• Corpus ID: 15417660

Map Projections and Visualization of Naviga¬tional Paths in Electronic Chart Systems

@inproceedings{Pallikaris2010MapPA,
  title={Map Projections and Visualization of Naviga¬tional Paths in Electronic Chart Systems},
  author={Athanasios Pallikaris and L. Tsoulos},
  year={2010}
}

• A. Pallikaris, L. Tsoulos
• Published 2010
• Computer Science

The purpose of this paper is twofold: firstly to present the current status and the weaknesses of the standards of the International Maritime Organization (IMO) on the employment of map projections in Electronic Chart Display and Information Systems (ECDIS) and other Electronic Chart Systems (ECS), and secondly to propose specific map projections for use in navigational systems. The paper starts with a brief overview of the IMO performance standards for ECDIS and the corresponding industry practice concerning the use of map projections and the visualization of navigational paths. Seventeen different map projections are evaluated and compared with respect to their fitness for use in the ECDIS/ECS environment. The comparison is supported by specially constructed maps portraying selected loxodromic and orthodromic navigational paths and distortion analysis data. The conducted study shows that improved visual perception is possible through the proper choice among five alternative map projections, depending on the location and the extend of the area portrayed on the chart. key words: ECDIS, ECS, map projections, visualization, loxodrome, rhumbline, orthodrome, marine navigation 1. THE IMO PERFORMANCE STANDARDS FOR ECDIS AND THE VISUALIZATION OF NAVIGATIONAL PATHS IN TYPICAL ECDIS AND ECS SYSTEMS Year 1996 marked a historical milestone in the evolution of marine navigation methods through the adoption of a resolution by the International Maritime Organization establishing the performance standards for the Electronic Chart Display and Information Systems (ECDIS) [1]. These performance standards, as they have been revised and updated [2], provide the basis for the automation of traditional methods for “route preparation”, “route planning”, “route execution” and “route monitoring” in contemporary and future navigational systems. The IMO performance standards for ECDIS [2] as well as the relevant ΙΗΟ product specifications for Electronic Navigational Charts [3], [4] that have to be used in ECDIS cover adequately many aspects of the portrayal of cartographic and navigational information on the screen such as the display of traditional and simplified symbols, setting screen colors depending on prevailing light conditions, choice and adjustment of the amount of the displayed cartographic and navigational information etc [5]. It is surprising though, that the international standards on ECDIS do not provide for specific requirements or recommendations on the employment of specific map projections, or the presentation of navigational paths, except for the following general requirements [2]: “ ..., “ECDIS should be capable of displaying all chart information necessary for safe and efficient navigation ... originated by, and distributed on the authority of, government-authorized hydrographic offices.” ....“It should be possible to carry out route planning including both straight and curved segments” ...“... the selected route and own ship’s position should appear whenever the display covers that area” In reality the lack of official standardization or recommendations on the use of specific map projections in ECDIS does not cause any real problem on its primary function to contribute to safe navigation. In ECDIS the solution of navigational problems is conducted analytically on the surface of the reference ellipsoid without any graphical work on the surface of the Mercator and Gnomonic projections, as in traditional navigation methods where the Mercator and Gnomonic charts have to be used due to their vital characteristics to portray Rhumb Lines (RLs)/loxodromes, and the Great Circles (GCs)/Orhodromes with straight lines [6]. Consequently in ECDIS as well as in other navigational systems it is not imperative to use particular map projections. Nevertheless the lack of standardization on the 3 INTERNATIONAL CONFERENCE ON CARTOGRAPHY AND GIS 15-20 June, 2010, Nessebar, Bulgaria employment of map projections in ECDIS has resulted in the relaxed use of projections in many commercial ECDIS systems that in some cases cause remarkable visual distortions and misinterpretations of the reality, as it is shown in figure 1, where: Placing the crosshair (cursor) in succession at the port of New York and the port of London to obtain the geographical coordinates of these two locations, shows that the port of London is quite far north from the port of New York (more than 10 o). Observing the position of the horizontal line of the cursor in either of these two points, give the wrong impression that the Port of London is quite far south of the port of New York. This wrong impression is enhanced by the normal practice in commercial ECDIS/ECS systems to not portray the graticule on the electronic charts covering extended geographical areas (i.e. in small scales). The visual impression is that New York Harbor is north of London harbor, whereas in reality it is south Figure 1: Example of poor visual perception in ECDIS The choice of the most appropriate map projection in ECDIS/ECS systems should be implemented in accordance with criteria and rules ensuring not only the maintenance of essential characteristics for navigation, but also the control of visual distortions within acceptable limits. 