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Old 06-06-2009, 10:41 PM   #1
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Navigation and charts

How accurate are your charts?

The year before we left to go cruising "for a couple years", I set out on an intensive education program for myself. I read Chapman's PILOTING from cover to cover. I did the same with DUTTON'S NAVIGATION AND PILOTING and a few heavy weather reference books. Didn't make me an expert on anything, but it did familiarize me to where I could go to find answers to my questions.

Although I believe that no book can teach you how to sail or read navigation charts, I used those references too, as a guide to my practical experience.

All this brings me to electronic charts and the accuracy of all charts, electronic or paper. I hear so many comments to the effect that with the accuracy of GPS electronic navigation is so much more accurate nowadays that it is silly to carry all those paper charts.

In the past several years we have made three or four trips up or down the entire Intracoastal Waterway (ICW). I have a navigation program for my computer with a GPS receiver that connects to the computer via a USB port, giving me real-time chart plotting the same as if we had a dedicated chart plotter. In some ways my system is somewhat more accurate than the chips inserted into the chart plotter because I download the charts from a NOAA site just before we head out, and the most recent notices to mariners are included.

Countless times when we’ve been in narrow sections of the ICW I’ll be startled to see on the chart that we’ve drifted out of the channel, only to look up and see that we’re approaching the next mark correctly after all. That’s an “error” of around 20 feet. Since we have been told that GPS fixes are accurate to within two or three meters, I began to worry about the quality of the charts and the GPS receiver that I was using and I was back to distrusting electronic navigation again. *

Fortunately I found Ocean Navigator magazine’s Spring 2002 issue in a cruising book exchange which had an excerpt from Nigel Calder’s latest book, “How to Read a Nautical Chart.” I’ll just quote one sentence and then direct you to the article itself.

“In the end, the absolute accuracy of the average harbor, approach or coastal paper chart is generally not less than one millimeter with respect to the chart datum, which is to say that the charted positions of features should almost always be within one millimeter of where they would be if the chart was completely accurate. Put another way, most of the time the cumulative errors will not exceed one millimeter x chart scale. For a 1:40,000 chart, this is 40,000 mm = 40 meters, or 44 yards. There will, of course, be many specific bits of data that have been derived from older and/or less accurate surveys than the norm, and which, as a result, fall outside these parameters, sometimes by a wide margin, as will almost all charts of areas beyond the immediate coastal belt.*

This is all a long-winded way of saying that the user of any chart should not be lulled into a false sense of security about its accuracy.”


Or, the width of a pencil line introduces an error size that is much greater than the accuracy of the GPS. *

Here’s the link: http://www.oceannavigator.com/ME2/dirmod.a...3FE786388483146
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Old 06-06-2009, 11:18 PM   #2
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I have a navigation program for my computer with a GPS receiver that connects to the computer via a USB port, giving me real-time chart plotting the same as if we had a dedicated chart plotter. In some ways my system is somewhat more accurate than the chips inserted into the chart plotter because I download the charts from a NOAA site just before we head out, and the most recent notices to mariners are included.
if u dont mind sharing jeannep , im trying to work out the computer gpd navigation thing myself. if i may ask, what software..version..and gps are you using? im trying to set up SeaClearII to work on my computer with my garmin 60csx reciever...not having any luck syncing SC2 with the 60csx.
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Old 06-07-2009, 03:44 AM   #3
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We love our computer with its GPS receiver, our AIS, our stand-alone gps chart plotters, and our radar that works in the pilothouse computer as well as outside. We also use paper charts. But recently we noticed that, according to two separate GPS systems, we were parked right in the middle of Isla San Fransciso (Sea of Cortez) when in reality we were 200 yards off the north shore in 20 feet of water. And back in CA, if we'd used the plotter instead of our eyes, we'd have run right up on land several times. As my husband says, "Trust your eyeballs and not merely your toys," a maxim he followed as a Navy pilot. Granted, that radar is awfully handy when the fog obscures vision...
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Old 06-07-2009, 04:00 AM   #4
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I forgot to mention that I always have paper charts as well sitting out on the chart table. *I will never surrender myself to using electronic means exclusively. *For me, that way lies disaster. *When we bought the MapTech chartpak for New England it came with a companion CD of electronic charts and a stripped-down version of their Offshore Navigator, called appropriately Offshore Navigator Lite. *It is extremely easy to use, and I don't need the full program so I'm happy with it. *

I just downloaded Sea Clear but I haven't tried to work with it yet. *My problem, also, is getting the program to recognize the GPS. *Since I won't be able to fiddle with it for a few days, I guess whoever figures it out first should give a shout.

