The Art of Dredging

Dredging and shipping

On the accuracy of nautical charts

Lies, bigger lies, statistics. And nautical charts, perhaps ?

After sailing ships for more than 20 years now,  I grew more and more wary of the "accuracy" of nautical charts …

Let’s start with some examples:

1.       Dubai Dryocks, U.A.E.,  Persian Gulf, 2005.

Dubai Drydocks. The whole west breakwater was not yet on the nautical charts, anno 2005.

Early 2005 “Gerardus Mercator” visited Dubai Drydocks, one of the largest ship repair complex in the world.

About two miles before we entered the breakwaters I figured out that there was one extra breakwater on the radar, and not on the nautical chart.

2.       Abu Dhabi New Port, Persian Gulf, 2007





Abu Dhabi New Port; the encircled area was a white spot in the charts, 2007.

We planned a routine visit for bunkers and repairs in this "new" port. 

The port was two years old, but still a white spot on the map.

To fill in the white, we used Google Earth and had our own survey done with a launch. The survey showed that we could only enter port with the ship completely deballasted.


Both examples of incomplete nautical charts are caused by not passing on information timely to British Admiralty. This is a typical phenomenom in the Persian Gulf, where construction rates boggle the human mind.

British Admiralty is aware of these problems and they publish notes on their Persian-Gulf-charts, basically telling: "Expect the unexpected !"


Kholmsk, Sakhalin Island, Russian Federation, 2004


Sakhalin Island is one of these forgotten backwaters, at the far east end of Siberia, where nobody was interested in for any reason, for many decades. 

It's an hydrographer's equivalent of a black hole.

The seas around the island were last surveyed just before World War II. 

Since gas was discovered off the coast, it became a different story...

 In 2004, a dreder flotilla set out to pioneer Sakhalin waters. Pioneering it was...

British Admiralty charts looked just inadequate. We managed to get a set of U.S. charts onboard, who showed more details. But we only got some to-the-point-data when a colleague of me boarded a Russian trawler in Kholmsk; and got some Russian charts photocopied. 

Anyway; the pilot book gave this very clear warning on Sakhalin, that "GPS-positions on the chart may be in error up to 10 nautical miles". They advised to keep out of the 50-meter-depth contour.


Kholmsk Fishing Port, Sakhalin, 2004. On this photo "Gerardus Mercator" is the large ship tied up in the northeast corner of the port.   

Turning inside the dock was impossible (too narrow), and the turning basin (at the underside of the photo) was a tight fit for a turn.


Our last port of call, before sailing to the project site, was Kholmsk, on the west side of Sakhalin.


We had only scanty information before arrival; including a few screendumps of Google-Earth.

The whole manoeuvre was strictly eyeballs only, and I had no idea about exact dimensions of the turning basin.

When we were all tied up in port, after a really tight manoeuvre, I realised that we came in safely, rather by chance than by design.

In hindsight, I should have refused to enter that port, with that ship: the margin of error on my information was to small to ensure safe port entry.

And charts or pilotbooks were no real help to get the info I needed. Google Earth was, once again.

On the web I could have found, a non-official source, but once again; is that reliable data ? They claim that the largest ship allowed in would be 500 feet, exactly the size of Gerardus Mercator. Anything less manoeuvrable than Gerardus would have made for a huge risk.

What's going on ?

If you get around with your car, do you use a roadmap dating back 20, 50 or 100 years ?

That's exactly what's we are doing aboard ships. It's even worse than that...

Mariner's tend to look to charts as if they were true, giving exact information.

This has been the root cause to a number ship groundings on shoals or reefs that were in the wrong place, according to the charts,  or not marked on the chart at all.

Well, nautical charts are not true, exact and unfallible, for a multitude of reasons.

Example: approach channel to Jebel Ali, Dubai.

The channel is 17 meter deep.  

We planned to enter the channel between buoy 1 and 2, coming from west, with a draught of 12 meter.  


Every nautical chart has a "source diagram", showing the source of survey data,used to draw the chart.

Left: sourcediagram for approach to Jebel Ali.

As you see: the approach channel has recently been surveyed (2006).

When you venture outside that channel, data dates back to 1970 ... which does not seems to be a bright idea if you have a small underkeel clearance, as we had...


The Mariner's handbook (British Admiralty NP 100) puts it:

"The maximum draught of vessels at the time of the survey should also be given consideration. The earlier surveyors were primarly concerned with the safe naviagtion of ships of their own era. (...) Draughts of 15 m were considered a maximum until 1958.

"Sidescan sonar came into general use (...) in 1973 enabling many wrecks and shoals lying between lines of soundings, which might otherwise not be located, to be detected." (Before that time, single-beam achosounders were used; allowing large gaps between the survey lines, with all kinds of undetected shallow dangers.

And we must add: GPS came only in use since the nineties. Expect errors in horizontal positions, in surveys before that decade.

For navigational purposes, in most cases, it's possible to keep a margin of safety, to err on the safe side.

The perceptionthat nautical charts are exact instruments, will be even more stimulated with ECDIS onboard.

Charts will be digitalised, computerised, psychologically lowering the critical sense of the users, not realising that the underlying data presented may be 50 or 100 years old, and liable to many mistakes, and no loner 100% trustworthy.

A nautical chart (ECDIS or paper chart) is -at times- a dim sketch of reality.


Marc Van de Velde 

Read also:     (essential stuff !)


All photos by Google Earth.






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