COMPARISON BETWEEN ABSOLUTE AND RELATIVE POSITIONAL ACCURACY ASSESSMENT - A CASE STUDY APPLIED TO DIGITAL ELEVATION MODELS

Abstract This paper presents a comparative study between the absolute and relative methods for altimetric positional accuracy of Digital Elevation Models (DEM). For the theoretical basis of this research, the definitions of accuracy (exactness) and precision, as well the concepts related to absolute and relative positional accuracy were explored. In the case study, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission (SRTM) DEM were used. In the analysis of the absolute accuracy, 6,568 ground control points from GNSS orbital survey were used, collected through relative-static method. In the relative accuracy, it was used as reference DEM with spatial resolution of 5 meters generated by stereophotogrammetrical process for the Mapping Project of Bahia (Brazil). It was concluded that, once the accuracy of the reference DEM is better than the other two evaluated DEM, the results of the classification for the PEC-PCD for the relative evaluation are equal to or better than the absolute evaluation results, with the advantage to being able to verify the pixel population of the evaluated models, which makes it possible to identify outliers, distortions and displacements, including delimiting regions, which is much less likely with a limited set of control points.

摘要 本文针对数字高程模型（Digital Elevation Models, DEM）的高程位置精度的绝对评估方法与相对评估方法开展对比研究。本研究的理论基础涵盖精度（精确性）与精密度的定义，以及与绝对、相对位置精度相关的核心概念。在案例研究中，采用了先进星载热发射和反射辐射计（Advanced Spaceborne Thermal Emission and Reflection Radiometer, ASTER）与航天飞机雷达地形测绘任务（Shuttle Radar Topography Mission, SRTM）生成的DEM。在绝对精度分析环节，使用了通过相对静态法采集的6568个全球导航卫星系统（Global Navigation Satellite System, GNSS）轨道测量地面控制点。相对精度分析则以巴西巴伊亚州测绘项目中通过立体摄影测量流程生成的空间分辨率为5米的参考DEM作为基准。研究结果表明，当参考DEM的精度优于其余两款待评估DEM时，相对评估所采用的PEC-PCD分类结果等同于或优于绝对评估结果，其优势在于可对待评估模型的像素群体进行核验，从而能够识别异常值、畸变与位移，还可划定异常区域——这一点在控制点数量有限的绝对评估中实现难度极大。