The Offshore Voyaging Reference Site

Electronic Chart Dangers

It seems like every time a new version of an electronic charting program comes out the developers have added another way to view the chart: 3D, BathyVision, whatever.

These features look cool and using them is tempting, but they can also be dangerous.

Here’s an example:

Last summer we were meeting our friend Ed (he of the Watt & Sea) in one of our favourite anchorages.

We got there early and he texted us his expected arrival time. I texted back:

Me: Great. Watch out for the rock in the middle on your way in. It really is there, impossible to see, and bigger than charted. Go right over the port hand side close to the moored sailboat.

Ed: Don’t even see a rock on the Navionics chart, so I will cheat heavily to port on the way in?

I was stunned since the rock is clearly shown on the charts and Ed is an experienced, prudent cruiser, and no fool.

Later we talked about it and he sent me the screenshot above. There are two dangerous “features” at work:

  1. The red crowd-source marker is obscuring the tiny rock symbol.
  2. Ed was using what Navionics calls “Sonarchart”, which produces a contour-lines visualization representing depth.

OK, I know, we can argue that Ed should have seen the 5-meter contour and then clicked on that crowd-source symbol or turned that layer off, yada, yada, yada.

To which I say, bullshit! It’s always easy to criticize others from the safety of an armchair, but I believe the issue is that the screen that Ed was looking at is hard to read and counterintuitive.

The problem is that these new ways of showing charts actually show too much information and most of it is extraneous to safe navigation.

Let’s dig deeper and in the process figure out the benefits and drawbacks of various chart presentations:

Login to continue reading (scroll down)

Inline Feedbacks
View all comments
Roland Stockham

I think this might also be a case of using a good resource for the wrong purpose. If I am sailing into that harbour then the traditional view is great but if I want to see where to drop the crab trap that is what the Bathay chart was created for and it is great. Horses for courses eh.

Arne Mogstad

Hi. I agree that electronic charts can be deceiving and sometimes obscure the real dangers, as I have noticed similar to your example on several occasions. A feature I like on TZ (both the app and the computer software) is the ability to show ZOC (zone of confidence) to give a level of the accuracy of the charts. Other ECDIS, apps and software may also have this, but I haven’t seen it in many other non-professional programs.

I think the visual presentation on electronic charts still has a long way to go! Especially the one for the leisure fleet. I am always a bit surprised when I see professional navigation software (both in aviation and marine ECDIS) by how dull and “unsexy” they look. But I think that is a good approach, as the real information seems to be less hidden.

I never trust it fully, but when scouting new anchorages, I use the bathyvision feature extensively. I put it highly zoomed in, and as I’m trawling back and forth checking sonar depths, I confirm them with the data already on the chart. I also use it for planning, but while underway I highly prefer a more traditional view and I generally try to remove as much unnecessary information as possible. Another great feature in TimeZero (the computer software) is the ability to make shortcuts. So I set up a shortcut for “offshore” that has a full preset loaded with only the necessary information, and another for “anchoring” that will show more contour lines and so on, but is only used zoomed in, so it doesn’t clutter much.

That said, at least in Norway, I find the accuracy of the bathyvision to be really good, and it gives a much better view of potential unknown anchorages. Especially as a lot of areas don’t have any information at all in the traditional charts. But I understand that over the last few years, there’s been a massive job with high tech equipment to chart most of the Norwegian coast, so this accuracy may be somewhat unique to Norway.

Arne Mogstad

I agree, underway, I don’t think it adds any useful data at all. And for roomier and decently charted anchorages I also find in pointless. But for the places where I cruise (northern Norway), there’s a lot of times I “need” to anchor in smaller coves that are not necessarily ideal with regards to depth, room, and so on. And many of these areas are either completely uncharted, or provide so little detail of the seabed, that it’s essentially useless.

The way I use it, is that I try to familiarize myself fairly well on the chart before arriving, and choose a place I want to drop the anchor (maybe an alternative or two as well). I then have an idea of what to expect. Then, upon arrival, I will do my usual careful back-and-forth sounding of the area. Then it’s quite easy to notice if things are not as expected, and to notice WHERE this unexpected shallow is, as opposed to doing the same on an essentially blank screen (in TZ, I can also drop a marker with just a button on the keyboard). I feel this gives me a better mental image of where I am on the water, in relation to any obstructions I might find.

