River Levels Derived with CryoSat-2 SAR Data Classification - A Case Study in the Mekong River Basin
River Levels Derived with CryoSat-2 SAR Data Classification - A Case Study in the Mekong River Basin
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DOI：
10.3390/rs9121238
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发表时间：
2017-11
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通讯作者：
E. Boergens;K. Nielsen;O. Andersen;D. Dettmering;F. Seitz
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作者：
E. Boergens;K. Nielsen;O. Andersen;D. Dettmering;F. Seitz
在这项研究中，我们利用湄公河流域的CryoSat-2 SAR(延迟多普勒合成孔径雷达)数据来估计水位。与传统的脉冲受限雷达测高相比，利用CryoSat-2SAR数据可以更准确地观测到中小尺度内陆水域，这是因为沿航迹足迹较小。然而，即使有了这些合成孔径雷达数据，估计中型河流(宽度小于500米)上的水位仍然具有挑战性，因为只有很少的连续观测。随着河流变小，使用陆地-水掩膜的目标识别往往会失败。因此，我们开发了一种分类方法，仅根据数据将观测数据划分为水和陆地回报。分类采用非监督分类算法，基于从合成孔径雷达和距离积分功率(RIP)波形中提取的特征。分类后，代表水和土地的类别被识别出来。将湄公河流域划分为上游、中游和下游三个地理区域，效果更好。下一步将使用被归类为水的测量来估计湄公河网络中每一条河流的水位。得到的水位被验证，并与测量数据、环境数据和使用陆地-水掩模得出的CryoSat-2水位进行了比较。通过分类得出的CryoSat-2水位比陆地-水掩模(1700个有2%的异常值，1500个有7%的异常值)在上游区域有更少的异常值，从而产生更有效的观测。验证中使用的年差异的中位数是在所有测试区域中，CryoSat-2分类结果小于Envisat或CryoSat-2陆地-水掩模结果(整个研究区域分别为0.76米、0.96米和0.83米)。总体而言，在拥有中小河流的上游地区，分类方法在获得可靠的水位观测值方面比在河流较宽的中游地区更有效。
In this study we use CryoSat-2 SAR (delay-Doppler synthetic-aperture radar) data in the Mekong River Basin to estimate water levels. Compared to classical pulse limited radar altimetry, medium- and small-sized inland waters can be observed with CryoSat-2 SAR data with a higher accuracy due to the smaller along track footprint. However, even with this SAR data the estimation of water levels over a medium-sized river (width less than 500 m) is still challenging with only very few consecutive observations over the water. The target identification with land–water masks tends to fail as the river becomes smaller. Therefore, we developed a classification approach to divide the observations into water and land returns based solely on the data. The classification is done with an unsupervised classification algorithm, and it is based on features derived from the SAR and range-integrated power (RIP) waveforms. After the classification, classes representing water and land are identified. Better results are obtained when the Mekong River Basin is divided into different geographical regions: upstream, middle stream, and downstream. The measurements classified as water are used in a next step to estimate water levels for each crossing over a river in the Mekong River network. The resulting water levels are validated and compared to gauge data, Envisat data, and CryoSat-2 water levels derived with a land–water mask. The CryoSat-2 water levels derived with the classification lead to more valid observations with fewer outliers in the upstream region than with a land–water mask (1700 with 2% outliers vs. 1500 with 7% outliers). The median of the annual differences that is used in the validation is in all test regions smaller for the CryoSat-2 classification results than for Envisat or CryoSat-2 land–water mask results (for the entire study area: 0.76 m vs. 0.96 m vs. 0.83 m, respectively). Overall, in the upstream region with small- and medium-sized rivers the classification approach is more effective for deriving reliable water level observations than in the middle stream region with wider rivers.