3 INTERNATIONAL CONFERENCE ON CARTOGRAPHY AND GIS 15-20 June, 2010, Nessebar, Bulgaria 2. CRITERIA AND RULES FOR THE SELECTION OF MAP PROJECTIONS IN ELECTRONIC CHART SYSTEMS The desired characteristics of the map projections for ECDIS/ECS that have been taken into account in the conducted study are: The control of the amount of distortion within acceptable limits (minimization and improvement of the distribution of distortion). The form (shape) of Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes The form (shape) of the graticule lines The criterion of minimization and distribution of distortions: The values of all kinds of distortions (linear, area and angular) depend on the scale distortion on the principal directions of the projection [7 pp 63-68]. Since for small values of angular distortion, the values of scale distortion on the principal directions are about the same value, it is sufficient to consider the values of surface distortion [8]. For these reasons, in the process of choosing the most suitable projection in ECDIS/ECS the maximum desired tolerances are set only for the angular and surface distortions. These tolerances are set to 8 ° 10 ° for angular distortion and to 10% -12% for the area distortion, in order ensure that no serious visual distortion is generated [9 pp295-98]. The criterion of the form (shape) of Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes: For better portrayal of the reality the lines showing Great Circles (GCs) have to be shorter than corresponding Rhumb Lines (RLs)/loxodromes. In addition Great Circles (GCs) should preferably bend towards the equator and Rhumb Lines (RLs)/loxodromes towards the poles. The criterion of the form (shape) of the graticule lines: In navigational systems it is very useful to ensure that meridians and parallels are portrayed as straight lines, intersected orthogonally. This allows direct conclusions concerning the relative geographic location of any point from the overall visual appearance of the portrayed region. These conclusions do not depend on the portrayal of the parallels and meridians, which also are not always displayed in ECDIS. Thereby there is a risk of creating false impressions, as in the example shown in figure 1. Realizing that some of the above-mentioned criteria for the selection of map projections cannot be met simultaneously, as in the case of the desired shape of graticule lines, Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes, the evaluation of map projections is conducted through three different approaches. With these three approaches, regardless of the degree of satisfaction of the criteria set, significant improvement of visual perception is achieved. The three different approaches that have been adopted for the evaluation and selection of map projections in ECDIS/ECS systems are: 1st approach: In this approach cylindrical, conical, azimuthal and pseudo cylindrical projections are considered in order to achieve the greatest possible reduction of area and angular distortions and better compromise the conflicting demands for the desired shape of graticule lines, Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes. 2nd approach: In this approach emphasis is given on the criterion for the desired shape of Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes. The selection is made from the conic and azimuthal family of projections in which Great Circles (GCs) are shown with shorter lines than Rhumb Lines (RLs)/loxodromes. In addition to this feature, these projections have the basic advantage that they reduce significantly the amount of angular and area distortions of the Gnomonic projection used in traditional navigation, as well as the area deformation of the azimuthal stereographic projection, which is used in many ECDIS/ECS systems . In this approach, the compensation cost of ensuring the desired shape of Great Circles (GCs) and Rhumb Lines (RLs)/loxodromes is that the selected projections do not fulfill the criterion for the desired shape of the graticule lines, thus leading to false impressions and conclusions, as in the example in Figure 1. 3rd approach: In this approach emphasis is given on the criterion for the desired shape of the graticule lines. For this reason the selection is made only from the family of cylindrical projections in which meridians and parallels are shown as straight lines intersected orthogonally as in Mercator projection commonly used in traditional navigation. In addition, these projections have the basic advantage over the Mercator projection of the smaller linear distortions whereas the angular distortion is within the specified tolerances. In this approach, the compensation cost of ensuring the desired shape of meridians and parall