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Old 04-04-2010, 11:32   #151
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Nick said, Yes, just like many paper charts are and most (90% or so?) sailors use non official paper charts.
Not disagreeing with your general sentiments Nick but in the UK, charts come from the Admiralty. They may have been republished by a reseller but the data comes from Admiralty charts.

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Old 04-04-2010, 12:07   #152
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Not disagreeing with your general sentiments Nick but in the UK, charts come from the Admiralty. They may have been republished by a reseller but the data comes from Admiralty charts.
P.
That's correct but having the Admiralty data doesn't mean they meet the requirements for commercial shipping. I think they don't. That's why they are often called Yachtsmen Charts or whatever variation. Imray is a good example: the perfect paper charts if you ask me but I don't think they meet the legal requirements for commercial shipping. And there are no requirements for charts for yachting that I know of.

Now, between you and me (the rest may read ;-), the Dutch and UK sailors all know how to navigate. We all studied the books and probably joined classes and passed exams. When we switch to digital chart plotters, we discover the gains that a computer adds to navigation and enjoy it, but we still know how to navigate. We can take a set of bearings and use them to plot our position on the digital chart plotter. But there are other (not to be named) nations where there are no requirements at all. Many sailors that go out to sea with a digital plotter have never put a pencil to a paper charts and think compasses are the round things in the cockpit. If you give them parallel rulers they have no clue what to use them for, let alone the more modern aids to paper chart navigation. Now, you don't really need those anymore but you still need to understand where you used them for. I have met many sailors that had expensive binoculars with integrated compass but without a clue that it can be used for navigation or collision avoidance.

All this doesn't mean that there are no competent navigators from those nations, to the contrary. But I think it is that group of competent navigators that is against electronic charting much more than us because they are right between many incompetent navigators and have to deal with their cluelessness (ha, that word wasn't in my spelling dictionary ;-) every day. I think they get frustrated because of all the sailors that can't navigate but go out to sea with their gps'es and chart plotters anyway. I can understand that after sailing through those parts of the world.
And then there's the old salts that will just never trust anything with a CPU in it and want to hold onto the old ways. Nothing wrong with doing that unless they start stating (wrong) facts about systems they never used nor understand ;-)

50 years from now the discussion will be much different and I can even see a future where no education in navigation is needed anymore. I wouldn't oppose that when I live to see it happen.

cheers,
Nick.
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Old 04-04-2010, 14:37   #153
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If your radar tells you the marker is at a bearing of 214 deg. and a distance of 103 meters, that is where it is regardless of what your paper chart and sextant tell you. The radar is right... again: THE RADAR IS RIGHT..
WRONG

The distance will be correct

The bearing accuracy will be dependent on the aerial beamwidth and the accuracy of the system providing bearing input, plus the accuracy of the system taking the bearing .

If all of these are cumulative, there may well be significantly more than 10 degrees of error
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Old 04-04-2010, 15:03   #154
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WRONG

The distance will be correct

The bearing accuracy will be dependent on the aerial beamwidth and the accuracy of the system providing bearing input, plus the accuracy of the system taking the bearing .

If all of these are cumulative, there may well be significantly more than 10 degrees of error
Every system of measing distance or bearing has some degree of inaccuracy. Radar within its limits of accuracy will always give the correct result.
Charts, paper or electronic, can contain errors which result in significant, unexpected mistakes. I can site several exaples of charts in error, as can any crusing sailor.
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Old 04-04-2010, 15:17   #155
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You don't have the laser rangefinder option bolted onto your portable GPS? But even if so ill equipped, surely you could use the radar to tell you that you are exactly 103 meters off that rock? ;-)

ciao!
Nick.
One small correction. The radar will tell you that you are exactly 103 meters off the portion of the rock visible above the water. Charts, paper or otherwise might be handy for giving you a clue about that part of the rock which lies below the surface!
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Old 04-04-2010, 16:07   #156
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WRONG [...]
The bearing accuracy will be dependent on the aerial beamwidth and the accuracy of the system providing bearing input, plus the accuracy of the system taking the bearing .

If all of these are cumulative, there may well be significantly more than 10 degrees of error
[... self moderated...] ;-)

However, I just checked the bearing accuracy specs of my 15 year old toy radar (Raytheon with 4 kW dome) and it is +/- 1 degree. All the heading sensors I know are better than 1 degree (within 45 degrees pitch/roll). When you take a bearing on your radar, you put the cursor on the echo and when you point right, it's a CPU doing the math with an error so small that it isn't a factor.
So it will be accurate to within a degree nominally but with a possible error of less than 2 degrees. Nothing like 10 degrees, that is just silly.

