On accuracy of nautical charts

Posted an article on the web about my personal experience with accuracy of nautical charts… Not so positive, I need to say.




Good article.

I had the same experience in Dubai with the extra breakwater, I was doing some head scratching while approaching the pilot station, one of those hazy days. The problem with a car ship is if you slow down too much the wind gets you. We don’t have access to the internet so no google earth.

As to Jebel Ali, port control has car ships stay outside the channel until buoy 4 (??) but with a draft of 8.5 to 9 meters I haven’t had any problem.

Places where I have either been told or have observed that the depths are greater then charted are Shuwaikh Kuwait, inside the port of Jeddah, the approach to Port Said. Out side of the Persian Gulf ME area, San Lorenzo Honduras, my Doppler showed a couple meters more then charted, the pilot verified this. I was told by the Sabine River pilot that some of the hazards marked on the chart in the entrance fairway are not there??

Its a good idea to try and gain as much local knowledge as possible from observation and pilots. Too bad we have no way to share what we have learned. I see cruising sailors have a wiki port guide, professional mariners should have the same thing.

Well, actually you have the same experience; water is deeper than mentioned on a nautical chart.

It works both ways; only tells me that the chart is inaccurate.

The sharing of local knowledge should be done through British Admiralty.
Unfortunately, it’s not that easy.

Once I had an urgent report on safety of navigation for shipping in the Persian Gulf, it took me two days to get through to them, firstly my emails rejected as spam, secondly it took them another week to get it on Navtex.

“Too bad we have no way to share what we have learned.”

If you send me some things you’d like to be able to share, I’ve been forming an idea for that and I may soon be in more of a position to get it going.

Full disclosure: I used to be a land surveyor, then a cartographer, then I went to work for NOAA on a hydrographic survey ship, all before the days of computers, GPS, ECDIS, etc.

Because of the unique path I took to becoming a mariner, I have always been aware of the limitations of the nautical chart. I vehemently agree that mariners are becoming more and more sucked into the “if it is digital it must be 100% accurate” syndrome associated with ECDIS, but I think that points more to complacency and poor situational awareness than anything else.

When NOAA conducts hydrographic surveys, they are extremely accurate. But only for that moment in time. Coastal waters are extremely dynamic. Natural shoaling occurs, earthquakes move sea beds, channels are dredged and new wrecks and obstructions are discovered. As you know, this information is promulgated as soon as possible in the US through Local Notice to Mariners and the NGA’s Notice to Mariners. It is up to the mariner to keep the charts corrected and updated.

That being said, a mariner must also take into account when a hydrographic survey was done, which you pointed out in your thread. Depths on any given NOAA chart may be based, in some areas, on hydrographic surveys conducted with leadlines prior to 1900, in other areas, on multibeam sonar surveys that attained full bottom coverage. Over 50 percent of the depth information found on NOAA charts is based on hydrographic surveys conducted before 1940.

NOAA has specified stringent accuracy standards for collection of data by its survey vessels. Currently, surveys are being conducted to DGPS accuracy. However, for surveys performed prior to the mid 1990’s (back in my day), the accuracy requirement was only 1.5mm at the scale of the survey. For example, on a 1:20,000-scale harbor approach survey, an accuracy of 1.5mm on the chart equates to 30 meters in real life.

This means to me that the prudent mariner should pass shoals or isolated dangers with utmost caution, no matter what navigation method is used.

Click here for a great web page on how accurate NOAA charts are, the limitations associated with survey data, and how blindly following a GPS trackline can get a mariner into trouble pretty quickly.

So if the paper charts are not accurate that must just complicate the transition to full ECDIS? In addition to accuracy, or lack thereof, there are other areas of caution. This recent news release (maybe a month or two ago), is probably well circulated to those in need.

NOAA Warns Mariners of Serious Display Issue with ECDIS

To significantly improve safety at sea, NOAA has led in the development of electronic nautical charts (ENC) that conform to electronic chart display and information systems (ECDIS). Following a recent notice issued by the International Hydrographic Office, NOAA and other hydrographic offices around the world are examining their ENC suites to uncover potentially serious issues with the display of some soundings on ECDIS. NOAA has also issued a notice to mariners to highlight the issue.

What is the problem?
When mariners use either the “base” or “standard” display in ECDIS, they turn off the soundings. When they use these display modes, navigation systems will not highlight isolated soundings that are shallower than the surrounding depth contours.