Geomod blog, Sailing

The arrival of electronic navigation charts (ENC) in the maritime world has brought a real gain in terms of navigation safety. The use of ENCs enables programming a large number of navigation alerts and provides additional information to those presented on paper charts or their digital raster versions (RNC). The use of ENCs is now pretty widespread and there is a growing demand for the most accurate and up-to-date port ENCs. However, the enrichment or creation of ENCs requires a specific expertise and a technology dedicated to digital marine mapping. As such, we are seeing the development of new partnerships between port services and consulting companies capable of providing both an expertise and a technology dedicated to marine geomatics.

Introduction

At the end of the 1980s, the arrival of the electronic navigation systems in the maritime world profoundly changes the way of sailing. Coupled to a GPS and based on digital marine charts, the electronic navigation systems enable real time monitoring of ship position on the marine chart as well as supervising a planned route.

The use of electronic navigation systems is quickly seen as a major step forward for the safety of navigation and its usage is formalized in 1995 by the International Maritime Organization (IMO). From this date, all electronic navigation systems respecting the IMO resolutions on the “Performance standards for electronic chart display and information systems (ECDIS)” becomes officially usable to navigate and it may no longer be required to use paper charts.

However, what do we mean by digital marine chart and on what type of digital marine chart should an ECDIS rely to exempt mariners from leaning on a portfolio of up-to-date paper charts? Broadly speaking, we can say that there are two very distinct types of digital charts: the raster charts and the vector charts. Only the vector charts based on the S-57 standard can prevent the mariners, operating a certified ECDIS, from maintaining a portfolio of paper charts.

The Raster Navigational Charts (RNC)

The raster navigational charts (RNC) are paper charts that have been scanned, georeferenced and encoded into a digital pixelated image (e.g. in arcs, bsb or geotiff format). Their resolution and their volume (weight in bytes) depends on the number of pixels inside the image. Higher is the image resolution, higher is its number of pixels and larger is its volume.

The advantage is that raster charts are quicker to produce, since they are no more than a color ‘photocopy’ of the paper charts, to which the cartographer has added some metadata including the chart number, the issue date or the code of the organization that produced it. Of course, this argument holds only if the raster chart is based on an already existing paper chart. Otherwise, it becomes necessary to produce the chart and there is no gain in time. Beyond this consideration, the raster navigational charts have no intelligence inherent in their structure. The use of raster charts has even several disadvantages…

A first drawback concerns the very large volume of data to manipulate and manage. A raster chart uncompressed in Geotiff format can easily reach hundreds of Megabytes (MB), which is considerable for a single chart. As a result, everyone can imagine the volume of a database composed of dozens of raster charts.

A second drawback relates to the charts update. Beyond the large volume of data to be updated, the raster charts structure makes difficult their incremental update and it is often easier to replace the modified charts by their newer versions. In this case, the mariner may no more visualize the differences between old and new charts.

A third drawback refers to the lack of additional information. A raster chart consisting of a single layer of pixels is not queryable. The raster charts do not include additional information or detailed descriptions of each of the objects making up the navigational chart. It also precludes to link these objects to external resources such as diagrams, sketches or pictures.

A fourth drawback stem from the unchangeable nature of the raster chart. Just like a paper chart, it is impossible to benefit from a dynamic display of the information presented, depending on the context of navigation. For example, screening objects depending on the scale of navigation is infeasible. Likewise, adapting the safety contour drawing, separating safe from unsafe waters, according to the draught of the ship is also infeasible.

The last drawback, and not the least, addresses the safety of navigation itself. The raster charts do not allow programming navigation alerts based on information coming from the chart. As such, it is impossible to raise anti-grounding alerts when the ship approaches an area whose depth is unsuitable to the vessel’s draught or when the ship is under way to a particular danger.

The Vector Navigational Charts (ENC)

The vector navigational charts are much more than simple digital reproductions of paper charts. They are structured dataset containing a lot of additional information compared to those presented on the raster ones. In other words, vector charts are not just a mere grid of pixels, but instead a real set of geometric primitives and geographical features allowing the on the fly generation of a digital image whose resolution depends on the quality of the screen on which the image is displayed and on the software that has generated it.

