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  #1  
Old 12-29-2010
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A good illustration of the pitfalls of chartplotters

From the latest 'lectronic Latitude...



Granted, the error may well be in the original chart...but still.
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Old 12-29-2010
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hard aground....
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Old 12-29-2010
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We have used Navionics charts in our Raymarine plotter from Northern Canada, down the US East Coast and throughout the Bahamas, and the entire Eastern and Western Caribbean. Once you leave the US, the UK and the French Islands the accuracy lessens until in southern Mexico there is no attempt to show anything more than the outline of the coast..

Anyone who thinks these things are accurate is heading for disaster.

Phil

Last edited by Yorksailor; 12-29-2010 at 11:53 PM.
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Old 12-30-2010
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Quote:
Originally Posted by Yorksailor View Post
We have used Navionics charts in our Raymarine plotter from Northern Canada, down the US East Coast and throughout the Bahamas, and the entire Eastern and Western Caribbean. Once you leave the US, the UK and the French Islands the accuracy lessens until in southern Mexico there is no attempt to show anything more than the outline of the coast..

Anyone who thinks these things are accurate is heading for disaster.

Phil
Well, they can be incredibly accurate most of the time, and that is the real problem...

And, lest anyone thinks you have to venture into the Third World before errors of such magnitude become likely, think again... Even in the superbly charted waters of our northern neighbor Canuckistan, such anomalies can easily take you by surprise...

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Old 12-30-2010
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According to my Garmin, I am on land, aground, while I know I am center channel, near my home, in 18' of water, in Louisiana.
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Old 12-30-2010
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Issues

I have also seen one area (southern tip of Matagorda Peninsula) where we were showing to be about 100 yards on land. Since the error wasn't repeated elsewhere (farther northeast) I think the error was with the chart and not an electronic issue. Not sure if the peninsula could have grown that much since the old charts were made although since the cut was made, a lot of shoaling has taken place as well as changes from hurricanes.

Here are a few other GPS error issues:

GPS explained: Error sources

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Originally Posted by Leocat66 View Post
According to my Garmin, I am on land, aground, while I know I am center channel, near my home, in 18' of water, in Louisiana.
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Old 12-30-2010
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If available radar overlay is very useful in revealing most electronic chart discrepancies. Its harder to check paper charts, which often have the same errors.
Unfortunately with chartplotters people often trust them too much and leave little margin for error.
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Old 12-30-2010
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Exclamation

Surly everyone knows that electronic charts are not to be relied on. They are a valuable aid to navigation but cannot replace appropriot paper charts, manual position checks and good common sense watch keeping while sailing.
A common failure is transferring GPS positioning of the vessel onto the chart. This is software driven and errors can occur for a variety of reasons such as offset settings and WGS interpretation. Actual positioning should always be checked against other visual aids.
Coast line and river beds in particular often change overtime and present a challenge to all charts.
Sand bars positions and bay depths can never be taken as accurate on any chart and must be updated regularly, as do navigation aids and known hazards marks.
Once the vessels course and position has been verified as reasonably accurate the skipper can relax and enjoy the ease of passage making inherent in the prudent use of a good chart plotter until he/she judges a position fix is again required for verification.
My laptop based chart plotter can pass a winters afternoon in comfortable summer cruise planning.

Happy sailing
Brian
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Old 12-30-2010
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[QUOTE]
Quote:
Originally Posted by centaursailor View Post
Surly everyone knows that electronic charts are not to be relied on. They are a valuable aid to navigation but cannot replace appropriot paper charts, manual position checks and good common sense watch keeping while sailing.
That is true, of course, but in the overwhelming percentage of cases, electronic charting is based upon a paper chart it is either duplicating, or “interpreting”… Amazing, how many people out there appear to think that C-Map or Navionics actually goes out there and conducts their own surveys of the world’s waters… (grin)

One of the biggest dangers of electronic navigation, IMHO, is that so many people don’t have an understanding of what they’re looking at when viewing an electronic chart, and what inherent inaccuracies exist when data has been transferred to a digital medium, then coupled with our current ability to place our position with a greater degree of accuracy than likely exists on our charts (Personally, I think it’s absurd that we’re able to establish our position within a circle of accuracy that is far smaller than the boats most of us sail, it only further heightens the absurd degree of "faith" many people are placing in the accuracy of such a means of piloting, but perhaps that's just me.)

I think the purveyors of electronic charting systems have done an extremely poor job of attempting to educate their end users regarding the nature of the medium they’re relying on… I like the analogy of viewing a TV screen to try to explain this: Most people think they’re seeing an actual image akin to a photograph when they’re watching TV, but as any still photo shot at a shutter speed higher than roughly 1/30 second will indicate, what we’re really seeing is a very incomplete image, comprised of nothing more than a series of continually scanning lines… I think it is very important for those using electronic means of navigation to constantly keep this in mind, that no matter how snazzy all this 3-D crap and whatnot may appear, it still presents a picture that is inherently very “incomplete”…

The best treatment of this subject that I’ve found is from Nigel Calder’s excellent work HOW TO READ A NAUTICAL CHART. Sadly, this is precisely the sort of book today’s kroozer thinks is “outmoded” – who needs to know all that stuff anymore, I’ve got this magic box that does all that stuff for me, and so on…

Here’s a relevant excerpt from his article years ago in OCEAN NAVIGATOR, sorry about the weird format, that’s the way it copied from their site… The link at the beginning will take you to the full text of his article...