I’m not saying it is in any way essential, but for my mostly single handed sailing, I see it as a pretty useful feature IN ADDITION TO the traditional and usually much better presentations. At least around here, where they actually provide pretty accurate data as opposed to no data. I think this is due to a lot of this being actual official data, and not just extrapolation of bad underlying data.

I could have added an example, but I didn’t see if I could add pictures to the post.

Olivier Le Carbonnier

on boating navionics I put a draft of 5m / 16.4ft sometimes 10m / 32.8ft (the actual draft is 1.53m / 5ft).

Olivier Le Carbonnier

I mainly put these drafts for the autorouting.
otherwise, I rarely use alarms, I prefer to monitor navigation than rely on alarms (I have experienced navigation without electronics = old school)

Ben Logsdon

Don’t forget to also check crowd-sourced points if you have them turned off. After finding an uncharted rock, I submitted a point to Navionics to warn others.

Charles Hendricks

We’ve recently ditched TZ for OpenCPN and find the ability to switch between various chart sources and high definition satellite views to be very valuable in areas where common chart source (navionics, c-map, etc) are horribly inaccurate such as San Blas. Unfortunately, it required a purchase of an android tablet but it’s been worth it.

Conor Smith

We love open CPN. We had OpenCPN on a laptop aboard. v5 onward has split screen to show same view on two chart sources. I usually always used a raster on one side and vector on the other. In san blas, OpenCPN would easily display Eric Bauhaus’s Raster charts on one side and a complimentary electronic chart on the other.

Sat imagery came from Google earth, taken from Sat2Chart with pre-created and shared kap files.

For 185 dollars, we bought an android tablet with waterproof cover and the android version of OpenCPN from play store. This allowed me to be familiar with one chart software, on two devices. Plus all of the charts can simply be copied onto the microSD card of the tablet (no subscriptions). While offshore we saved power and just used the tablet. When approaching shore we fired up the old power hungry Lenovo with OpenCPN for redundancy. (last time I checked, the android version did not have split screen however, just one of the other).

Matt Marsh

This article is really bringing out the stark difference between a recreational chartplotter (which can be whatever the manufacturer wants it to be) and a commercial ECDIS (which must meet specific, legally mandated standards).

IHO S-52 was very carefully written, through hundreds of consultations over many years, to ensure that any navigator can sit down in front of any ECDIS on any ship and see exactly the information they need, in a familiar form.

The issues John raises in this article are serious enough that, at least in Canada, electronic navigation systems that do not precisely follow the S-52 requirements for chart content and display are explicitly prohibited from being used as the primary charting and navigation device on commercial craft. (See SOR/2020-216 §143(1).) They can be carried as toys or for convenience, but if the main ECDIS on the bridge of a commercial vessel does not display the charts per the IHO standard, the owner will be cited for a Canada Shipping Act violation.

The fact that we encourage recreational electronics manufacturers to do “creative and pretty” things with falsified (sorry, “interpolated”) data and ridiculous display schemes, that would be literally illegal to do on a commercial ship because of the potential confusion they cause, is worrying.

And, while I generally like Navionics, their insistence on integrating the ActiveCaptain crowd-sourced layer during navigation is, I think, irresponsible. ActiveCaptain was originally designed to be a voyage planning and services/facilities awareness tool. It was never meant to take the place of an actual chart, or to supercede the data on the chart, and it’s really gone down hill since the corporate buy-out.

Matt Marsh

You can turn off the Navionics ActiveCaptain layer easily, but it often turns itself back on when there’s an update or any other change that triggers a reset-defaults.

A lot of what’s in ActiveCaptain now is just dumb crowd sourced re-hashes of information that’s already on the chart. “There’s a shoal here with green markers, safe channel is to the north” — congratulations, AC contributor, you can read the chart that shows a big green shoal marked “Rock Shoal” with lightstation and buoy symbols on it, white to the north, and a solid blue shelf to the south. Good for you, here’s a lollipop.

