This information paper focuses on the importance of understanding ENC compilation scale and the safety implications of using ENC data beyond its intended usage, during both the Electronic Chart Display and Information Systems (ECDIS) route planning and checking and route monitoring phases of navigation.
The paper provides ECDIS users with information regarding the process Hydrographic Offices employ to transform the physical world into a 2D Electronic Navigational Chart (ENC) that can be used in an ECDIS.
Within the paper the following topics are covered:
- Cartographic generalization practices
- ENC Compilation Scale
- ECDIS safety checking functions
- ENC over-scaling
- Conclusions and recommendations
note : while this article applies to ENCs and ECDIS, it is also appropriate for recreational users with ECS or mobile apps.
For centuries marine cartographers have been using generalization techniques to transform our view of the world from a true three-dimensional reality to a scaled, two-dimensional abstract view.
Many aspects to generalization are used by Hydrographic Offices when creating navigational products: classification, simplification, exaggeration, and symbolization.
Simplification: Features are simplified by either smoothing or compacting. Smoothing is generally used for linear features such as depth contours and coastlines where each curve cannot be depicted because of scale or because the detail would clutter the chart.
IHO Chart Specification S-4 states ‘Contours should be smoothed only where it is necessary to remove intricacies which would confuse mariners.
In compacting, if there are many features in a small area, such as isolated rocks which will just be dots at chart scale; those features may be grouped (compacted) within a single obstruction area.
Exaggeration: Due to scale, certain features must be shown larger than their actual relative size. Dangerous features such as rocks, wrecks and obstructions would at certain scales be unreadable if shown at their correct size, so they are exaggerated enough to be recognized and to show their relationship to other similar features.
Symbolization: Symbols are used on charts to inform the Mariner what features are.
Globally accepted cartographic practices include the use of point symbols to represent real- world area features when the scale of the product is reduced but the importance of the feature is such that the cartographer wants to retain that information.
ENC Compilation Scale
The viewing scale of a paper chart is determined and fixed by the cartographer at the chart compilation stage, so symbols are typically larger than the extent of the real-world feature they represent and do not change.
ENC producers use a variety of methods to define the compilation scale of their ENC data, but for safety reasons these will always take into account the scale at which the source information was captured.
To ensure consistency, and thus contribute to improved display, most ENCs are assigned to one of the IHO’s recommended standard compilation scales.
The various compilation scales define the level of detail that can be included, and how that detail is depicted.
Charted point features only indicate that a certain feature object exists in a given point location.
Images show survey data (left), section of ENC (centre) and ENC superimposed on survey data at compilation scale (right).
Source: Australian Hydrographic Office (AHO) and ATSB (Australian Transport Safety Bureau)
These images show the same ENC displayed at two differing scales.
Source: Electrotech, annotations by the AHO.
ECDIS safety checking function
Since July 2018 all SOLAS vessels of 500GT and upwards are required to be using ENCs created by Hydrographic Offices in type-approved ECDIS equipment.
IMO Resolution A.893(21) adopted on 25 November 1999 Guidelines for Voyage Planning states that;
This clause requires vessels to carry all appropriate scale ENCs for their intended voyage, thus minimizing any effects of generalization and ensuring the ECDIS can alert the Mariner to dangers by using the largest scale data available.‘(2.1) All information relevant to the contemplated voyage or passage should be considered.The following items should be taken into account in voyage and passage planning: appropriate scale, accurate and up-to-date charts to be used for the intended voyage or passage, as well as any relevant permanent or temporary notices to mariners and existing radio navigational warnings.’
IMO Performance Standard for ECDIS (11.4.6) requires;
‘An indication should be given to the mariner if, continuing on its present course and speed, over a specified time or distance set by the mariner, own ship will pass closer than a user-specified distance from a danger (e.g. obstruction, wreck, rock) that is shallower than the mariner’s safety contour or an aid to navigation.’The route checking functions built into ECDIS to check and monitor a route for dangers is a fundamental safety benefit for Mariners.
Where passage planning is conducted on ECDIS, use of the route checking function is a key component of the overall process of checking the suitability of a planned route and complements the visual check of that route.
The route checking function is dependent upon a number of parameters set by the Mariner as part of setting up the ship’s ECDIS for the voyage.
Figure 3 shows the minimum considerations when determining what allowance should be made for charted dangers on or near a planned route.
