In the study, the authors used the MODIS satelliteimagedata and the thermal inertia method to analyze the permafrost on the northeastern Qinghai-Tibet Plateau.
研究中利用MODIS卫星图像的数据,借助热惯量方法对青藏高原东北部的多年冻土进行了分析。
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Free images can also be old and if countries want a certain image it may well be outside the remit of free data access, so they have to pay for a satellite to take it.