## 6 Conclusions  
Optical remotely sensed images in mountainous areas are subject to distortions induced by topographic effect. It impacts accuracy of image applications in various fields such as biophysical parameter inversion, time series data analysis and change detection to a large extent. Especially when time series images are employed for seasonal change information extraction, topography-induced reflectance distortion may mix with and even obscure the real seasonal changes. Thus, topographic correction is a preliminary and necessary step for image quantitative application. This paper proposed an improved physics-based topographic correction model under consideration of anisotropic behavior of surface scattering. Direct and diffuse solar irradiance and neighboring terrain irradiance received by rugged surfaces are computed based on 6S radiative transfer model and Hay model. NDVI thresholds are utilized to divide land targets into eleven groups, and each group employs corresponding averaged MODIS BRDF model parameters to solve the equations and gain the corrected reflectance. Topographic correction model proposed was tested with two Landsat-5 TM scenes of severely rugged mountainous areas acquired under different sun elevation angles, and compared with Sandmeier model (representative of models with Lambertian assumption) and Li model (representative of models without Lambertian assumption). Experiment results showed that model proposed performed very well, with reflectance in sun-averted slopes being enhanced and topographic effects being greatly suppressed. Also, correlation coefficients between NIR band reflectance and illumination condition reduced almost to zero, and coefficients of variance for illuminated forests decreased. Nevertheless, there are still some sources of uncertainty for the model proposed here. For instance, the model relies on MODIS BRDF products to account for the non-Lambertian property of land surface, and the accuracy of topographic correction depends largely on MODIS BRDF model parameters and DEM. Along with the development of high spatial and temporal resolution images, such as images from HJ-1 A/B and Sentinel-2 A/B, some of these problems may be solved, and the model proposed here may be further modified and operationally employed in the future.  
This research was funded jointly by the National Natural Science Foundation project of China (41271433), the International Cooperation Key Project of CAS (Grant No. GJHZ201320), the International Cooperation Partner Program of Innovative Team, CAS (Grant No. KZZD-EW-TZ-06), and the \"Hundred Talents\" Project of Chinese Academy of Sciences (CAS). We are grateful to all the contractors, image providers and the anonymous reviewers for their valuable comments and suggestions.  
All authors have made major and unique contributions. [PERSON] designed the framework of this research work, and finished the final version of this paper. [PERSON] performed the data generation, data validation analysis, results interpretation, and drafted the preliminary version of this paper. [PERSON] took part in data comparison analysis, results interpretation, and manuscript revision. [PERSON] performed the data collection and preprocessing.  
The authors declare no conflict of interest.