摘要
NSIDC发布的MODIS全球积雪面积产品采用的NDSI阈值为0.40,但在我国并没有验证,在区域积雪制图中仍然需要进行NDSI阈值选取的试验.选择祁连山中部山区常年积雪区作为研究区,利用SNOMAP方法从Landsat-ETM+影像中提取积雪图.通过与目视解译获取的积雪图作比较,该方法提取积雪面积总体精度超过96%,可将其作为地面真实积雪.然后选用MODIS 1B资料,采用NDSI方法得到研究区积雪图,通过改变NDSI阈值得到不同的MODIS积雪图与假设真实值Land-sat-ETM+积雪图进行对比.比较结果表明:NSIDC发布的MODIS积雪面积产品采用的NDSI阈值0.40偏高,造成研究区积雪面积的低估;通过对3个子研究区积雪图对比及统计分析,得出该区域的合理阈值为0.33.
Currently, the MODIS （Moderate Resolution Imaging Spectroradiometer） snow cover products have being widely applied to many researching fields, such as NWP （numerical weather prediction）, hydrology, climatology, and so on. The MODIS snow-mapping algorithm is essentially designed for the evaluation of the threshold value of the NDSI （Normalize Difference Snow Index） threshold value. The NDSI threshold of the MODIS snow cover products issued by the NSIDC （National Snow and Ice Data Center） is 0.40. The NDSI values of the MODIS scenes greater than or equal to 0.40 represent snow cover pixels. However, validation of the current NDSI threshold has being accomplished only by the measurements in the United States and Europe. In China there is not reliable NDSI threshold for the MODIS snow mapping and a credible threshold should be established. In the paper, the snow cover area in the middle Qilian Mountains was selected for stud- ying. First, the Landsat-ETM＋ snow cover maps were produced by the mothod of the ,SNOMAP. Then, the maps were compared with the ones derived by the manual photointerpretation classification technique. Overall agreement from a simple comparison of the number of snow pixels is high, about 96%. Thus, the Landsat-ETM＋ snow cover maps can be served as the ＂groudtruth＂, with which the snow cover maps of the study area extracted from the MODIS 1B measurements by NDSI method were compared. For the MODSI snow cover maps of the study areas, the NDSI threshold value for snow increases gradually for 0.25 to 0.40 in steps of 0.01. At last, the comparisons focused on comparing the MODIS snow cover maps following with NDSI threshold value and the Landsat-ETM＋ snow cover maps serving as absolute standard. The result suggests that the NDSI threshold for the MODIS snow cover products issued by the NSIDC, 0.40, is underestimated in the snow-covered area of the study areas. In the study areas, the credible NDSI threshold value is approximately 0.33.
出处
《冰川冻土》
CSCD
北大核心
2008年第1期132-138,共7页
Journal of Glaciology and Geocryology
基金
国家自然科学基金项目(40671040)
中国科学院西部行动计划(二期)项目(KZCX2-XB2-09-01)资助