As for the navigation raster charts, vector navigation charts may exist in different formats. It may well include the S-57 format from which are produced the electronic navigational charts (ENC) or the additional military layers (AML). It may also include the “vector product format” (VPF) from which the digital nautical charts (DNC) of the “National Geospatial Intelligence Agency” (NGA) are produced.

The S-57 charts are the only navigational charts to be officially recognized by the International Hydrographic Organization (IHO). As a matter of fact, the S-57 format actually refers to the S-57 standard published and maintained by the IHO, and on which rely upon various cartographic products—the most common one being, from far, the electronic navigation chart (ENC). The acronym ENC denotes the cartographic product as well as a specification also published and maintained by the IHO. This specification lists all of the objects (land area, sea area, bathymetry, aids to navigation, etc.), the attributes of objects (depth value, minimum display scale, object name, description, etc.) and rules that the cartographer must use to produce a proper ENC. The basic unit of the geographical coverage displayed on a screen (the equivalent of a paper chart) is called a cell. This coverage can be composed of several cells or cell fragments. The use of ENCs has several advantages over the use of their raster counterparts…

A first advantage is the reduced size of the cells. The ENC specification does in fact require that the size of an ENC remains smaller than 5 MB. In practice, a cell often weighs much less than this. As such, a set of ENCs is much thinner than a set of RNC.

A second advantage is the ability to easily integrate updates. The differences between the base cell and those resulting from the updates are stored and thus easily accessible. The ECDIS also grants the possibility to create (or remove) its own manual updates. In this case, the content of the chart is not changed. Instead, it just adds a visual artifact.

A third advantage is the possibility of using a customizable portrayal. The S-57 standard applies only to the data structure and does not, in any way, include display rules. The ECDIS display of an ENC is driven by the IHO S-52 standard. However, it is quite possible to apply other types of portrayal. The use of the S-52 portrayal rules is mandatory only if the mariner wishes to operate its electronic chart system (ECS) in ECDIS mode. The S-52 relies on the topology of the S-57 standard to implement display priority rules (e.g. when a geometry is shared by several objects). For any other type of electronic chart system, such as those used for recreational navigation, each manufacturer is free to apply its own display rules. Technically, an ENC may very well be displayed with the same look and feel as the paper charts.

A fourth advantage, still related to the display, is the configurable nature of this display. The user can choose to display any given object according to their preferences. ECDIS happens in fact with 3 display modes, ‘Base’, ‘Standard’ and ‘Full’. The ‘Base’ mode corresponds to the minimum display. Objects grouped in the ‘Base’ profile can never be hidden. The ‘Standard’ mode is the default and recommended display configuration to navigate. It contains all of the objects considered as critical for the safety of navigation. The ‘Full’ mode displays all the available objects. Generally, manufacturers include a fourth ‘Custom’ mode that allows the user to create and apply specific configuration.

A fifth advantage relates to the queryable nature of all the objects making up the chart, giving access to a whole lot of additional information. These pieces of information may take the form of symbology descriptions, depth values indications, navigation instructions, guidance, and so on. It may also take the form of diagrams, drawings, photos and any other types of media available as external data source linked to the queried object—as long as it is packaged in tif, gif, jpeg, txt, or pdf format.

A sixth advantage relies on the configurable nature of the safety contour. Each ship has a specific draught that may evolve depending on its load. It is very useful for mariners to be able to configure this draught in the ECDIS and benefit in return from an adapted safety contour and a suitable differentiation between safe and unsafe waters (i.e. white and blue colors).

Finally, one last advantage, and probably the most important one, refers to the possibility of setting up navigation alerts. These navigation alerts may depend on the configured safety contour as well as the information contained in the ENC. As such, wrecks, obstructions and other dangers, restricted areas, or bathymetry data are as much information that can be used to raise navigation alerts.