I would love to hand you my Steiner binoculars and see of you get it more accurate with some nice roll and pitch ;-)

Again, on a boat the radar is the most precise instrument to take a bearing with and what it shows is how it is as far as navigation is concerned. When your chartplotter shows you are gonna clear that rock but it shows dead ahead on your radar, you better turn.

In Holland some time (hummm 20 yrs) ago, to pass the test to get your radar permit, you were put at the helm of a small tugboat with the bridge windows blinded. You had to do many maneuvers incl. docking and everyone I know taking the test passed it without crashing into the wall. Radar is that accurate.

ciao!
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Old 04-04-2010, 17:28   #157
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That's correct but having the Admiralty data doesn't mean they meet the requirements for commercial shipping. I think they don't. That's why they are often called Yachtsmen Charts or whatever variation. Imray is a good example: the perfect paper charts if you ask me but I don't think they meet the legal requirements for commercial shipping. And there are no requirements for charts for yachting that I know of.
That correct Nick, British Admiralty charts are recognised as meeting the requirements of commercial shipping, in fact in most instances they are setting the benchmark to other hydro office's around the world, also many nations around the world contract the British Admiralty to chart their coastlines for them, Admiralty data being applied by a re seller is not recognised....

Correct Imray are not recognised as mentioned above, at least on IMO registered vessels as it is a recognised fact in commercial shipping that you must be carrying the official up to date and corrected charts for the area(s) you are operating in, and these must come from the official Hydro office of that country, I might also point out that even if you are carrying the correct charts for the area, if they are not the latest edition and corrected up to date you are in breach of Survey and Port State requirements and the ship can be detained....

All the above obviously pertains to commercial shipping only and the recreational area is a whole different ballgame....

Cheers
John
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Old 04-04-2010, 17:30   #158
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One small correction. The radar will tell you that you are exactly 103 meters off the portion of the rock visible above the water. Charts, paper or otherwise might be handy for giving you a clue about that part of the rock which lies below the surface!
Spot on...!!!
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Old 05-04-2010, 02:45   #159
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Originally Posted by s/v Jedi View Post
I just checked the bearing accuracy specs of my 15 year old toy radar (Raytheon with 4 kW dome) and it is +/- 1 degree. All the heading sensors I know are better than 1 degree (within 45 degrees pitch/roll). When you take a bearing on your radar, you put the cursor on the echo and when you point right, it's a CPU doing the math with an error so small that it isn't a factor.
So it will be accurate to within a degree nominally but with a possible error of less than 2 degrees. Nothing like 10 degrees, that is just silly.
.
I am assuming you have a RD424 4KW Radar Dome

The specifications on this set are for an aerial beamwidth of 3.9 degrees. However, this is a much larger aerial than used on most yachts, and the size of the aerial has a lot to do with its beamwidth.

The next size down raymarine dome has a 5 degree beamwidth.

The Furuno 1623 has over 6 degree beamwidth

The JRC 1000 has a 7 degree beamwidth (and I recall that the original 1000 had a 10 degree beamwidth)

Thus any measurement of the bearing COULD be out by as much as the beamwidth, and if the system does not have an Electronic Bearing line the bearing will be an estimate with an accuracy of at best 5 degrees. If your radar is head up, like most then this will add in further errors.
Another source of errors is the alignment of the radar to ships head - again a probable source of errors up to 2 degrees (and sometimes more)

Military radars are re-aligned frequently to resolve this problem.


Now it is unlikely that all these errors are cumulative, but anyone using radar has to be aware of their equipment capabilities, and radar range is good, but bearing needs to be taken with a great deal of knowledge.

In a court of law, three radar ranges creating a fix will establish a position. Two ranges and a visual bearing is also fine. Two ranges and a radar bearing would be unacceptable.

If it is all you have then use it, but if there are alternatives to radar bearing, then use them first.
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Old 05-04-2010, 04:06   #160
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I find this debate interesting and Nick’s psychological analysis of why many of us old time professionals still maintain a decent chart portfolio rings quite true.

Still, while trying to come to grips with the way I view electronic charts and their readers/inputs vs. paper charts I come back to the old warnings taught to me in marine college… when it comes to determining your position.

The difference between a “Navigational Tool” and a “Navigational Aid”?

Tools like pencil, parallel Rules or Triangle are just that…. Free from fixed or variable errors and can be depended upon if the user knows what he is doing and is practiced.

Whereas Navigational Aids like Compasses, Radar, GPS, Floating Beacons should always be considered suspect since they can take on errors from a myriad of causes.

Hydrographic Charts (whether paper or electronic) can also have many omissions and positional errors however, we use them as a recording tool during our passage to assume a geographical position of a surveyed area.

What keeps me holding on to my paper charts is that ability to personally scan a whole detailed (large scale Harbor chart) to check; transits on peaks; bearings to points; variety of details outlined in the Pilots/Sailing Directions to make sure that the chart and Pilotage information gels with the chart.