I got a hint that you have to download and install G7towin.exe (I found it by putting that into Google search). *I've done that, but I'm not quite sure whether I should paste G7toWin.exe into the same folder as SeaClear_2.exe - I think that's what I'm supposed to do.

Sea Clear seems to recognize the GPS on COM Port 1, but my GPS (Deluo USB connected receiver) installs itself wherever - right now it's on COM Port 7. *Since I can't trust it to work I won't try to fiddle with it and use the program until we're stopped somewhere for a full afternoon. *A couple days, perhaps.

So, first one to get a fix wins.

J
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Old 06-07-2009, 04:08 AM   #5
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*"Trust your eyeballs and not merely your toys," *
Absolutely! *There is a lot more to Calder's article and he goes into the reasons that GPS is a wonderful tool that can lead you into misery. *

I *hate to tell somebody something that is known to them, but how do we know what others may, or may not, know? *So. *All charts printed since GPS became widely used will provide offsets and datum that one needs to enter into the setup of the GPS to correct for differences between the charts and the actual coordinates. *I've noticed that many people don't notice them on the charts, particularly places like the Philippines and the S. Pacific Islands.
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Old 06-07-2009, 07:08 AM   #6
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Granted, that radar is awfully handy when the fog obscures vision...
Exactly the point. RADAR is a great tool and, as it shows you the land and navigational marks without linking them to a GPS position (unless using more modern RADAR equipment) it is really presenting you with the eyeball picture in a different format.

GPS is a wonderful tool and, I am convinced, it has reduced the number of SAR missions drastically. However, when we connect a GPS to a chart we may be linking 21st Century technology to surveys done well over 100 years ago, especially in the more remote parts of the world. The surveyors and cartographers from James Cook onwards did a wonderful job but they had very limited means. They produced astonishingly accurate charts: charts which were made to a degree of accuracy to which the average navigator of the age could never aspire. This was the case until the introduction of GPS; a tool which really allows pinpoint navigation and which is more accurate than the charts.

The major inaccuracy in the old charts was is establishing the position of a reference point from which all data was plotted. The reference point could be several cables out. In fact, even in well charted and frequently navigated waters one can find discrepancies. The position of the lighthouse on the island of Vinga, the landfall light when approaching Gothenburg, differs between the Swedish and Danish charts. Nonetheless, the charts were perfectly good for navigating as the absolute positions of rocks and other hinders to navigation was unimportant as everything was relative and so, the position of the ship plotted by, say, terrestrial bearings, was correct relative to everything else. The errors become noticeable first when positions are plotted from systems not related to the chart in use. We then discover the discrepancies but, by applying the correction stated on the chart, we end up with a perfectly usable fix.

In theory, the use of digital charts, i.e. plotters and PC based charting, should reduce the errors. If using, for example, British Admiralty charts in their digital form, then it does as the correction putting the chart in the correct lat and long is done for us. A problem arises though when using other than official charts. In this case the chart publisher is taking the data from the official publication or surveys and publishing it digitally in his own format. The major problem here is that there is scope for error. I have seen a chart produced by a well known manufacturer of plotters and charts which showed no small skerry off Gottskär (south of Gothenburg) where the official Swedish chart did. Now that, to me, is a very serious omission and one which could cause a lot of grief.

When I was a cadet I had it drummed into me that such things as RADAR, compasses, sextants etc were only aids to navigation. By using a combination of these aids, keeping an account of one's position and plotting DRs or better EPs on the chart and comparing these with the positions found using the aids then one could claim to be navigating cautiously and safely. GPS is a wonderful tool as are chart plotters. I always amazed as I see my position changing against the background of a digital chart but never forget that even this, for all its advantages, is still just an aid to navigation.