Christopher Barnes

“interpolated” data reminded me of a similar issue with GRIB data where the software interpolates the data between data points. It is always a good idea to toggle interpolation off and on if you can to make sure one isn’t being lured into a false sense of confidence…

One of my Fundamentals when using apps and the like is to be wary/mindful of interpolated data presented the same way as the actual data (soundings or wind flags in a GRIB)

Over time, my affinity for unadorned data presentation (just the facts, please…) has grown to exceed my desire for pretty… it’s like pie charts, a nearly useless graphical presentation of data, whereas a simple table would be much more informative…

END grumpy “okay boomer” rant…

Eric Klem

Hi John,

I generally agree and we have both raster and vector in our plotter and I look at raster on a proper computer screen or paper before we leave.

That being said, I find the Navionics Sonar layer helpful in a few key instances. I keep it installed on my phone so do not have the ability to do it on the plotter and never use it for primary navigation. My understanding of this is that they are combining existing data with customer sonar data ( I have watched people participate in it but I have no idea how widespread it is, I assume it is mainly recreational fishermen who do this. You can also create your own sonar charts for just your use if you have the appropriate hardware (we do not). I have actually found the data to be pretty accurate and where it differs significantly from government charting, it is usually better. Examples of when we look at it are:

  • When considering an anchorage with marginal depths, we use it as a second source to see whether it is worth going in and trying. Sometimes it is also really helpful about which part of an anchorage to try.
  • When in a deep anchorage, we use it to help try to find an area of reasonable depth and not too steep to anchor in.
  • When there is no charting of an area, we look at it although rarely venture into those spots.

While I use it for these things, I am very aware that it is imperfect and here are some things that I keep in mind:

  • We never navigate like it is a trustworthy chart. If we are even remotely using it, the light is good, someone knowledgeable is on the bow and we are proceeding at <2 knots. Ideally we explore it by dinghy first at low water.
  • The source including date and how many customer sonar plots were included is not known.
  • How customer data is validated is unknown, a miscalibrated depth offset could be a real mess. As a result, we check the general trends against a government chart and also against our depth sounder.
  • There definitely seems to be some data smoothing so individual and pillar rocks seem to be missed sometimes.
  • Based on an area with shifting sand that we transit a lot, I can tell that the data isn’t getting updated often enough to keep up year to year.

A quick example with pictures. This is a spot where we really enjoy anchoring but the original survey data is terrible and we only went in due to local knowledge. The NOAA chart gives very little idea of what is going on in here and I would never dream of going in on that poor data. And in fact, if you did you would likely get into trouble. It just shows 5′ at the entrance and there is a 5′ spot that I have hand-sounded right in the middle but in truth you can just sneak around it. If you try to head to the north end of the anchorage, where the NOAA chart suggests it is reasonable to keep going, you will run aground on a pretty steeply shelving rock jumble that the sonar chart has correctly. Then as you head south into the area where people commonly anchor, the sonar chart correctly has you hugging the west side against a ledge to avoid shallows to the east which NOAA doesn’t tell you anything about. However, this is also the place where the sonar chart has issues as while it correctly identifies shallower water, it misses 1 big rock and a few smaller ones on the shelved area that stick up probably 4-5′ above the soundings it is giving. I just looked at Active Captain and in the comments, none other than Ben Ellison reports the big missing rock complete with a picture of it.

Don’t get me wrong, I agree with your main point, it is just that I do use the sonar chart in some very limited instances. I probably look at it twice a year so could easily do without and would lose maybe 3 anchorages we have used over the years. This all probably makes me sound more adventurous than I am, my wife always jokes that I still think that all boats I am on are 12’+ draft.


Eric Klem

And the NOAA chart.

Matt Marsh

The ones that are based on GPS-referenced, tide-corrected depth sounder data from actual boats (whether crowd-sourced from users, or from survey vessels contracted to the chart vendor) are, I think, quite valuable in many ways. Particularly so when the official chart is based on a century-old survey using lead lines, referenced to a sextant-and-theodolite datum of uncertain quality. I have used these many times on lakes for which no government charts exist, or on charted lakes where the official data is really sparse and the area I’m interested in is just marked as 50 acres of solid light blue “well, we didn’t survey this in detail because why would you ever take a ship in there” on the official charts.

