Project

ENC Sounding Selection Validation

Goal: The development of algorithms for the validation of the shoal-biased selection of charted soundings.

Date: 1 December 2017

Updates
0 new
0
Recommendations
0 new
0
Followers
0 new
5
Reads
1 new
60

Project log

Christos Kastrisios
added 2 research items
Timely and accurate identification of change detection for areas depicted on nautical charts constitutes a key task for marine cartographic agencies in supporting maritime safety. Such a task is usually achieved through manual or semi-automated processes, based on best practices developed over the years requiring a substantial level of human commitment (i.e., to visually compare the chart with the new collected data or to analyze the result of intermediate products). This work describes an algorithm that aims to largely automate the change identification process as well as to reduce its subjective component. Through the selective derivation of a set of depth points from a nautical chart, a triangulated irregular network is created to apply a preliminary tilted-triangle test to all the input survey soundings. Given the complexity of a modern nautical chart, a set of feature-specific, point-in-polygon tests are then performed. As output, the algorithm provides danger-to-navigation candidates, chart discrepancies, and a subset of features that requires human evaluation. The algorithm has been successfully tested with real-world electronic navigational charts and survey datasets. In parallel to the research development, a prototype application implementing the algorithm was created and made publicly available.
The compilation of nautical charts comprises a number of tasks that are often monotonous, time consuming, and, as such, prone to human error. A long-term goal of the hydrographic community has thus been the automation of the process due to the unquestionable advantages of automation to the accuracy, reliability, and consistency of products for a reduced cost-to-productivity ratio. One of the tasks that has received the attention of automation efforts is the compilation of bathymetry on charts and more precisely the generation and generalization of depth curves and the selection and validation of charted soundings. There is, however, much room for improvement with current methods. Charted soundings and depth curves complement each other in maintaining and emphasizing the morphological details and characteristic features of the seafloor on charts. They are derived from a more detailed dataset, either survey data or a larger scale chart, with generalization. Once the cartographer delineates the depth curves, he/she makes the selection of soundings to be charted following established cartographic practice rules (see, e.g., IHO 2017; NOAA 2018). Both tasks may be performed either fully manually, or partially manually through a computer-assisted method. The goal is for the cartographer to retain the morphology and the characteristic features of the seabed and at the same time to honor the overarching principle that the expected depth (based on the charted bathymetric information) must not appear, at any location, deeper than the source information. According to S-4 (IHO, 2017), for well surveyed areas the “shoal-biased” pattern of charted soundings is achieved through the “triangular method of selection”, where: • No actual sounding exists within a triangle of selected soundings which is less than the least of any of the soundings defining the edges of the triangle (hereinafter: triangle test); and • No source sounding exists between two adjacent selected soundings forming an edge of the triangle which is less than the lesser of the two selected soundings (hereinafter: edge test). Motivated by the need for automated tools that perform consistently and satisfactorily in every geographic situation, and in the context of a developing project for a fully automated solution in nautical chart production, this paper presents our research work on the development of a comprehensive solution for the validation of the shoal-biased pattern of charted bathymetric information. This includes an algorithmic implementation of the triangle test that eliminates the false positives of previous implementation efforts near and within linear features (see Figure 1), as well as the first automated implementation of the edge test in the literature. For the above implementations we incorporate the available bathymetric information on the chart, e.g., coastlines (natural and man-made), depth curves, soundings, obstructions, and wrecks (see Kastrisios and Calder 2018; Kastrisios et al. 2019). The presented work also illustrates the importance of the edge test in the validation of the charted bathymetric information as it can identify shoals that the triangle test may not identify (see Figure 2), thereby proving that the edge test must not be disregarded by cartographers in the validation process. However, the two tests share the intrinsic limitation of missing discrepancies between the charted and source bathymetric information, even if they are significant (see Figure 3). Thus, a fully automated solution based on a verbatim interpretation of the two tests as written in S-4 does not seem feasible. Therefore, we propose a surface-based validation test that will account - at the appropriate charting resolution - for the configuration of the seabed that would be mentally reconstructed by the mariner from the charted bathymetric information. This is expected to lead to a more realistic representation of the submarine relief and its navigationally significant features on charts from the available source information (see Kastrisios et al. 2019).
Christos Kastrisios
added a project goal
The development of algorithms for the validation of the shoal-biased selection of charted soundings.
 