The ECDIS safety checking function verifies the user-defined safety corridor against the entire chart database in the ECDIS for dangers, not just against the extent of visual point symbols displayed on screen.
ECDIS safety check only verifies data along the user-defined corridor; the width of the corridor is set by the Cross Track Distance (XTD).
The two following fictitious examples show how a hazardous point feature could be missed if the correct ENC scale charts are not loaded in the ECDIS and route XTD is not adequately set.
In the first example (Figure 4), the charted position of the ‘isolated danger’ point feature representing the reef lies about 55m to the east of the planned route and falls within the route safety region.
Source: DigitalGlobe, Esri, modified and annotated by the ATSB and the AHO.
In the second example (Figure 5), the planned route lies 55m further to the west.
In this situation, if the vessel has not taken into account the possibility of isolated reefs within the region, and resultantly extended the XTD to at least account for the horizontal accuracy component of the underlying quality information (CATZOC), there is a possibility the danger could be missed during the visual inspection and the vessel could potentially run aground without the ECDIS indicating the danger on the planned route.
Figure 5 shows a similar planned route and route safety region, 55m further west, near the same point position used to represent the reef within the ENC.
Source: DigitalGlobe, Esri, modified and annotated by the ATSB and the AHO.
Given the size of the reef in the examples, it must be stressed that it would typically warrant capture by the cartographer as an area feature within an ENC compiled at the scale of the examples; and only at significantly smaller compilation scales would it be captured as a point feature.
A similar scenario and associated safety implications equally applies to the ECDIS look-ahead function and XTD once the ship is underway and monitoring along the planned route.
A key difference to note between charted area features and point features on an ECDIS display is that area features change size in proportion to the scale at which the ENC is being viewed, whereas point feature size remains constant irrespective of display scale (see Figure 2); in other words they are not enlarged as viewing scale is increased.
Additionally, the size and shape of the point symbol does not necessarily represent the size or shape of the physical, real-world feature it is depicting.
Traditionally, nautical cartographers have sought to ensure that the symbol on the chart is larger than the real-world feature it represents when seen at the chart’s compilation scale.
However, when the ENC is viewed at scales progressively larger than the compilation scale, the intended relationship between the point symbol and the area feature it represents is broken; as the ENC is progressively ‘over-scaled’ on screen, the symbol represents a progressively smaller proportion of the real-world feature, such as a reef area, on the ECDIS display.
As a point feature, a reef is charted in a specific latitude/longitude position on the ENC, typically representing the centre of the area of the reef.
The ECDIS has the functionality to allow ENCs to be displayed at scales larger than the original compilation scale.
- Over-scale indication shown within the graphical user interface
- Over-scale (jail bar) pattern
Figure 7: Over-scale indication and over-scale pattern on ECDIS
It is important to also note that the ECDIS will provide an indication if the ship’s position is covered by an ENC at a larger scale than the current ENC being used in the ECDIS display.
Conclusions and recommendations
With many additional ENC tools capable of planning routes the Mariner must still be aware that only the ECDIS is certified for carrying out route planning and monitoring.
There is a common misconception by some Mariners that zooming in beyond the compilation scale of the ENC allows for greater accuracy – however, this is not the case. In reality zooming in beyond the intended maximum display scale of ENCs may be misleading and dangerous, particularly for ‘isolated dangers of depth less than the safety depth’.
The risks associated with over-scaling the ENC within ECDIS are two-fold:
Secondly, but most importantly, because the text and point symbols stay the same size within the over-scaled ENC, any sense of appropriate distance from a potential danger is no longer intuitive and can result in a false sense of safety that does not reflect reality.
Mariners are strongly advised not to zoom in ECDIS beyond the compilation scale to a point where the ECDIS over-scale indication or pattern are triggered.
Some ECDIS allow the operator to turn off over-scale warnings.
Familiarization with all the core functions of the ECDIS are mandatory requirements within STCW and are essential for safe navigation.
- GeoGarage blog : Nautical charts : zones of confidence / Chemical tanker grounding and ENC data ... / Lessons learned after passenger ship hits ... / Investigation finds unintended risks with ... / After crash in Volvo Ocean Race, a team ... / Independent report into the stranding of Vestas Wind / Questions asked about Volvo Ocean Race boat grounding / Questions about electronic charts in a twenty-first century ship grounding