Standardization of figures and assessment procedures for DTM verticalaccuracy

Abstract

Digital Terrain Models (DTMs) are widely used in many sectors. They play a key role in hydrological risk prevention, risk mitigation and numeric simulations. This paper deals with two questions: (i) when it is stated that a DTM has a given vertical accuracy, is this assertion univocal? (ii) when DTM vertical accuracy is assessed by means of checkpoints, does their location influence results? First, the paper illustrates that two vertical accuracy definitions are conceivable: Vertical Accuracy at the Nodes (VAN, the average vertical distance between the model and the terrain, evaluated at the DTM's nodes) and Vertical Accuracy at the interpolated Points (VAP, in which the vertical distance is evaluated at the generic points). These two quantities are not coincident and, when they are calculated for the same DTM, different numeric values are reached. Unfortunately, the two quantities are often interchanged, but this is misleading. Second, the paper shows a simulated example of a DTM vertical accuracy assessment, highlighting that the checkpoints' location plays a key role: when checkpoints coincide with the DTM nodes, VAN is estimated; when checkpoints are randomly located, VAP is estimated, instead. Third, an in-depth, theoretical characterization of the two considered quantities is performed, based on symbolic computation, and suitable standardization coefficients are proposed. Finally, our discussion has a welldefined frame: it doesn't deal with all the items of the DTM vertical accuracy budget, which would require a much longer essay, but only with one, usually called fundamental vertical accuracy.

Assessment of Leica CityMapper-2 LiDAR Data within Milan’s Digital Twin Project

“…It is important to know that relying solely on independent DSMs/DTMs may not always provide accurate or reliable results [40,41]. This approach introduces additional inaccuracies related to discretization and interpolation, and it is necessary to carefully evaluate the error budget concerning the location of the ground point [20]. Similarly, surveying isolated points using a conventional GNSS NRTK strategy may not achieve the necessary level of accuracy for the ground truth [27,28].…”

“…This method exploits a small number of isolated points to qualify millions of LiDAR points, which results in a less accurate process. Furthermore, the incorrect use of this approach, which confuses, for instance, the accuracy at the nodes with the accuracy at the interpolated points, results in different numeric results [20]. Historically, this method was chosen due to the relatively sparse spatial density of LiDAR points.…”

Preface to the special issue: the role of geomatics in hydrogeological risk

“…photogrammetry (Borgogno Mondino 2014;Scaioni et al 2014), structure from motion (Forlani et al 2013), optical remote sensing (Franci et al 2014), laser scanning (Barbarella et al 2013), global navigation satellite systems (GNSS) (Cina & Piras 2014), synthetic aperture radar (SAR) (Frangioni et al 2014;Nascetti et al 2014;Vittuari et al 2014) and geographic information systems (GISs) as decision support using advanced models (Cencetti et al 2014;Federici et al 2014). A digital terrain model (DTM) is fundamental to successive analysis; Casella and Franzini (2014) have taken the vertical information and investigated, respectively, on high-resolution DTM accuracy and the dynamic aspect (movement) of the vertical component of the vertical component of the DTMs.…”