Christos Kastrisios
added a research item
Charted bathymetry encompasses the cartographer's effort to accurately describe submarine topography based on the source information. A key factor in the selection process is that, at any location, the charted features do not give a false impression that the water depth is deeper than it actually is. Our research work focused on development of an automated solution for the identification of locations where this goal is violated. We present the research work from conceptualization to the development of an ArcGIS Toolbox named "Sounding Validation Toolbox" that consists of three automated tools, as well as the challenges and results in the process of making it operational with the Marine Chart Division of NOAA/Office of Coast Survey.
Christos Kastrisios
added a research item
The nautical chart is one of the fundamental tools in navigation used by mariners to plan and safely execute voyages. Its compilation follows strict cartographic constraints with the most prominent being that of the safety. Thereby, the cartographer is called to make the selection of the bathymetric information for portrayal on charts in a way that, at any location, the expected water depth is not deeper than the source information. To validate the shoal-biased pattern of selection two standard tests are used, i.e., the triangle and edge tests. To date, some efforts have been made towards the automation of the triangle test, but the edge test has been largely ignored. In the context of research on a fully automated solution for the compilation of charts at different scales from the source information, this paper presents an algorithmic implementation of the two tests for the validation of selected soundings. Through a case study with real world data it presents the improved performance of the implementation near and within depth curves and coastlines and points out the importance of the edge test in the validation process. It also presents the, by definition, intrinsic limitation of the two tests as part of a fully automated solution and discusses the need for a new test that will complement or supersede the existing ones.
Christos Kastrisios
added a research item
The selection of soundings to be shown on nautical charts is one of the most important and complicated tasks in nautical cartography. From the vast number of source soundings the cartographer is called to select all those important for the safety of navigation and to verify the “shoal biased” pattern of selection against the source soundings. A long-term goal of the cartographic community has been the automation of the tasks involved in nautical chart production, including that of the selection and validation of charted soundings. With the aim to contribute to that effort, this paper presents an implementation of the triangle test for the automated validation of selected soundings which has improved performance on the detection of shoals near depth curves and coastlines.
Christos Kastrisios
added a research item
Depth curves and charted soundings are two structural components of nautical charts, both derived from a more detailed dataset through generalization. Once depth curves are generated, the cartographer makes a selection of soundings that complements the depth curves and other features carrying bathymetric information in the adequate representation of the seabed morphology at the scale of the product. The selection of charted soundings – which is currently either performed fully manually, or partially manually using a computer-assisted solution – must meet the safety constraint, i.e., that no source sounding exists that is shoaler than what the mariner would expect by mentally interpolating the charted bathymetric information. According to International Hydrographic Organization S-4 publication, for well-surveyed areas that is achieved through the “triangular method of selection” and consists of two tests: the Triangle Test and the Edge Test. There are currently no fully automated solutions, so, with the ultimate goal of supporting “cartography at point of use”, we consider the problem of automating the validation of shoal-selected soundings. We discuss an implementation of the triangle test with improved performance near linear features, the first automated implementation of the edge test, and that the edge test may identify shoals that the triangle test fails to detect, confirming its significance in the validation process. We demonstrate an “intrinsic” limitation of the two tests that makes infeasible a fully automated solution based solely on the two tests. Finally, to overcome the intrinsic limitation, we propose a new validation test, named Nautical Surface Test, that captures the local morphology at the appropriate charting resolution as the solution for the automated validation of the charted bathymetric information.