为了抑制采样点中粗差对数字高程模型(digital elevation model,DEM)建模的影响,以较高精度的多面函数(multi-quadric,MQ)为基函数,由改进Huber损失函数和权重惩罚项组成目标函数,发展了MQ抗差插值算法(MQ-H)。通过优化MQ-H目标函数,采样点权重计算最终转换为方程组求解。以数学曲面为研究对象,将MQ-H计算结果与传统MQ及最小绝对偏差MQ(MQ-L)进行比较,结果表明:当采样误差服从正态分布时,MQ-H计算精度与传统MQ相当,而远高于MQ-L;当采样误差服从拉普拉斯分布时,MQ-H计算精度略高于MQ-L及传统MQ;当采样点被粗差污染时,MQ-H计算精度远高于传统MQ及MQ-L。在实例分析中,以无人遥测飞艇立体像对获取的地面离散高程点为基础数据,基于MQ-H构建测区DEM,并将计算结果与传统插值算法,如反距离加权(inverse distance weighting,IDW)、普通克里金(ordinary Kriging,OK)和专业DEM插值软件ANUDEM(Australian National University DEM)进行比较,结果表明,传统插值方法在不同程度上受采样点中异常值或偶然误差影响,而MQ-H受异常值影响较小,且能准确捕捉到地形细节信息。
In this paper,we propose a robust multi-quadric method(MQ-H)based on Huber loss function to conduct interpolations of contaminated spatial points,especially those derived from remote-sensing techniques.The objective function of the MQ-H has two main parts;an improved Huber loss function and a regularized penalty term used to improve robustness and avoid overfitting,respectively.A mathematical surface,subject to model error with different distributions,was employed to comparatively analyze the robustness of the MQ-H,the classical MQ,and a least absolute deviation based MQ(MQ-L).The results indicated that when sample errors follow a normal distribution or a Laplacian distribution,the performance of MQ-H is comparatively better than those of MQ,and more accurate than MQ-L.For sample errors with a contaminated normal distribution and Cauchy distribution,MQ-H is more robust than MQ-L and MQ.Moreover,MQ with the improved Huber loss function is superior to MQ with the classical Huber loss function.A real-world example of DEM construction with stereo-image-derived elevation points indicates that compared to the classical interpolation methods including IDW(inverse distance weighting),OK(ordinary Kriging)and ANUDEM(Australian National University DEM),MQ-H has a better ability to reduce the impact of outliers while maintaining subtle terrain features suitable for qualitative analysis.