Study of estimating directional vegetation fractional cover using remote sensing method

As an important index of measuring vegetation's density, vegetation fractional cover (VFC) has been an important subject of vegetation remote sensing. It is useful not only in land use mapping and supervising environmental change but as a input parameter, it is of great significance for transpiration model and land surface temperature model. There are two factors that we must take into account when we retrieve e VFC by means of remote sensing. One is land surface reflectivity and the other is vegetation spectrum. However, these two factors are all extremely influenced by vegetation structure, solar zenith angle and view angle, etc. So in some cases, we must consider the changes of VFC which are caused by the change of solar angle, view angle and vegetation itself. We call this VFC as directional vegetation cover. The study of directional VFC will improve the precision of relevant models and will contribute to multi-angle vegetation remote sensing. An intensive experiment was carried out in 2003 in YuCheng experimental station In our experiment, directional digital pictures of maize with four distribution (north-south row edge, center, equality) were required at five viewing angles of 4-5,60,90,120,135 from east horizontal direction by a digital camera. At the same time, vertical reflectance from nadir direction and leaf area index (LAI) was also required. Vertical vegetation covers were estimated by using the existing models. In contrast with the true values which were got from our digital pictures worked, we found the better model estimating vegetation fractional cover. In our work, because it was difficult in differentiating between shadowed vegetation and shadowed soil with traditional supervised classification method, the true VFC values were obtained from digital pictures which were disposed of by Adobe Photoshop and the absolute precision were limited within five percent. In addition, we analyzed some influencing parameters of directional vegetation cover including LAI, view zenith angle and vegetation distribution. In a word, our study only was the primary study of directional vegetation cover. It still was affected by some other factors and needed further research.