Vegetation Index Algorithm for Vegetation Monitoring in an Arid and Semi Arid Land

在利用卫星数据计算NDVI（归一化植被指数）或SAVI（土壤调整植被指数，Huete, 1988）估算植被覆盖度时，像元内的植被密度会影响结果。为了消除这种不良影响，提出了最优植被指数（OPVI）。在该方法中，每个像元内的植被覆盖度由ASTER的波段4与波段5 （TM5和tm7，对于Landsat TM）的辐射反射率比计算。然后，如果反射率大于或小于某个临界值，则选择NDVI或SAVI计算每个像元的植被指数。为了确定临界值，在实验室中，从植物和土壤的光谱反射率作为植物覆盖面积与土壤覆盖面积之比的函数，研究了植被覆盖度与反射率的关系。然后，通过比较…More NDVI和SAVI得到的结果确定临界值。利用塔克拉玛干沙漠绿洲内及绿洲周围的卫星数据，将OPVI方法与NDVI和SAVI方法的结果进行了比较，因为预计塔克拉玛干沙漠的植被变化范围很广。图中为本研究的伪彩色图像以及目标区域的NDVI、SAVI和OPVI地图。在NDVI图中，绿洲内大部分区域的植被指数高于OPVI图。绿洲外植被密度较低，植被指数较高。另一方面，即使在绿洲内部，整个SAVI的植被指数也被评价为较低。特别是，即使在实地调查证实的果树园和棉田等植被密度高的地区，从SAVI获得的植被也过低。结果表明，OPVI比单一的NDVI或SAVI能更好地反映干旱区的实际植被覆盖度。此外，OPVI还可以有效地调查荒漠化问题，这在世界上许多干旱和半干旱地区正在成为问题。少

In estimating the vegetation coverage from NDVI (Normalized Vegetation Index) or SAVI (Soil Adjusted Vegetation Index, Huete, 1988) which are calculated from the satellite data, the vegetation density within a pixel affect the results. To eliminate this undesirable effect, an optimum vegetation index (OPVI) is presented. In this method, the vegetation coverage within each pixel is calculated from the reflectance ratio of radiance in band 4 band to that in band 5 of ASTER (TM5 and TM 7, in case of Landsat TM). Then, if the reflectance ratio is larger or smaller than a critical value, select either NDVI or SAVI for calculating the vegetation index in each pixel. For determining the critical value, the relationship between vegetation coverage and the reflectance ratio are studied from the spectral reflectance of plants and soils as a function of ratio of coverage area of plants to that of soils in the laboratory. Then, the critical value is determined by comparing the results obtained by … More NDVI and SAVI.Comparison of results obtained by OPVI method with those by both NDVI and SAVI are made by using the satellite data collected in and around oases in Taklimakan Desert in China since the vegetation is expected to vary over the wide range. Figure shows the false color image and the NDVI, SAVI, and OPVI maps of the object area for this study. Most of the area inside the oasis in the NDVI map shows higher vegetation index than in OPVI.Further, a high vegetation index in the NDVI is shown outside the oasis which has a lower vegetation density. On the other hand, the vegetation index in the SAVI, as a whole, is evaluated to be a low even inside the oasis. Particularly, even in a high vegetation density such as fruit gardens and cotton fields as confirmed by in situ survey, the vegetation obtain from the SAVI is too low.The results show that OPVI gives a better indicator of actual vegetation cover in an arid area than single NDVI or SAVI alone. Further, OPVI becomes effective to investigate the desertification, which is becoming at issue in many arid and semi-arid areas in the world. Less