How Accurate Are Digital Charts?

[Nausikaa]

I was just looking yesterday at a digital chart covering all of Denmark, the Oslo fjord and the west and south coasts of Sweden.

How very convenient to have everything in one SD card. However, be warned. I zoomed in to an area I know very well; Nordströmmerna in the Swedish province of Buhuslän. I was reliving many a voyage through this picturesque, narrow and winding lead, in my minds eye ticking off the landmarks and buoys as I passed them on my immaginary voyage when I discovered that a danegerous rock in the middle of the channel and at its narrowest point was missing from the chart.

Now, I must check this against a printed chart but, unless the rock has been dynamited and this I doubt as it is in a nature reserve, it is still there and still dangerous.

I never rely on digital charts with the exception of those produced by the British Admiralty, i.e. scanned copies of Admiralty charts. I am going to continue using digital charts for voyage plotting and also on board to get an overview of m position but the plot will still bekept on normal, reliable paper charts.

Anyone else have experiences of discrepancies between official charts and digital charts compiled by non-official bodies?

Aye // Stephen

[Istioploos]

This is a fair warning.

I too have been using C-Map digital charts (vector) on the GPS/Chart Plotter but always in conjunction with paper charts. In general I find the C-Map charts quite accurate BUT like Stephen's case at high zoom-in they are not reliable. Often while peacefully and blissfully sipping a glass of ouzo and admiring a lovely anchorage my boat is well on the rocks according to the chart!

Fair winds

Vasilis

[JeanneP]

I don't know on what datum your electronic charts are based, but if you have the wrong datum entered into your GPS you can be as much as half a mile or more off in your position. And that is true for paper charts as well. Many charts, paper or electronic, will give offsets to use when setting up the GPS to use with the charts. There are numerous reasons for this, including areas that haven't been surveyed in a century or more, the error that a thin line will create when that dot or line on a small-scale chart can equal 500 feet. And don't forget human cartographic errors. So for everyone, when using paper charts, make sure that the datum and/or offsets given on the charts are entered into the setup of your GPS. There are lots of explanations of this on the Internet, here's one of them

A good explanation of Raster charts and ENCs can be found at the NOAA Office of Coast Survey

In the US, at least, paper charts are printed from the same digital files as are available as Raster charts used in many computer navigation programs.

The raster charts that we have downloaded from the NOAA site are the same charts that one buys from NOAA as Print On Demand, paper charts. I like paper charts and find them much easier to use for route planning and general "look forward" as we go. If we insisted on following the electronic charts blindly we would be running into channel markers, rocks, and lots of shoals. Again, part of that is simply the discrepancy that scale creates GPS accuracy may be 3 meters, but chart accuracy just isn't that great.

However, for a hazard to not be on the chart at all is a bit worrisome. Isn't such a hazard identified on the water with some sort of buoyage/mark? And if the mark isn't on the chart also I would be very worried. Since electronic charts are created from paper charts, I would be contacting the supplier of the electronic chart to find out how such a well-marked hazard was missing.

[Coyote]

Wise or foolish, I trust my electronic charts, more or less, on the west coast of the US. Other areas, I might not trust any charts, paper or charged particles.

[Nausikaa]

Originally Posted by Coyote' date='14 May 2010 - 07:19 PM

Wise or foolish, I trust my electronic charts,

I too completely trust me British Admiralty ARC charts to the same degree as I trust Admiralty paper charts.

What I was getting at is that when some manufacturers of digital charts for plotters transfer the information they get into their own format something may dissapesar or hasve the position altered.

The plotter/chart manufacturer in question claims to, in the case I mentioned, have used official Swedish charts (which are at least as good as BA charts)and then transfered that information to their own charting system, obviously loosing something on the way.

I am just wondering if anyone else has had similar experiences?

Aye // Stephen

[Coyote]

As a software engineer, I would never be surprised at anything done by software. I am often surprised that anything works ever.

I recently found a two bugs in a piece of code that has been used regularly by millions of people for at least 6 years.

