Relative Spectral Mixture Analysis (RSMA) - MODIS, Australia coverage

RSMA measures change in the relative contributions of photosynthetic vegetation (PV, or GV green vegetation), non-photosynthetic vegetation (NPV) and soil reflectance compared to a baseline date. These spectral changes correspond to changes in fractional cover relative to the baseline date. Full details on the RSMA method are presented in Okin (2007). One of the key advantages of the RSMA, its insensitivity to changes in soil spectra, is a result of the fact that it does not require us to know the soil reflectance profile for a region. This strength is also the cause of a major weakness in RSMA. Since the measure is relative to a baseline date, and the absolute cover levels for every pixel are unknown at the baseline, the RSMA does not convey the absolute cover levels at any other point in time. However, if the absolute cover levels are known at any point in time, it is theoretically possible to convert the RSMA to absolute relative spectral mixture analysis (ARSMA).

As with all products derived from passive remote sensing imagery, this product represents the world as seen from above. Therefore, the cover recorded by this product represent what would be observed from a bird's-eye-view. Therefore, dense canopy may prevent observation of significant soil exposure.

相对光谱混合分析（Relative Spectral Mixture Analysis, RSMA）通过与基准日期对比，量化光合植被（photosynthetic vegetation, PV，即绿色植被GV）、非光合植被（non-photosynthetic vegetation, NPV）与土壤反射率的相对贡献变化。此类光谱变化对应相较于基准日期的分数覆盖度变动。有关RSMA方法的完整技术细节已在Okin（2007）的研究中详细阐述。RSMA的核心优势之一是对土壤光谱变化不敏感，这一特性源于其无需获取某一区域的土壤反射率剖面。但该优势同时也成为RSMA的重大局限：由于该度量方法以基准日期为参照，且基准日期下各像素的绝对覆盖度未知，因此RSMA无法表征任意时刻的绝对覆盖水平。不过，若已知任一时刻的绝对覆盖度，理论上可将RSMA转换为绝对相对光谱混合分析（absolute relative spectral mixture analysis, ARSMA）。
与所有源自被动遥感影像的产品一致，本产品呈现的是自上而下观测到的地球场景。因此，该产品所记录的覆盖度对应鸟瞰视角下的观测结果。浓密植被冠层可能会遮挡显著的土壤裸露区域。