The ones that are just interpolated from sparse data from an old survey are, I think, likely to do more harm than good.

Unfortunately, it is not always easy, or possible, to tell which kind of data you are looking at.

And you do need to be able to turn excess detail off when it’s not needed. While navigating, “Can I hit this, Y/N?” is sufficient, and if the answer is “N” then I don’t want to know about it. While fishing, or while dropping ROVs over the side, it’s nice to have much more detail.

Eric Klem

Turns out that Panbo has an article on adding sonar logs for the exact anchorage I used as an example. It also has some earlier (2014) screenshots so you can see the progression of the development of the chart as more logs are added.


Paul Browning

Where to start! First I completely agree with John about the dangers of vector charts which have always appalled me. I just can’t believe how easily we were all seduced by them such that we thought it not such a bad thing that critical hazards disappear from the chart unless you are zoomed right in. Not such a bad thing? CATASTROPHIC I’d say! It should have rung alarm bells with fishing & sailing associations and coastguard and sea rescue authorities years ago.

Then there is the attitude of the Australian Hydrographic Service which makes it incredibly difficult and expensive to purchase electronic raster charts. I bought a set (every Australian and NZ chart) through Memory Maps for $50-odd AUD 7 or 8 years ago. Ridiculously cheap. But now not available through Memory Maps because the AHO charge some $250 AUD PER CHART for the 1,000+ charts EVERY YEAR for electronic raster charts. By comparison, NZ charges nothing. You just download them for free. I think US likewise. I know very few other sailors who have the Memory Maps pack and none who have any other form of electronic raster chart for Australian waters. And now the AHO wants to phase out paper charts altogether as well, which will leave recreational boaters in Australian waters solely dependent on vector charts.

John, as a techie, lets the IT industry off the hook like everyone else in society, by applying a much lower standard to them. If civil engineers were held to similar low standards there wouldn’t be a bridge still standing anywhere!

We have computer reliant systems everywhere that are simply unsafe or at the very least, are unreliable. Whether it’s the “just two lines of faulty code”, as the Boeing witness described to a Congressional hearing, that rendered Boeing 737 Max airliners such death traps (costing 647 lives); or the so easily breached security of email (don’t put anything in an email you wouldn’t be happy to see go viral or lead the evening TV news or morning paper); or the now climaxing British Post Office scandal that’s been building for two decades, involving Fujitsu’s dreadfully flawed accounting software that saw postmasters accused and jailed for frauds they never committed that were all the result of flaws in the software. Hundreds of innocent people had their lives destroyed, lost their homes and reputations, were ostracized in their own communities. At least 4 took their own lives. The anger in Britain about this at present is absolutely seething and palpable right across society, especially since the recent airing of a four part dramatized documentary on the matter.

I run a business that arranges the settlement of real estate transactions which requires me to handle hundreds of thousands and millions of dollars of other people’s money on a daily basis. I know I can NEVER rely on email to securely manage this process. The risk of a malicious third party intercepting and intervening in email correspondence to mislead my clients (typically ordinary mums and dad) or my staff into paying enormous sums to the wrong people is too great. We are back to face to face dealings, sophisticated ID techniques and handwritten bank account numbers, such is the stratospheric nature of the risk. And that doesn’t include the very common glitches in banking software that sees them write off tens of millions every year and sees enormous stress incurred and energy expended on the part of their clients and staff. Nor does it include the hoax of cyber security, there is now no if, only when. Online digital systems, all of them, are fundamentally insecure. It is built into them and there’s nothing we can do at the user level to make them safe.

All this in systems we all use every day and take for granted. If there were non-digital solutions available we should use them in preference, but increasingly there are not. Having always been an early adopter of technology and a regular accuser of slow or non adopters as luddites, I’m just appalled at how badly let down we are by the IT fraternity and how seduced we are by the convenience of digital solutions with zero consideration of the flaws, which will be increasingly to our great cost. It’s absolutely time for us to hold tech to a much higher standard.

Starting with vector maps.