We call it "Computer Science." That is a lie. We call it "Software Engineering." Another lie. Software creation is more art and black magic than anything else.

As Reagan said, "Trust, but verify."

That said, I have never seen an error in my electronic chart and I trust it.

[chriscolum]

In any way you should never trust and completely rely on digital maps and/or gps plotters. Navigation is a serious thing, and one should always have printed charts issued by an official institution of a certain country.

[Spike_dawg]

Actually the e-charts are quite often more accurate then the printed charts when found on cruising yachts. All US charts (ENC) are FREE and you can download the latest edition. Other area's of US influence are also free (Guam, Puerto Rico, Marianas, etc.). Most cruisers are working from very old paper charts, due to expense, but they can update their ENC charts at anytime. Real paper charts are very expensive and many cruisers settle for only a few scales where the electronic charts will have all available charts for that area. Some people use chart pacs (Bellingham primarily), B&W, which is difficult to discern and about useless.

C-maps typically require an offset in the Pacific, for countries like Kiribati, Tuvalu, Marshall Islands. The basic surveys were done 50 to 100 years ago and WGS-84 datum (GPS standard) wasn't around then. The paper charts have the same problem if you take a fix with a gps unit. You have to change the GPS datum. If you take a celestial fix you won't be close enough to worry about the offset. JMO

[Aquaria]

Just read in a sailing magazine that a german participant in the ARC Round the world race put his 63ft boat on a reef in the western part of the Fitschi-Islands. The skipper relied on his electrononic charts!!! in an area where it Â*is known that charts are not very reliable....Â*

The older the the sources of data the charts are made of, the less reliable they are!

Paper charts published by the national official hydrografic offices mostly state two things:

- On maps of bigger scale you find a small chartlet somewhere on the corners survey data of the last depths soundings - and I am always surprized that these data are mostly out of a time before GPS!!! Â*I guess that these data also concern the shore lines (land surveying)?

- And most of the official charts still have this Notice/Warning:Â*Keep in mind that maps in some cases are not as exaxt as the postions derived from modern satellite based navigation aids. Therefor navigation must always be done with a safety margin taken into account. Think that 1mm in the map of a scale of 1:50.000 corresponds with 50m in reality.Â*I found this notice in the latest chart sollection covering the swedisch waters south of Stockholm (and I hope I translated it right).

Now we have the situation that we navigate with electronic charts mostly published by private companies. And these companies rely on official data (produced by the national hydrographic offices). So, these new electronic charts never can be better or more exact that the official charts.

So, I would not rely on a position shown on the PC - not even in swedish waters (where the data are excellent) and NEVER EVER between the reevs of the Fitschi-Islands.

Uwe

SY Aquaria

[JeanneP]

Originally Posted by Aquaria

Therefor navigation must always be done with a safety margin taken into account. Think that 1mm in the map of a scale of 1:50.000 corresponds with 50m in reality.Â*[/i]I found this notice in the latest chart collection covering the swedish waters south of Stockholm (and I hope I translated it right).

Now we have the situation that we navigate with electronic charts mostly published by private companies. And these companies rely on official data (produced by the national hydrographic offices). So, these new electronic charts never can be better or more exact that the official charts.

So, I would not rely on a position shown on the PC - not even in swedish waters (where the data are excellent) and NEVER EVER between the reevs of the Fitschi-Islands. {JeanneP note: = Fiji Islands}

I think that electronic charts and organized rallies, such as the ARC Round the World Rally, are two aids to "navigation" that can be the most helpful and/or the most dangerous for cruisers. (I don't like using " " marks, they are overused and are mental stop signs to reading text in a smooth manner, but in this case, I think/hope it's appropriate).

The problems with the above are that less experienced cruisers are likely to be lulled into a false sense of security, first by traveling in company with other cruising boats in an organized rally; and secondly by using what they consider the most up to date accurate charts. There is something seductive about seeing an icon of your boat on the chartplotter screen moving as you go along and appearing to have seen all the hazards and obstacles that exist. There are no bright red warnings popping up saying "we really don't know what's here, so be very careful and look at the water!"

