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

在利用卫星遥感数据计算植被覆盖度时，像元内的植被密度会影响计算结果。为了消除这种不利影响，最佳植被指数（OPVI）。在该方法中，每个像元内的植被覆盖度由ASTER（TM 5和TM 7，对于Landsat TM）的波段4波段与波段5的辐射率的反射比计算。然后，如果反射率比大于或小于一个临界值，选择NDVI或SAVI计算植被指数在每个像素。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之比的关系。然后，通过比较由以下方法获得的结果来确定临界值： 关于我们 利用中国塔克拉玛干沙漠绿洲及其周边地区的卫星遥感数据，将OPVI方法与NDVI和SAVI方法的结果进行了比较。图为本研究对象区域的假彩色图像和NDVI、SAVI和OPVI图。绿洲内部的植被指数在NDVI图上大部分高于OPVI图，绿洲外部的植被密度较低，植被指数也较高。另一方面，在SAVI的植被指数，作为一个整体，被评价为低，即使在绿洲内部。特别是在植被密度较高的地区，如果园、棉田等，SAVI得到的植被覆盖度较低，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成为有效的调查荒漠化，这是在世界上许多干旱和半干旱地区的问题。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