Lastly, as regards the system I use, I have paper charts but use British Admiralty and official Danish and Swedish charts in digital format on my laptop with Navmaster Offshore software. This is far more expensive than plotters and their charts as the software was not cheap and the BA charts come at the normal price but it does give peace of mind.

Aye // Stephen
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Old 06-08-2009, 02:26 PM   #7
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To continue the discussion of the reality of nautical charts - as was mentioned by others most of the worlds nautical charts are at least 40 years old - See the NOAA disclaimer page for downloading e-charts. In remote areas of the world it is true that the latest survey was done by Capt Cook back in the 1800's. In a lot of areas in the Pacific - especially off the commercial shipping routes - the charts are blank and do not show hundreds of reefs and atolls at all. If nobody (i.e., commercial or military) shipping go there - who cares enough to spend the money to survey the area . . .

For the Pacific there are supplements available on the internet like the "Pinniped waypoints" that fill in with GPS locations of the "missing" hazards, etc.

Normal "Map Datum" to GPS position errors vary from 1 nm in the USA and other first world countries to over 10 nm in the remote area like the South Atlantic ocean.

When "vector charts" and plotters were first introduced the errors/ omissions of critical navigation data was serious enough to cause many collisions and groundings with geographical obstacles. The early vector charts were "interpretations" of actual paper charts. Now they are better but still lack the details and information on actual paper charts.

"Raster Charts" are actual computer scans of real paper charts so are valid substitutes for paper charts. You can always print out selected areas from your electronic raster chart if you want a "hard copy" in case the electronic system fails at the most inopportune time.

However, knowing how to navigate the "old way" with eyeballs, magnetic bearings and radar is - in my humble opinion - critical to survival on the seas. Since raster charts have geographical data that is decades, if not centuries old - relying upon your GPS "boat" symbol overlaid on your e-chart can lead to disaster at worst, or frustration at best.

I have a complete inventory of over 14,000 e-charts of the whole world on my hard drive which I can call up and display and use with my ancient "The Capn version 6.1" system. Keeping that many charts in paper would probably sink my sailboat due to weight. - - And - - paper charts crumple and tear after a few years or decades whereas the e-charts stay fresh.

So you have to learn "how to use" and mentally "re-calibrate" your charts / e-charts to present to you a "semi-accurate" presentation of where you are and what the hazards are around you. On a bleary, crossed eye-balls morning being able to look at an electronic presentation of your position and track as opposed to trying to shoot bearings and translate that to a paper chart can make the difference between life and death. Electronic chart readers for lapbook computers and other computers range from free to prices about half of the cost of the fancy marine chart plotter systems. And the charts are free from NOAA for the USA.

The only "accurate" - really accurate charting system is any computer navigation system that allows you to record and recall your past "tracks" as you enter and exit harbors and other areas. These "tracks" are a series of lat/long points stored on the computer from your GPS as you sail. They are then recalled and displayed on your computer the next time you visit that area - and if you didn't hit anything last time, it is highly likely if you follow your past "track" that you will not hit anything this time. It is not uncommon to see my past tracks displayed on an e-chart sailing right through a mountain or other coastal obstacle on the displayed e-chart. That is because as mentioned above, the original charts have minor to significant Lat/Long "offset" errors. Some charting programs allow you to "re-calibrate" your e-charts but most do not and anyway it is a time consuming process. So keep your eye-balls open and use the chart display as the guide to your destination - not the authority.
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Old 06-10-2009, 06:39 AM   #8
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Used Calder's book to sail around Cuba. Amazing detail.

