How accurate are nautical charts?

Australian charts are all printed with ZOC tables and we have been using them for many years. I’m surprised that this is new in some places. We have huge areas that are either unsurveyed or inadequately surveyed largely because the imperative has not been there for either the safety of ships passaging along the coast or smaller craft inshore. The ZOC table is essential in many less frequented areas.

I have told my story here at this forum of my unfortunate relocating a rock pinnacle in the Gulf of Carpentaria which is almost totally unsurveyed but I would not have gone through that particular strait between a couple of islands had I had the chart made of the area by Matthew Flinders during the first circumnavigation of Australia in HM Sloop Investigator in 1802-3 which I came across many years later. Regrettably, the danger line Flinders had dotted on his chart was not retained on the modern chart which was simply blank. The danger became visible an hour or so later as the tide started to run and the effect could be seen on the surface. Many such rocks there are named after the patrol boat that bumped them.

So old soundings are not necessarily unreliable, simply less precise. Many of the more remote offlying Australian Coral Sea islets might have soundings ascribed to whomever first visited and made a sketch. Nobody has since bothered to revisit and do it properly.

I do agree that local knowledge is best in those areas but the fishermen who have the best knowledge never tell anybody what they know as their information is a valuable secret from their competitors.

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Vokeo Island off the North coast of PNG was known in Wewak as “Walkabout Island”.
If you took a bearing of it from the anchorage in the morning and again in the afternoon, when the 3 o’clock rainstorm had cleared, the difference was several degrees.
In the late 1960s it was marked on the charts with a stippled line and the note; “Position uncertain”.

Here is a later edition of the BA Sailing Direction for the North coast of Papua New Guinea:

By now the shape and position of the Schouten Islands are a bit more certain, but still reported to be; “1-2 n.miles from charted positions” in different directions.

Who produces nautical charts and who funds them?

Is that a trick question?
The positions of quite a few small islands off the west coast of Mexico are way off. Probably not surveyed since the rule of conquistadors.


Nope, not a trick question. Just asking why people run into things if they travel in uncharted waters and then compliain.
If the waters are not charted why go ? If one runs into something, whose fault?

Update: USS Connecticut Hit Uncharted Underwater Seamount

According to NOAA, only about five percent of the global ocean has been mapped by modern multibeam sonar systems to provide detailed information about the seafloor.

All waters were once uncharted. Why go? Sounds like you’ve spent your seagoing career plodding along well established sea routes which attract better surveys, made safe by someone or many going before you.

And there’s a difference between uncharted (not many areas like that with) and unsurveyed, which can exist in widely different accuracies on the same paper chart.

Went up to Istanbul in the early 90’s
Current UK chart was surveyed in 1890 something
We used the same chart as used in WW1 lol
The bottom/southern part of the Gallipoli Peninsula said its 5nm north of where marked on the chart.
GPS useless in these areas, its puts you on the land.

Actually, I was thinking about the US Navy’s propensity for running into things and each other on the surface. One would think they’d be extra cautious under the surface. :slightly_smiling_face:

I agree. They are cautious, I’m sure. But a submarine cannot maintain its covert cover if it goes around pinging ahead of itself looking for sea mounts that aren’t on the chart, so there’s a trade off. There’s a huge amount of work to be done to make the charts fully safe for ships with a 1000 foot draught.

Most deep oceans ore only sounded by the echo sounders of ships passaging on the surface rather than deliberate hydrographis surveys using special stabilised narrow beam sounders that cover a far wider sweep. Most echo sounders don’t reach the depths of deep oceans (and thus the shallower depth over sea mounts isn’t noticed) and most surface ships aren’t interested in recording and reporting such soundings.

Submarines do run into things and they do have excuses. I’ve told the story in this forum of a mate of mine here who was an engineer officer aboard a British nuke making a high speed run across the Atlantic at about 30 knots. They hit a seamount at that speed but luckily only a glancing touch, being deflected upwards. Not good for the nerves.

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Zones of Confidence in Australia are just as new as they are on BA and NOAA charts. ZOCs originated with the IHO.

This is from “Mariner’s Handbook for Australian Waters_Week 33_NtM Edition 17_20 Aug 2021”


Nevertheless, I use paper charts and the ZOC diagram has been on them for years and prior to that the source diagram. This didn’t start in August 2021.

Perhaps it is simply that this system which had been previously introduced in the UK and Australia has been standardised internationally.

This is good illustration showing the possibility for errors in old hand lead surveys: from the same pub as above:

The original soundings in fathoms are shown written by hand with a newer survey printed in meters.


The 2.1 meter spot in the blue area was missed in the 1889 survey.

While useful in many situations I still maintain vector charts are an abomination.


See my old post here >>>

In a far away region, the Lesser Sunda Islands, this ship proceeded with ECDIS only.
While the original old paper chart showed the submarine rock, the ECDIS chart did not!

Noting the topical grounding of a submarine and the general acceptance that charts are not necessarily accurate for all purposes, it might be interesting to get the feedback from this forum to the questions :

Do you run (and monitor) your echo sounder when on long ocean passages?

Can your echo sounder reach say 1000 feet? and

Would you report an observed sounding of, say, 900 feet if it was in an area of deeper charted depths?

yes, about 30yrs ago

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From a quick look at the report it doesn’t seem to be a simple matter of the hazard being missing from the ECDIS chart.

Seems to be related to the “Isolated Danger Symbol”. There’s been a lot of discussion about this:


From 10 things they should have told you about ECDIS

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I always assumed the 400-600 foot reading I got sometimes in waters charted at 6,000+ feet deep were thermoclines or plankton layers.