I feel it’s extremely important for anyone using electronic charts to understand what he’s saying here… If you do not after a first read, read it again, and again, until you do… (grin)

I've bolded a portion of his text which I think reflects the most important conclusion/lesson for a prudent navigator to draw for all of this... This, in a nutshell, is why - to use one recent example - it still would have been incredibly risky for the skipper of RULE 62 to have entered the North Bar Channel at night, even in a flat calm/ideal conditions...

Quote:


Ocean Navigator | The magazine for long-distance offshore sailing and power voyaging

Survey accuracy

The extent of bottom coverage is one issue. The level of accuracy with which data is collected is another. Pre-GPS inshore surveying had a customary positional accuracy within 1.5 millimeters times the scale of the survey. Offshore (out of sight of reference points on land), positional accuracy was far less.

What this means is that given a survey at 1:20,000, the level of inshore accuracy in charting soundings and features would be within 1.5 x 20,000, which is 30,000 mm or 30 meters. It was common practice to survey at twice the scale at which the data was used — e.g., a 1:20,000 survey would be used to construct a 1:40,000 or smaller-scale chart. So now we have a level of accuracy of +/-30 meters on our 1:40,000 chart. Thirty meters at this scale is 0.75 mm on the chart. Given traditional plotting techniques, no navigator could plot with a precision greater than this, so all the pieces fit together.

With the introduction of GPS and electronic charts, the pieces started to fall apart. Any modern off-the-shelf GPS has a positional accuracy of 10 to 15 meters. If you add wide-area augmentation system (WAAS) corrections, this goes to 2 meters. So long as plotting is done on paper charts, the fact that the GPS is more accurate than the placement of details on the chart is more or less irrelevant because the accuracy of a fix would continue to be determined by the accuracy with which it could be plotted. However, electronic charting with electronic position plotting provides a degree of precision that the underlying chart data simply does not support.

Current International Hydrographic Organization survey standards call for a positional accuracy of +/-2 meters for all critical soundings (harbors and channels), and +/-5 meters for most other inshore soundings, with less accuracy required offshore. Although these standards are no better than a WAAS-corrected GPS, the pieces fit once again. Unfortunately, this is somewhat irrelevant, because hydrographic offices worldwide have shifted resources into digitizing existing databases without much new survey work!

Compounding the problem

The picture with the existing Databases is a bit more complicated than this. The printing process used to manufacture paper charts
results in an image display resolution equivalent to around 1,000 dots per inch (dpi). Modern computer and charting display hardware
has a screen resolution that varies from a low of 40 dpi (some cheap chart plotters) to a high of around 100 dpi.

Because of the much lower screen resolution as compared with a paper chart, if you display the fine details of a paper chart on a screen at the same scale as they are shown on the chart, you won't be able to read them. At the electronic chart display and information system (ECDIS) level, which sets the standards for electronic charting on big ships, the industry seems to have settled on a zoom level of 1:1.7 for making electronic chart displays readable. This is to say a paper chart at 1:20,000 is displayed electronically at a scale of 1:20,000/1.7, or 1:11,765. At the recreational level, for reasons I do not have the space to explore in this article, the relationship between a paper chart and its electronic display is often 1:6.25. That means a 1:20,000 paper chart is now displayed at a scale of 1:3,200.

Let's consider an area of an original survey that was done at 1:10,000, with a positional accuracy within +/-15 meters. This area was found to be foul with rocks. The chartmaker, working at a scale of 1:20,000, showed a couple of these as close together as possible using traditional drafting techniques, which is to say about 2 mm apart (representing 40 meters in the real world). No sane navigator plotting on this paper chart would try to take a vessel between these displayed rocks. But now this chart gets digitized and displayed at 1:3,200. The space between the rock symbols on the display has just increased to 2 x 6.25, or 12.5 mm. All of a sudden, it looks like I can take my boat between these rocks, especially when its position, based on my WAAS-corrected GPS, is displayed with pinpoint accuracy. Unfortunately for me, there is no gap between these rocks — just another rock the chartmaker could not show at the original chartmaking scale of 1:20,000.

Overzooming a chart — i.e., using it at a scale for which it was not designed — is one of the cardinal sins of navigation. All electronic charts are overzoomed to some extent, with those found in the recreational marketplace typically grossly overzoomed. To compensate for this, in theory, any navigator using electronic plotting should place an imaginary circle of possible error around the boat's plotted position. This circle should have a radius equal to the allowable error that was used in plotting the features on the chart. Note that this circle of error does not represent errors in plotting the position of the boat, which will be phenomenally accurate, but instead represents the extent to which the features around the boat may be out of place on the chart. All potential hazards should be kept outside the circumference of this circle, or, put another way, if any comes inside this circle, we are clearly into the realm of eyeball navigation.

So how do we determine the radius of this circle? We need to know two things:

# The level of accuracy of the survey on which the chart is based.

# The scale at which the chart is being displayed.

Unfortunately, neither is readily available for most electronic charts supplied to the recreational marketplace!

Many (but by no means all) paper charts have the first part of this information included in a source diagram, and the scale is, of course, clearly stated beneath the chart title. A source diagram is typically a small inset chart that breaks up the area covered by the chart into sections according to the age of the data on which each section is based, the scale at which this data was collected, and the method used to collect it. When navigating in any of the waters covered by the chart, a quick glance at the source diagram will provide a good sense of the accuracy and reliability of the underlying data for that section of the chart.

Last edited by Faster; 12-30-2010 at 09:28 AM. Reason: fix formatting
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