There are many dangers to which their inexperience leaves them vulnerable.

Tropical cruising means a remarkably changing environment. Coral grows constantly and it is near to impossible to keep charts up to date even if there were an agency with the budget to attempt it. In Fiji there is nobody there to do it, so charts are outdated though generally accurate. How, though, can a novice know that if his research has been focused on the places to go shopping in Suva, the parties that are being thrown by the rally participants, and the support provided by the rally organizers?

These round the world rallies are designed to get a lot of boats around the world in a very short time. Peter and I had as much time to research - for example - Fiji as these people would have to research the entire round the world trip. With such limited experience they don't even know what they don't know.

I don't mean this as a criticism of the people who participate in these rallies, but more of a speculation of what can go wrong, and why. Cruisers on their own tend to gravitate to groups of other cruisers with varied levels of experience. There the cruisers can share knowledge and information.

What I see as both a plus and a disadvantage of rallies such as the ARC Round the World rally is that this "group" of boats will descend upon a destination port and generally hang out with other members of the rally group. In such a case, the participants will all be sharing a common fund of knowledge and ignorance. Not all members will be like that, but too often that is the case with the majority of such entrants.

For all that, I love most regattas and rallies for the camaradery they provide, and the fun they can be. I just think that people should be careful when joining them and not rely on others' knowledge too heavily.

[MMNETSEA]

Found this site which has FREE navigation charts for US inland waters and foe coastal waters also for the USA and elsewhere :-

C L I C K

[JeanneP]

Most of this information is on the Cruising Wiki, HERE

I have a small quibble with the copyright notice on his page. BSB is the format in which all raster charts are formatted, including all provided for free by the US. I know there are Maptech copyrights, but I believe they refer to the changes that Maptech has made to many of the NOAA charts, not to the BSB format.

For those who can run OpenCPN software on their computers, I am pretty sure that even the inland waterway charts are readable by that software, though I'd appreciate confirmation of that.

With regard to accuracy of electronic charts, there is the same problem with them as with paper charts. Because electronic charts chips are quite expensive, there is a lot of trading and swapping of the chips going on among cruisers, just as there has always been with paper charts. Where one can get into trouble is with all the changes that happen with land and ocean topography. I have no doubt that most cruisers have not updated their charts of the Gulf of Mexico post-Hurricane Katrina, and the changes there are dramatic. There is a new island in the South Pacific that appears and disappears as the underlying volcano burps - I doubt that there are any charts showing it. New volcanic island Cruisers post lists of new underwater hazards in the area, as mentioned in this topic.

Be careful.

[SeaVenture]

In the Sea of Cortez, the paper charts, from which digital chips were copied, are off, but if one uses eyeball navigation and radar (to keep a good 2 miles off a coast where a rock lies 3 feet under the surface and is not charted anywhere), then sailing the area is no problem. I remember navigating through a dangerous area between islets, heading between recommended waypoints but keeping a close lookout. Suddenly, off the starboard bow appeared some very skinny water. I turned to port and avoided an uncharted rock. We go VERY slowly in dangerous areas and pick our way past rocks and bars, using our eyes and great care. Subsequent trips had the exact location of the nasty bit of shallow water marked on our chart, so we never again came close to it.

Waypoints in guidebooks and paper/electronic charts are great aids, but nothing should take the place of sailing with caution. Here, in North Carolina coastal waters, shoals change with each season. The shallows aren't always easy to see, but slow going until one knows that year's channel changes is a must.

[truda]

By the way, is there a particular digital chart brand/system which is more accurate for the Pacific islands, in particular those farther from the beaten track? I suspect not, but would welcome opinions from people who´ve been using that kind of stuff there.

Thanks,

José Truda & Nalu

[redbopeep]

Originally Posted by truda

By the way, is there a particular digital chart brand/system which is more accurate for the Pacific islands, in particular those farther from the beaten track? I suspect not, but would welcome opinions from people who´ve been using that kind of stuff there.

