ABSTRACT

Global digital elevation models (GDEMs) are one of the most important sources of elevation data. Like other spatial datasets, GDEMs are inevitably affected by various types of errors in both vertical and horizontal dimensions. One of the main sources of horizontal errors is georeferencing error which in steep areas intensifies the vertical error. Since horizontal and vertical errors are interwoven with one another, any error introduced in horizontal dimension influences its vertical counterpart. In this paper, for the first time, we intend to answer this question: to what extent the impact of georeferencing error is really important in vertical accuracy of GDEMs and their vertical comparison. Hence, the main purposes of this study are evaluation of georeferencing effects on (i) the vertical accuracy of GDEMs and (ii) vertical comparison between GDEMs. In this regard, Advanced Spaceborne Thermal Emission and Reflection (ASTER), Shuttle Radar Topography Mission (SRTM), and Advanced Land Observing Satellite (ALOS) World 3D-30 m (AW3D30) GDEMs over three different study areas (BumeHen, TazehAbad and Sainte-Maxime regions) were used and assessed. In order to assess the effects of georeferencing error on GDEMs accuracy, studies were conducted in two general categories, namely Local to Global (LG) and Global to Global (GG). In the first category, these effects were studied on vertical and horizontal differences of ASTER, AW3D30, and SRTM GDEMs with their corresponding reference DEMs (REFDEMs). In the second category, these effects were assessed on vertical and horizontal differences of ASTER and SRTM GDEMs with AW3D30 GDEM. To this end, prior to making GDEMs georeferencing consistent, they were vertically assessed. For the sake of consistency of GDEMs georeferencing, a registration process was performed and then, the registered GDEMs were vertically and horizontally assessed. After the registration process, in the first category, 4.4%, 13%, and 3.3% and in the second category 2.3%, 5%, and 1.2% improvement occurred on average root mean square error (RMSE) over all GDEMs for BumeHen, TazehAbad and Sainte-Maxime regions, respectively. These results refer to the fact that GDEM registration always increases their vertical accuracy, which is in fact originated from the improvement in GDEM georeferencing consistency. Experimental results also indicate that the improvement rate of vertical errors after the DEM registration process is directly related to terrain slope angle. From another perspective, we illustrated that in general the amounts of achieved improvements in GDEMs vertical accuracy are much smaller than their nominal accuracies. Finally, our results show that the effect of horizontal error on vertical comparison between GDEMs is unavoidable, but this amount of error can be ignored because the ratio of improvements occurred in the GDEMs with respect to their nominal accuracies is very low.

摘要

全球数字高程模型 (GDEM) 是高程数据最重要的来源之一。与其他空间数据集一样，GDEM 不可避免地受到垂直和水平维度上各种类型的错误的影响。水平误差的主要来源之一是地理配准误差，它在陡峭地区加剧了垂直误差。由于水平和垂直误差相互交织，水平尺寸中引入的任何误差都会影响其垂直对应物。在本文中，我们打算第一次回答这个问题：地理配准误差的影响在多大程度上对 GDEM 的垂直精度及其垂直比较非常重要。因此，本研究的主要目的是评估地理配准对 (i) GDEM 的垂直精度和 (ii) GDEM 之间的垂直比较的影响。在这方面，三个不同研究区域（BumeHen、TazehAbad 和 Sainte -最大区域）被使用和评估。为了评估地理配准误差对 GDEM 精度的影响，研究分为两大类，即局部到全局 (LG) 和全局到全局 (GG)。在第一类中，研究了 ASTER、AW3D30 和 SRTM GDEM 与其相应参考 DEM (REFDEM) 的垂直和水平差异的影响。在第二类中，这些影响是在 ASTER 和 SRTM GDEM 与 AW3D30 GDEM 的垂直和水平差异上进行评估的。为此，在使 GDEM 地理配准一致之前，对它们进行了垂直评估。为了保证 GDEM 地理配准的一致性，进行了注册过程，然后对注册的 GDEM 进行了垂直和水平评估。在注册过程之后，在第一类中，BumeHen 的所有 GDEM 的平均均方根误差 (RMSE) 分别提高了 4.4%、13% 和 3.3%，第二类提高了 2.3%、5% 和 1.2%，分别是 TazehAbad 和 Sainte-Maxime 地区。这些结果是指 GDEM 配准始终提高其垂直精度的事实，这实际上源于 GDEM 地理配准一致性的提高。实验结果还表明，DEM 配准过程后垂直误差的改善率与地形坡度角直接相关。从另一个角度来看，我们说明一般而言，GDEM 垂直精度的改进量远小于其标称精度。最后，我们的结果表明，水平误差对 GDEM 之间垂直比较的影响是不可避免的，但是这个误差量可以忽略不计，因为 GDEM 相对于标称精度的改进比率非常低。我们说明，一般而言，GDEM 垂直精度的改进量远小于其标称精度。最后，我们的结果表明，水平误差对 GDEM 之间垂直比较的影响是不可避免的，但是这个误差量可以忽略不计，因为 GDEM 相对于标称精度的改进比率非常低。我们说明，一般而言，GDEM 垂直精度的改进量远小于其标称精度。最后，我们的结果表明，水平误差对 GDEM 之间垂直比较的影响是不可避免的，但是这个误差量可以忽略不计，因为 GDEM 相对于标称精度的改进比率非常低。