How accurate are nautical charts?

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.

I agree, but lately I have been making myself use them all the time because the paper charts and their digital equivalents are going away.
Vector charts introduce whole new layers of inaccuracies.

Why would that be the case?

I think you have that backwards, that’s why vector is useful on occasion.

Here is why:

  1. You may not understand the algorithm that determines how details come and go as you scale in and out. There was a multiple fatality some years ago that involved a boat on autopilot trying to sail through an island not visible when scaled out to about 100 miles. This stupid sailor trick has been done more than once.
  2. The program itself may have a defective scaling algorithm. I recently discovered that AquaMap was selectively removing buoys closer in and leaving ones farther out. It was totally nuts, at one point I had it removing just half the channel markers leading into Boston and this is at scales you would be using on the way in :fearful: I hated to get rid of Aquamap, but the COE downloads for ICW work didn’t make up for this glaring flaw.
  3. Various issues with how companies make their vector charts. You can dig around and find places where Navionics shows a rock and C-Map doesn’t and vice versa. I know some ICW regulars now have two tablets running two different programs for this reason.
  4. They all usually have functions to turn various layers on and off, you may not understand them all in detail and remove things you don’t want to remove.

I run SeaIQ now, ( and am somewhat happier now. It uses the same NOAA ENCs that OpenCPN does and the detail settings are all similar, so at least I miss the issues with proprietary charts and can make my tablet and nav computer look the same if I want to. I heard this program is popular with harbor pilots too, so that was another thing in its favor.

I still far prefer the look of paper charts and the RNCs made from them.

  • another thought - Are the days when charted islands and reefs could be miles off gone, or is there still some of that going on? I knew someone who worked making charts at NOAA. He said the relative accuracy was always quite good, but the absolute lat-lon from the 19th century maybe not so much.
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You answered your own question a few posts previously:

12 posts were split to a new topic: Team Vestas Hitting Reef

Both terms, “accuracy” and "layers’ have specific meanings wrt ENCs so I didn’t understand how vector charts could add layers of inaccuracies.

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On standby at a rig for weeks, nothing to do so office says get all the charts out and update as vessel owner might pay a visit.
Drag out lots of west coast africa.
Updates are a few years out of date as boat has not been there.
I knew how to use the chartco app so I was printing all the stuff, I noticed that many updates were shifting the old charts to and from wgs84 and this had been done many times over on the same chart.

No point having nice chart if you dont know where the lat lons refer to

Of your list only #3 has to do with the accuracy of the charts. With ECDIS only official charts are allowed to be used so Navionics vs C-Map etc is not an issue.The more appropriate comparison would be official raster charts vs official vector charts .

For mariners not using ECDIS and official vector charts than perhaps using raster charts might be a better choice, especially if they have little understanding of how their system works

ECDIS has a “route checking feature” that can be used during planning to check for hazards along the planned track inside the XTD.



In the grounding of the tanker Pazifik the XTD used for the route check was 0.1 miles so the rock they struck was not flagged.

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Both SeaIQ and OpenCPN use the official charts and both have layers that can be turned off and hide things you might want to see.
Sure that is user error more than inaccuracy, but that is something you can’t do on RNC charts unless you are doing something like coming into Norfolk using the chart for the entire east coast :roll_eyes:

  • this is why I am making myself use the ENCs even though I don’t prefer them, current RNCs will vanish sooner or later :frowning:

My C-Map plotter does that, I recently got a distress call on DSC and it popped up on the plotter with a warning that a course to the boat in distress included obstacles, which in this case was because the boat was inland up a bendy creek, so a straight line crossed several farms and a highway.

All charts, with a few exceptions, both commercial and government, use data produced by government hydrographic agencies. Commercial entities do not collect their own survey data.

The requirement for ECDIS is that that both raster and vector charts must be official charts, just using charts created with data from official sources is not sufficient.

With ECDIS turning layers off and on is not allowed, or only allowed to a very limited extent.

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Nearly all pleasureboat plotters say when you boot them up not to used for navigation.
Due to the drawings based on??
Most punters using them think the whole world has been re surveyed thats why they have nice scalable continuous drawings.
Imagine the pleasureboat system showing a warning that this drawing is based on a survey from 1890?
It should imho.
Hence yachts run up on reefs on charts in the middle of the ocean but ships normally dont.

Ships generally don’t attempt getting in and out of the places yachts do, I have never run aground going into Baltimore or Norfolk either :roll_eyes:


Recommended Reading :

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Aquamap is the worst one I have ever seen:

The green 3 vanishes with tiny changes in scale while other marks farther out stay on the display. Coming into Boston the channel markers on just one side of the channel vanish. It is really too bad because they will overlay current USACE surveys on the charts way before that info gets into the official charts.