Mark Wilson

I agree entirely about those red crowd source markers. Unnecessary and dangerous. And they only make life harder for newcomers to electronic navigation: what is this strange red cross that I have never seen on a paper chart ?

From my flat in central London I have just opened up Navionics on my Ipad and discovered I have it set to Nautical Chart, No Overlay. I seem to have stumbled onto your way of doing things. This from trial and error. Mostly error.

My worst error was trusting the Automatic Routing function resulting in me using the wrong entrance into Barcelona and coming face to face with a low bridge. This got me a severe ticking off from the Guardia Civil – not an organisation you want to get on the wrong side of. If I had read the sailing directions more thoroughly this would not have happened. Mea Culpa entirely. I had already noticed that Automatic Routing tries to direct you far too close to the shore for my liking when making coastal passages but was surprised to be directed under a bridge suitable only for a motor boat when my profile on the system is of a sailboat with a 17 metre max height.

Dick Stevenson

Hi John,
Excellent article: thanks for putting it together.
One take, for me, is the seductiveness, without reflection, of labor-saving gear (not to mention the complicity of those flogging the gear). In my review of things, most labor-saving (or comfort enhancing) devices and gear, while saving physical effort and making life easy, have as the price you pay for muscular ease and greater comfort is increased mental effort in the form of greater discipline, diligence and attentiveness. This is especially the case when fatigue enters the picture,
Exemplary in this regard is, for me, the electric winch: clearly labor saving while at the same time capable of inflicting serious injury. It is hard to hurt yourself with a conventional winch, even when used in a fire-drill of some sort or by inexperienced and fatigued crew. Not so with an electric winch where having your wits ready-at-hand while using is necessary.
It is hard to name a labor-saving device that does not demand great mental effort. I am thinking of roller furling sails, full cockpit enclosures (comfort inducing), integrated instruments (take an autopilot connected to the chart plotter), davits, swivels, etc. etc. There are also, it seems to me,  too many skippers relying on AIS and radar, and their alarms, rather than regular visual sweeps of the horizon. The best labor-saving devices extend our senses, but do not replace them.
Random thoughts, Dick Stevenson, s/v Alchemy

Mark Wilson

Hi Dick
All good points.
As someone who gave up voyaging in 1988 and restarted in 2018 I am totally gobsmacked by how things have changed. And overwhelmingly for the better. I can’t get my head around some of them. Davits, full cockpit enclosures, swivels to name three. But people have circumnavigated with all of these. I haven’t and maybe never will.

Dick Stevenson

Hi Mark,
And, to add to John’s comments, some of the draw-backs are readily apparent, others are not. And the most under-appreciated drawback, to my observation, is the increased mental discipline and diligence that is necessary with some of these labor-savings and comfort-enhancing gear and devices to run a safe and seamanlike boat.
My best, Dick Stevenson, s/v Alchemy

Mark Wilson

Hi John

For the sake of clarity I was not voting for any of the above. Just saying that a lot of people seem to think they are a good idea.

There’s popular yachts and then there are Proper Yachts.

Matt Marsh

I have a banged-up outboard prop with six repair stamps, and a gouge in the tip of a keel, testifying to my occasional mistakes in reading charts or in going where there are no charts….

Re. Auto routing. I’ve been severely unimpressed by every single one of these I’ve ever tried. Maybe they might work for open-water passages, but around here, they always seem to pick impossible head-to-wind courses through channels where there’s no room to tack at the necessary angles. “Oh, you draw 6′, I can send you through this lovely little gap” — no, the buoys and sailing directions say to go far outside that shoal for a very damn good reason.

Dan Manchester

I very much agree in general John, however there are a number of scenarios where crowdsourced sonar data like that in Navionics is incredibly useful:

  • When trying to understand the bathymetry of wide shallow sandy anchorages, how close you should get to shore, and where it makes sense to safely drop anchor without potentially swinging onto a sand bar or mound.
  • In areas where shifting sands mean that crowdsourced data is always more current that official surveys. A good example of this is through the Great Sandy Strait in Qld where the sand moves constantly, channels are narrow, passage is tide dependant, and official data is useless; but the abundance of fishermen means that crowd sourced sonar is updated almost daily. (side by side image for comparison.)

