 0.1 for almost all intervals after topographic correction by Li model and model proposed. This implies subtle correlation still exists between forest reflectance and IC for each aspect interval, which may be attributed to the insufficient topographic correction or the influence on vegetation growth states by abiotic factors [58]. Model improved in this paper employs MODIS BRDF products as ancillary data to account for the anisotropic behavior of surface reflectance recorded in TM images. It provides methodology of considering BRDF effects in topographic correction for moderate and high-resolution images. However, spatial resolution difference may impact the topographic correction effects to some extent. A MODIS pixel with nominal resolution of 500m at nadir (corresponding to about 278 TM pixels) is often mixture of different land cover types in TM scene [59]. Though linear additive kernel-driven model is applied to generate MCD43A1 products, reflectance of natural land targets in real scene may not simply be represented by the sum of products of end-member reflectance and corresponding area [60], taking into account the topographic relief and mutual-shadowing effect. This will potentially influence the model's ability to reduce topographic effect. Apart from this, [PERSON] reported (2013) that MODIS BRDF parameters may capture no local information about BRDF effect on TM pixel scale due to their large resolution difference [54]. Though the report is based on the tests solely performed on rather flat areas, it may be also the case in rugged areas. For mountainous areas, the angular configurations of the sun and sensor relative to the land target vary more severely. Therefore, much bigger part of the BRDF shape being modeled by the MODIS BRDF processing will be necessary when using MODIS BRDF parameters to account for BRDF effect in mountainous TM images. This is not like the situation in which only a very small part (mostly central part, near nadir) of the BRDF shape is used for BRDF correction of TM images of flat areas, as described by [PERSON]. Thus, uncertainties of employing MODIS BRDF parameters to account for angular effect in Landsat TM imagery still exist and need further research. In the future, it is anticipated that BRDF information matching TM spatial resolution will be derived from satellite constellations with short revisit cycle and multi-angular viewing ability, like China's HJ-1 A/B [61, 62] and the forthcoming European Space Agency (ESA)'s Sentinel-2A/B [63, 64]. It will be advisable to interpret BRDF effect in TM scenes using matching BRDF data. Besides, bandwidths of MODIS images are narrower than corresponding TM ones and their spectral response functions differ, which may also potentially affect the accuracy of methodology proposed [24]. Typically, there will be several land cover types in the domain of a TM scene as to mountain ecosystem. For instance, TM scenes covering the study area selected in this paper include forests, shrubs, grasslands, cultivated lands, residential lands and barrens (Figure 1b). Different land covers usually show different BRDF effects [65, 66], which should be considered in topographic correction. However, the availability of matching and precise BRDF information and land cover map is restricted.