Yes you can do it also on a 12- 14” screen but the full size paper chart makes it easier for me to assess a new anchorage.

I love the electronic convenience but when scrutinizing a remote anchorage, the detailed paper chart just feels better.
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Old 05-04-2010, 05:04   #161
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Excerpted from Chapter 13, Bowditch (American Practical Navigator)

“Radar determines distance to an object by measuring
the time required for a radio signal to travel from a
transmitter to the object and return. Such measurements can be converted into lines of position (LOP’s) comprised of circles with radius equal to the distance to the object.


Since marine radars use directional antennae, they can also determine an object’s bearing. However, due to its design, a radar’s bearing measurement is less accurate than its distance measurement.* Understanding this concept is crucial to ensuring the optimal employment of the radar for safe navigation...

... Resolution in Bearing.
Echoes from two or more
targets close together at the same range may merge to
form a single, wider echo. The ability to separate targets
close together at the same range is called resolution in
bearing. Bearing resolution is a function of two
variables: beam width and range to the targets. A
narrower beam and a shorter distance to the objects
both increase bearing resolution ..."

Here ➥ http://www.nga.mil/MSISiteContent/St...N/Chapt-13.pdf

* Notwithstanding, A Radar bearing is likely to be just as accurate as any visual bearing taken from a small vessel (when possible, at all).
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Old 05-04-2010, 06:35   #162
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A Radar bearing is likely to be just as accurate as any visual bearing taken from a small vessel (when possible, at all).
When the radar unit is properly set up, aligned, and functioning correctly, then yes. But manual positioning procedures are much easier for a navigator to check in the heat of the moment than a radar unit.

The point is not that radar and GPS are not incredibly useful for positioning, but that they portray a virtual reality, not the real world. If their portrayal and the navigator's interpretation of their information is faithful to the real world, then the difference is negligible. If not...

[Irrelevant aside: One of my favorite e-nav teaching anecdotes is about the two single-handed sailors who collided in dense fog when they came around a point of land in close quarters from opposite directions. They had been glued to their radar units.]
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Old 05-04-2010, 09:15   #163
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A Radar bearing is likely to be just as accurate as any visual bearing taken from a small vessel (when possible, at all).
This is so much nonsense, that I would not have expected it to come from Gord.

I have taken visual bearings for a few years from big ships, small ships, boats, yachts etc using hand held compass, binocular compass, azimuth rings on big ships, and the old autohelm digital compass. The only sort of radar that can do better for accuracy is a military weapon tracking radar.

Epecially when so many of the radars on small yachts are head up displays! rather than gyro stabilised north up.

If you truly think that radar on a small yacht is more accurate than visual, someone on your side of the pond is doing some pretty inaccurate teaching.
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Old 05-04-2010, 10:06   #164
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... If you truly think that radar on a small yacht is more accurate than visual, someone on your side of the pond is doing some pretty inaccurate teaching.
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... Notwithstanding, A Radar bearing is likely to be just as accurate as any visual bearing taken from a small vessel (when possible, at all).
I'm certainly not an expert Radar user, so would take expert instruction; which would begin with an accurate representation of what I said.
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Old 05-04-2010, 10:25   #165
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Now that the world has gone digital there is a presumption that things are "identical" when they are replicated. But with printed charting, that's not necessarily so. There are physical deformities in the printing process, i.e. the paper stretches as it goes through the printing presses, and if the charts were printed with photographs of original charts (as US "ChartKits" used to be) there are more distortions in the photographic reproduction process. Heck, all you have to do is take a paper chart, crumple it up, roll it, fold it a few times, and then compare it to a mylar film of the same chart. Folding the paper stretches it, the chart no longer is the same.

So paper charts, no matter how precisely they are made, are simply susceptible to error. "Original" government charts are usually more expensive because they are literally made to the highest (most expensive) standards in manufacturing, including the invisible ones like the quality and stability of the paper (or better still, plastic) stock.

I have to laugh at the idea of a GPS "knowing" where I am. It doesn't work that way at all. All the GPS "knows" for sure, is that a signal from two or three or four satellites took "this long" to come from each satellite. After that the GPS is really just making at educated guess that the satellites might be in position, the signal might not be degraded by atmospherics (which is what WAAS corrects), and the mathematical assumptions about the shape of the earth and how all the numbers translate into a position usually translate into a reasonably accurate guess about where you are.

There are any number of assumptions and reliances in the GPS position. I still get a belly laugh out of seeing the time on a GPS, knowing that for under $100 I have a non-certified non-chronometer displaying the results fo a dozen atomic clocks worth millions of dollars. And, it usually is the right time.<G>
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