An integrated management of the world production (WEND)

The world production management has also significantly changed since the appearance of the ENC. In the past, each producing agency could propose marine charts all over the planet, even if those were overlapping with other charts produced by other agencies. Besides, the responsibilities in case of mapping errors could be somewhat hazy.

Since 1997, the IHO resolution on the “Principles of the Worldwide Electronic Navigational Chart Database (WEND)”, defines the obligations of the producers of official ENCs:

“to ensure a world-wide consistent level of high-quality, updated official ENCs through integrated services that support chart carriage requirements of SOLAS Chapter V, and the requirements of the IMO Performance Standards for ECDIS.”

Accordingly, the producing agencies of official ENCs have found themselves forced to share the global production, so that the mariners may benefit from the most up-to-date ENCs on the largest part of the globe. This resolution helps avoiding ENCs duplication as there may only be one ENC per given area. If the geographical footprints of two cells of the same scale band overlaps, only one of both must contain data on the overlapping area. Cartographers use the meta-objet M_COVR and its CATCOV attribute equal to 1 to indicate an area containing data, and CATCOV equal to 2 to indicate an area of no data.

In addition, any official ENC is necessarily published under the authority of an IHO Member State. Indeed, since the liability of the authority publishing an official ENC may be engaged in case of mapping error, and considering the cost of the damages that may represent the grounding (or the shipwreck) of a ship, it is understandable that only a State may be allowed to take this kind of responsibility.

Therefore, there are two types of ENC: official ENCs produced under the authority of a State through its national hydrographic agency and unofficial ENCs produced by other organizations. It is worth noting that a private company may still be mandated by a national hydrographic service to help in the production of their official ENCs.

Towards an increase of ENC dedicated to port services

A need to enrich the official ENCs portfolio by more specific ENCs was issued by many port services, including French ones. This is, for instance, the case of the port pilots of Marseille-Fos and Nantes – Saint-Nazaire or the technical services of the Nantes – Saint Nazaire Port Authority, the Ports of Normandy Authority (PNA) and the Grand Port Maritime du Havre (GPMH).

However, modifying, enriching or making an ENC requires a very specific expertise in digital marine mapping in order to meet the quality requirements of the ENC specification. For instance, an ENC that does not respect the topological structure required by the S-57 standard may appear correct at first glance, but alter the proper functioning of the ECDIS and Pilot Port Unit (PPU). This type of expertise is generally only available within national hydrographic offices or within agencies specialized in marine geomatics.

In addition, modifying, enriching or making an ENC also requires to rely on a technology dedicated or at least adapted to this activity. It is worth noting that the traditional geographic information systems natively lack features dedicated to the publication of S-57 ENCs and do not implement the quality control procedures defined in the standard S-58.

Therefore, we see that port services are starting, more and more, to develop partnerships with consulting companies able to provide both an expertise and a technology specialized in marine geomatics. This is typically the case of Geomod who intends to support ports services by providing its expertise around the S-57 standard, its  ENC editing software (PortSide), its generator of bathymetric ENCs (Ulhysses), and some custom-built applications (e.g. ePilotBook).

The activity of ENC production support for harbor authorities has started by addressing a request of the Marseille-Fos port’s pilots who needed to integrate the centimeter level accuracy of the port’s topographical survey in the ENCs^* used in their aid to pilotage web application based on ePilotBook.  We may well include the Nantes – Saint Nazaire Port Authority for whom 7 “Berthing” level ENCs^* have been initiated in order to cover the ascent of the river Loire. We may also include the Le Havre port’s pilots who needed an ENC covering the end of the Grand Canal.

The activity of ENC production support for harbor authorities is now continuing at the request of the technical services of the Grand Port Maritime du Havre (GPMH), that Geomod helps in the production of 15 “Berthing” level ENCs^* covering the entire port domain (the port of Antifer, the port of Le Havre, the Grand Canal and the Tancarville canal.

Port of Le Havre – Géoportail

Port ENC of Le Havre – Geomod

^* It should be noted that these ENCs  are “first release” only meant to provide assistance to the internal ports services.