The Art of Dredging

Dredging and shipping

ECDIS update 2012


ECDIS, ......  What You See Is What You Get ?

ECDIS is not yet a mature system. This may sound odd, since electronic charts have been around since the nineties. Here are some loose ends:



The Electronic Navigational Chart (ENC) on your ECDIS system is a database, from which the navigator builds a map, selecting or omitting data, to suit the situation.

Some of the data there was surveyed tens of years, or hundreds of years ago. These ENC's are compiled with the same data used for your old paper charts, and in a few cases this data was compiled by good old captain James Cook himself, with his sextant and leadline, no doubt.

So far for horizontal/vertical accuracy of your ECDIS.

Underlying hydrographic data may be unreliable, and blunt acceptance -WYSIWYG-  is potentially dangerous.

Status of hydrographic surveying and nautical charting worldwide does NOT justify great  confidence in ECDIS. Users need to allow for the limits of underlying survey data by constantly monitoring their position relative to the uncertainty of the charted information.

Is there a proper appreciation of the quality and reliability of the data behind the display ? 

In paper charts, relaibility of survey data could be read from the source diagram. In ENC's, reliability is categorised by aZone of Confidence Category (CATZOC).

 An ECDIS user has to interrogate his system, to find the CATZOC classification of the area he is navigating at that moment.

The funny symbols on the electronic chart (left) refer to the CATZOC system, an indication for accuracy of underlying survey data.


CATZOC has five categories (ZOC A1, A2, B, C, D) with a sixth category (U) for data which has not been assessed. The categorisation of hydrographic data is based on three factors (position accuracy, depth accuracy and seafloor coverage), as specified in IHO publication S-57.

For the exact "CATZOC" definitions, check out:


A nautical chart is an assumed model of the seafloor. It may be incomplete. No chart is perfect; it only reflects what is known, measured and reported. It is merely a rude sketch of reality.

I wrote up some personal experiences on the accuracy of charts here.


Consequently, an essential requirement for mariners is the ability to interpret the quality indicator (CATZOC) in ENCs. Information on survey methods, chart compilation, horizontal datums and CATZOC is provided in e.g. the UKHO Mariner’s Handbook NP100.     (downloadable .pdf,with some exerpts from NP100).

The IHO (International Hydrographic Organisation) is well aware of shortcomings in global coverage and accuracy, but hydrographic survey capacity is declining worldwide. Accuracy problems are here to stay.


Stay critical, in front of your ECDIS display. "Digital ? Yes !  Accurate ? Probably not..."


Some ECDIS systems do not correctly display critical safety features. This is due either to anomalies with the ENC data or within the ECDIS equipment. Read more here.

When isolated shoal depths are encoded in a particular way i.e. shoaler than the range of depth of the surrounding depth data, they will not display when operating ECDIS in Base or Standard display mode.

Isolated shoal soundings may not trigger anti-grounding alarms in any mode of display.



Ships compliant with ECDIS, should carry two ECDIS systems, for redundancy.

The ECDIS setup onboard a ship will most probably be: two identical computers, running the same software, and tied up with a link between them.

That is not the best redundancy achievable: if one system fails, there is a fair chance the other system also goes down. ECDIS-computers can go down for a multitude of reasons, simple hardware failure, a lack of "digital hygiene" (e.g. using infected USB-sticks), reported errors on uploading chart updates, etc...



The whole global transportation system is heavily addicted to GPS. Does GPS deserves  that kind of trust ?

1. ECDIS relies heavily on the old, but robust GPS system; there is no redundancy in position input. Alternatives for GPS are not yet operational (Galileo, Chinese COMPASS), or will not be projected worldwide (COMPASS, Indian IRNSS). GLONASS , the Russian alter ego for GPS, is fully operational and available for civil users, with an acceptable accuracy. Russian authorities are committed to maintaining this system.

However; as things go: no international regulations prescribe the use of a redundant source of position input, thus no company will voluntarily install them. But: alternatives to GPS are available, and redundancy is on offer today, and more systems are in the pipeline.

2. The old navigation skills (chartwork, sight bearings, radar and celestial navigation) are no longer drilled into new recruits; and we have come to the point that some junior officers are no longer capable of making a radar fix. Take away GPS, and they are irretrievably lost. Old school navigation abilities are becoming history, and will soon be a thing of the past.

3. GPS jamming and spoofing are latent dangers.

Jamming is the local blocking of reception of GPS signals. This has become a criminal trick in road transport; e.g. jamming the GPS signal of a stolen truck. It might become the next terrorist thing in the maritime and aviation world.

Spoofing takes this one step further: sending imitation-GPS-signals which causes wrong position and time readings on the GPS-receiver. 

4. During the second half of 2012, and into 2013, we will see a peak in solar activity; an increase in sun activity will increase radiation and impact GPS satellites. As solar activity sweeps through Earth's atmosphere, it can cause distorted GPS signals and accuracy problems.


Conclusion: the ECDIS system has added many technological layers to ship's navigation; the problems of which are not completely understood yet.

There is hardly any redundancy to be found in ECDIS-systems, especially what concerns navigation input. A failure of the GPS-system would have tremendous concequences.



Marc Van de Velde,  August 2012






Subscribe to our newsletter:

Write us: (at)

Manu's scripts

- a sailor's fifth column


Oops! This site has expired.

If you are the site owner, please renew your premium subscription or contact support.