基于ICESat/GLAS的山东省SRTM与ASTER GDEM高程精度评价与修正

SRTM3和ASTER GDEM V2数据具有较高的空间分辨率和广泛的覆盖范围,对于地学研究具有重要意义;但在不同地形复杂度和地面覆盖物区域,两类数据的误差分布并不均匀。SRTM3和ASTER GDEM V2数据自公布以来,其精度修正一直是研究热点。然而大范围区域精度验证缺乏有效手段,传统方法可靠性差且数据获取成本较高。自ICESat-1数据公开以来,它们已成为SRTM3和ASTER GDEM V2精度评定的主要检核点。为此,本文以山东省为研究区域,借助ICESat-1评估了SRTM3和ASTER GDEM V2的高程精度,并根据插值误差曲面对两种DEM进行了修正。分析表明,原始SRTM和ASTER高程中误差分别为5.57 m和7.20 m,均高于标称精度;随着坡度的增大,高程精度呈降低的趋势。通过分析土地覆盖类型与误差分布关系表明:农田、灌丛土地类型精度较高;森林、湿地精度较低。分别采用反距离加权、普通克里金、地形转栅格和自然邻域插值方法构建误差曲面。结果表明:不同的插值方法构建的误差曲面的特征和精度也不同。其中,反距离加权修正的效果最佳,其次是地形转栅格和自然邻域,而普通克里金修正的效果最差。

Elevation Accuracy Evaluation and Correction of SRTM and ASTER GDEM in Shandong Province based on ICESat/GLAS

Shuttle Radar Topography Mission(SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) GDEM have a high spatial resolution and wide spatial coverage, which play an important role in many Earth researches. However, their error distributions are heterogeneous on different terrain types. In order to assess the elevation accuracy of the two DEMs, data from Geoscience Laser Altimeter System (GLAS) carried on the Ice, Cloud, and land Elevation Satellite (ICESat) are often used as the checkpoints due to their high accuracy. Taking Shandong Province as the research area, the accuracy of SRTM and ASTER GDEM are first evaluated by ICESat/GLAS in the years of 2003-2010 in this paper. Results indicate that the root mean squared errors (RMSEs) of SRTM and ASTER are 5.57 m and 7.20 m, respectively, which are much lower than the nominated accuracy. We further analyzed the effect of terrain slope and landscape type on the accuracy of SRTM and ASTER GDEM. Specifically, the study area was first divided into different sub-regions according to slope ranges (0~5°, 5~10°, 10~15°, 15~20°, 20~25°, 30~35°, 35~40°, 40~45°) and landscape types (farmland, shrub, forest, grassland, wetlands, water body), respectively. Then, the RMSE of each sub-region was computed and analyzed. We found that with the increasing of terrain slope, the accuracy of the two DEMs decreases, and under different land cover types, they also have different accuracy. More specifically, the two DEMs have a higher accuracy on farmland and shrub; while have a lower accuracy on forest and wet lands. To improve the accuracy of SRTM and ASTER, their error surfaces were first produced by interpolating the elevation differences between the DEM and randomly selected ICESat/GLAS data with the proportion of 90%. The interpolation methods include Inverse Distance Weight (IDW), Ordinary Kriging (OK), terrain-to-grid method (T2G) and Natural Neighborhood (NN). Then, the interpolated error surfaces were added to the original DEMs. Accuracy assessment of the improved SRTM and ASTER using the remaining 10% ICESat/GLAS demonstrates that IDW with the RMSEs of 2.20 m and 5.31 m is more accurate than the other interpolation methods. IDW is closely followed by T2G and NN. It is surprised to see that OK produces the worst results. Hence, SRTM and ASTER GDEM are improved with the IDW-based error surfaces. The ICESat-2 satellite was launched on September 15, 2018. It can emit 10,000 laser pulses per second, monitoring the height of glaciers and land in unprecedented detail. ICESat-2 collects elevation data over all surfaces spanning the world's frozen regions, forests, lakes, urban areas, and more. Thus, further researches will focus on improving the accuracy of SRTM and ASTER with the ICESat2 data.