Thanks,

José Truda & Nalu

I don't know the answer to that one. I suspect that, as SeaVenture noted, the digital charts are only as good as the paper charts and would expect that to mean that if a reef is missing or in the wrong location it will be the same for all chart programs. Hopefully someone else can opine on this shortly.

Fair winds,

[Nausikaa]

Well, the answer to that one is yes and no.

Yes, they can be as good as the official paper charts and, in some cases, are simply the scanned version of the official charts. However, they can contain errors that the official charts do not have. Unless the chart maker has access to his own surveys the digital charts can never be better than the official ones.

What digital charts can do is programme in the WGS 84 offset so in thhat respect they are easier to work with than some paper charts.

Aye // Stephen

[truda]

Many thanks for your comments! Seems there´s a consensus on digital charts x paper charts being more an issue of user common sense than trust in accuracy.

Cheers,

José & Nalu

[Aquaria]

No matter what: official paper charts are best concidering the point of actuality at low costs.

Meanwile it is no problem any more to get up to date Â*chart correction services offered by almost all national hydrografic offices online and for free.Â*

Well, it takes some time to apply the chart corrections on a chart that is outdated for, let's say 4 years, but with a little patience it should be accomplished. But most or us can not correct a digital chart this easy (maybe computer/programming gigs can...).

And except in tidal swept wadden seas with a fast change in topography and when major man made changes in water ways took place, the number of changes per chart are not as much as to keep charts updated over many years. Â*It's just Â*for example loading down the last weekly

notices to mariners

Â*- here I chose the UKHO, as they still cover alot ouf our planet, even the Pacific as far as it is covered by the British Admirality charts. (Click on WEEKLY and just choose Â*a pdf and select/download a week's content!).Â*

I was not yet in the need of the correction services by other major hydrografic servics in the Pacific area, but Â*there are Â*similar services in Â* Â* Â*Australia Â* , Â* NZÂ* , Japan and maybe other nations. Â*

So, with some patience and time Â*it is no problem at all to have up to date paper charts world wide. Download the free pdf s, draw/write the small changes into your chart (like new lighting characters of buoys) or print out the small chartlets with the more complex changes and glew them into your chart. If using a laser printer the correction is even as water resistant, as paper charts can be.Â*

Uwe

SY Aquaria

[timtwickham]

On the subject of chart errors and accuracy, some of you may be interested in the TeamSurv project (www.teamsurv.com).

This is a crowd sourcing project to create more up to date and accurate charts, using depth and GPS data logged by boats.

Using a hardware or software data logger, you collect data whilst doing your normal sailing activities, then upload the data to the web. There, tide height and other corrections are applied, and you can see the results online.

We are just proceeding to the next stage, where charts will be available for download in areas where we have sufficient data.

[redbopeep]

Originally Posted by timtwickham

On the subject of chart errors and accuracy, some of you may be interested in the TeamSurv project (www.teamsurv.com).

This is a crowd sourcing project to create more up to date and accurate charts, using depth and GPS data logged by boats.

Using a hardware or software data logger, you collect data whilst doing your normal sailing activities, then upload the data to the web. There, tide height and other corrections are applied, and you can see the results online.

We are just proceeding to the next stage, where charts will be available for download in areas where we have sufficient data.

Cloud sourcing and cloud data is all very interesting. However, the result is going to have a great deal of uncertainty in it. Given enough data in a particular location, you will have a reasonable picture of what is going on. However, it will take huge numbers of data points to get a chart reliable enough for use in navigation.

What sort of corrections to GPS data? Are you collecting which GPS system and what triangulation/sat's are in play when the data is being collected? Depth--how will you actually know that the boater is providing you with accurate depth information rather than simply depth below the transducer?

Again, cloud sourcing is a wonderful way to get large and helpful/useful datasets. It isn't necessarily a good way to pin point something with accuracy or precision.