Each form of navigation has it's good and bad. It's true that the thickness of a rhumb line can represent 20 - 30 meters. Thats why the prudent captain uses all forms of navigation and sends a watch forward in tight areas. as far as errors on GPS, be careful how you interpret the error. We once slipped through a break in the reef south west of Isla de Juventude at three in the morning, mostly by Lowrance GPS and radar but with me conning on deck. I heard the reefs breaking on the one side and knew we were fine. But then the next day, when tied up in Neuva Gerona, the GPS showed we were on land. So why so accurate in one instance and not in the other. Well we took the Lat & Long the GPS was putting out and figured we were actually fairly accurately represented. It was the mapping that was wrong. And thats why you use everything you have to cross check and cross reference. Grenada was always a little off for us, Hogg Island was misrepresented and some parts of antigua. But if you read the chart, watch the contour lines, watch the depth, read the GPS and look out the window, there's no reason to fly into a mountain, or sail into it.
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Old 06-10-2009, 07:57 AM   #9
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One important point to consider.

The accuracy of your GPS is limited by the accuracy of the GPS antenna, its view of the sky, and (very importantly) its siting on the boat.

I have seen a few boats where the GPS antenna has been mounted high, in the rigging, or on the mast. This is a very very bad idea. You will introduce a large error here by the effect of "signal bounce", which is the satellite signal bouncing off the water and being received by the antenna some off the water. To reduce the effect of signal bounce it is important to have the GPS antenna mounted low, along the gunwale or at the pushpit, as close to the waterline as possible (while maintaining an uninterrupted view of the sky).

A GPS manages to get its accuracy by averaging out the signal path of 6, 8, 12, or sometimes as many as 20 (although in practicality rarely more than 14) satellites. A single signal path being out by 30 or 40 metres can reduce your GPS accuracy to +/- 10 metres or more depending on the number of satellite fixes you are currently receiving.

You also need to take into account the signal bounce caused by nearby boats and geographic features. For example cliff faces, buildings, etc, rarely a problem at sea but don't trust your GPS in enclosed areas -- even trees on the shores of rivers (and inter-coastal waterways) can be a problem.

I have experimented with a Garmin GPS 76 (one of the newer model receivers) and a Garmin antenna connected via a long length of coax cable which allows me to site the antenna anywhere. I can easily introduce a signal path variation of +/- 60 metres by deliberately siting the antenna in a bad location either in the rigging, under the barbecue on the back of the boat, inside the boat (with only a small view out of a porthole), or below the gunwale with its view of the sky obliterated by the steel hull of the boat.

Also, beware of any of the small USB "mouse" type GPSes. These are not nearly as accurate as a proper ships GPS with an external antenna. The internal antennas of these GPS devices rarely have sufficient accuracy especially if sited inboard on a vessel with its view of the sky obscured by the ship's hull and decking.
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Old 06-11-2009, 04:19 AM   #10
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Some folks are just missing the point of the original question - - There is nothing wrong with your GPS. If it is reasonably modern and has differential/WAAS capability it is accurate enough to plot where you keep stuff onboard. What is inaccurate are the nautical maps! As has been stated, the most recent versions are about 40 years or so old when it comes to geographical data. The "revisions" have only to do with commercial harbor markers/bouys and relocated channels which are used by commercial ships. Everywhere else the geographical data has not be updated because there is not money allocated for the survey ships. This is described in the NOAA disclaimer (not the money part, I got that first hand from a NOAA rep at a boatshow).

The GPS gives you an accurate Lat/Long but without a map that matches the Datum (WGS-84) and current geographical survey, those numbers are meaningless or at best misleading. There are over 400 different DATUMS in use worldwide - 124 of them are in the USA. The money to correct old charts to WGS-84 does not exist so you can expect to see from minor to excessive plotting errors when comparing your GPS position, chart position and eyeball outside position.

Any nautical navigation system will get you to the "vicinity" of your destination and then you must use your eyes, ears, and brain to "correct" any offset between the e-nav systems and reality. I mentioned before that if your e-nav system has "track" storage and re-display you can actually take full advantage of the GPS system's accuracy. Of course, you have to have been into the area before in order to record a track. But thereafter any time you return, you can display your previous track and be assured that you will not hit anything, if you didn't hit anything the first time. Of course this excludes other vessels or any man-made objects that are either new or been moved. And if it has been a significant length of time since you last entered the area, Mother Nature has the nasty habit of moving channels, bars, and other natural obstacles. Common sense is always the by-word for navigation.
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