Otherwise clarity > detail.

Dan Manchester

And the OFFICIAL offical chart is even worse.

Paul Gudelis

Heads-up, NOAA raster charts will be no longer in 2025.
On the NOAA website: All traditional NOAA paper nautical charts will be canceled by January 2025NOAA has already started to cancel individual charts and will shut down all production and maintenance of traditional paper nautical charts and the associated raster chart products and services by January 2025.
Mariners and other users of nautical charts are encouraged to use the electronic navigational chart (NOAA ENC®), NOAA’s premier nautical chart product.
Read more information about the Future of NOAA Charts.

Matt Marsh

The fact that NOAA publishes all their vector ENCs, and all updates to those charts, for free and in full compliance with IHO standards, makes me not worry about this at all. The BSB rasters were only ever intended as an interim solution while the official vector charts were being fully developed and cross-checked.

It’s more of a concern when a decision like that happens in a place such as Canada, where the hydrographic office locks the charts and their updates behind a paywall. Official Canadian ENCs cost $25 per file or $600 per region. The whole national set is $5400, you have to go through private-sector dealers to get “licensed for navigation” versions, and updating is very much manual. When Navionics or TimeZero charge you $25/year including regular automatic updates to use their charts on a set of iPads, and the government wants you to pay $1800 via a third party to get the “official” versions of the same charts and take care of updating them yourself, it’s no wonder that most people who don’t have a legal requirement to carry the official ENCs are going 3rd-party.

Alastair Currie

I agree that Navionics symbols for dangers to surface navigation are very small, dangerously so in my mind and the contrast red dots around black crosses (rock awash I think), I find difficult to focus on; it is just a poor design. I have stopped using Navionics as a result of this and switched to MemoryMaps and UK Hydrographic Office Admiralty charts in raster format (from my iPad). I currently use Raymarine’s Lighthouse charts, on an Axiom +12 and have found their symbol size that much larger. Raymarine also allow the symbol size to be increased through a menu option.

For decades I have used paper charts and have a full set for the West Coast of Scotland which I maintain up to date. Often people say rocks do not move, but there has been a significant increase is surveys for marine aquaculture, renewable energies and larger ferries servicing the islands. Many rocks have moved quite a distance with these surveys and some have had their depths changed as well.

I wonder if some of the blind trust issues associated with leisure based plotters and chart packages is psychological. I started navigation without access to any form of electronic position fixing. I was taught and developed to have a high degree of chronic unease about my position. As a result, studying charts and reading pilot books was mandatory to glean as much information as possible, as was plotting a course and updating position regularly. I believe I had a greater understanding of the threats and uncertainties around me. Today, I can be incredibly lazy with my navigation, but that chronic unease still sits on my right shoulder whispering in my ear, “are you sure?”

However, there is no doubt that electronic navigation is boon to safe sailing, folks just have to be aware of the limitations.

Bruce Brown

As a very amateur mariner, I have read the above Tip and responses and have tried to quell my impulse to run for the highest point of land and stay there. Trying to digest and render relevant to my sailing the information in the discussion reminds me of the challenge of mastering my new EV and its 400 page manual.

The history of navigation prior to the 20th century is the story of accumulation of data that reduced the likelihood of randomization and encounters with obstacles. However, in the 21st century, we appear to have reversed that trend (in almost everything) so that the usefulness of data is now less even if the volume is more and, as commenters have pointed out, so psychologically challenging in its volume and diversity that it becomes difficult to use. (Does this sound like your latest struggle with the TV remote?)

In my own life, I find sailing to be a struggle between enjoyment and what appears to be the almost-impossible mastery of technology and certainty. It’s tiring, frankly.

Nonetheless, the discussion above is worth a year’s subscription in both epistemological reflection and practical advice.

Stanley White

John you are 100% correct on coders not sailors developing the software. Check out the B&G systems controls (attached). Think about being in a seaway and needing to adjust the brightness or night mode. The Power Off switch is adjacent and easy to hit as your hand is bouncing around. The brightness is a simple slide control and, again, easy to accidentally turn the brightness all the way down.