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Old 16-02-2016, 12:18   #16
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Re: Paper chart v Plotter

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Originally Posted by Mike1956 View Post
I have just worked out if my 5 inch GPS plotter zooms in to about 20m (top to bottom of screen) which I believe it does, a chart that covers 53M N to S to accommodate the same level of zoom would need to be 613m top to bottom. Would need a big chart table!.

Would someone check my maths please.

Mike
Mike,

You didn't say where you are. Books of charts can be bought for US coastal waters. It's a pain in the ass watching side ZZ with side ZZ in page whatever. but cheaper than big charts. Us the chart plotter but have the paper charts, some parallels and a compass. For an ocean crossing I don't have the slightest?

I have a basic mistrust for electronics and I had been in that field for years.

JMHO
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Old 16-02-2016, 12:25   #17
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Re: Paper chart v Plotter

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My paper chart would be 278,27 high.
math looks sound based on those assumptions
but the paper chart hight only depends on how many times you fold it.
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Old 16-02-2016, 12:42   #18
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Re: Paper chart v Plotter

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Mike,

You didn't say where you are. Books of charts can be bought for US coastal waters. It's a pain in the ass watching side ZZ with side ZZ in page whatever. but cheaper than big charts. Us the chart plotter but have the paper charts, some parallels and a compass. For an ocean crossing I don't have the slightest?

I have a basic mistrust for electronics and I had been in that field for years.

JMHO
I absolutely detest electronics. I am expecting nay hoping that someone will soon announce this world wide web thing has been a big experiment that has gone wrong and switch it off.

Mike
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Old 16-02-2016, 12:47   #19
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Re: Paper chart v Plotter

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I will disagree, as a 5'' plotter in my book refers to the DIAGONAL not short side measure of a plotter.

I also keep my charts HORIZONTAL on my table and so I keep (most of the time) my plotter. I think depth sounders and radars are often held in vertical orientation but plotters are more commonly found in the horizontal setup.

Here my take:

5'' screen is a DIAGONAL, how the hell do we know the height of the screen without knowing the ratio?

Basically though, for a 16:9 (+/- 2:1) screen:
- 5'' dia = 127 mm dia
- 127 mm dia at 2:1 ratio = 56.7 mm height,

Now, what is 53M. Do you want to say 53 Nm?

If so, then 53Nm x 1852m = 98156m / 20m = 4907.8 screen heights x 56.7mm / 1000 (mm to meter) = 278,27m

My paper chart would be 278,27 high. Mind I keep the plotter horizontal and so is my chart. Other assumptions will yield other results.

So my table would be only a quarter of your size?

Otherwise, we agree.

Let's synch up. Go thru my maths, I will go thru yours. We will find our bugs and agree on the table then.

b.
Thanks, You are probably correct about the size. The infernal gadget is kicking around somewhere, next time I trip over it I will pick it up and measure the screen. Will need a lot less plywood for my chart table now.

Mike
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Old 16-02-2016, 12:49   #20
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Re: Paper chart v Plotter

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Originally Posted by mikecambrai View Post
Care needs to be taken when using plotters due to the way that they are made up. ...

Don't forget that few people can update their electronic charts, and papercharts are only as good as the survey information on which they are written and the date of the last Notice to Mariners corrections.
Paper charts are only as good as the survey information on which they are based and the date of the last Notice to Mariners corrections AND MANUALLY APPLIED TO THE CHART. But remember that Notices to Mariners are only issued after someone notices an error or someone changes the position of a buoy etc. They don't mean that the chart is now completely error free.

I know that I have some old charts in my cupboard for areas where I haven't sailed for a while. When I last needed to update my folio for the Ionian Sea (Greece), because I was going back there for a trip I had 50 pages of corrections to apply!

The cheat in the UK is to order new Admiralty charts (the full size ones, and not the Small Boat Folios) from a registered Admiralty Chart Agent. They have to employ people who will apply all the published corrections to the charts before they send them to you.

Their corrections are also a lot neater than I can do when I apply any later changes.

My chart plotter (OK it is only a handheld Garmin, but has the same charts as their larger models) can really get confused when the chart data that they have based the electronic version on was not compatible with the WPS84 spheroid used for GPS. (You know the old charts that tell you that GPS positions need to be moved x east and y south before being applied to the chart. I have had my plotter showing my track going the wrong side of a lighthouse when I was going into a harbour in Croatia, difficult to do in practice as it was on the mainland!

On another trip if I had based my approach to a harbour on the west coast of Scotland on the electronic chart as shown on the plotter I would have gone straight through the harbour wall instead of through the entrance.

As my first sailing instructor said to me in Greece, many years ago, "If you have brought a GPS with you it will tell you exactly where you are on the planet, but not where you are on these charts. They are still based on a survey taken by HMS Hydra in 1870, and that was done using manual triangulation and lead lines!"

Only a year or so ago the Admiralty issued a "Notice to Mariners" alerting shipping to a sea-mount that had just been discovered between the Orkney Islands and the Shetland Islands in the UK. It was discovered after a heavily laden container ship reported hitting an object in that position resulting in damage to its bottom. They dispatched one of the Royal Navy survey vessels to investigate and there it was, an underwater spike that no-one knew existed.

I would recommend that anyone who can get hold of a copy reads a book called "How to Read a Nautical Chart" by Nigel Calder. Some of the examples of old surveying errors that he shows (and explains the reasons for) will make you cringe, and wonder how much you can really rely on any chart that has not been based on a survey done recently with modern equipment.

Personally I keep both electronic and paper charts and use both, OK I probably use the electronic ones most of the time but the paper ones make it easier to see the big picture. But I don't rely on automatic course corrections at my way-points and always keep a good lookout. I also know that when the visibility goes down then I cannot necessarily rely solely on the electronic charts to get me out of trouble. They are an aid but I still plot my position on my paper chart and keep my eyes open and the echo sounder pinging away.
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Old 16-02-2016, 13:25   #21
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Re: Paper chart v Plotter

I agree with the interesting comments of Goodal M1. In addition I would say that from your log you should be able always to be able to plot your course and position historically should anything happen to the chart or the electronic display. Thus you can work up a DR position and add tides, leeway to get an EP.
Bear in mind that there are many ways to plot your position. I think that I identified some 20 when I last taught navigation many years ago.
I too love my plotter but it a tool and the information is not sacrosanct. You need at least 3 independent sources of data to even begin to guarantee that you know exactly where you are. Using the planning chart in the first place at least you know exactly where you are not! ie in a cross hatched hazard that you previously identified and marked.
Too many vessels have come to grief using electronically devised errors.
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Old 16-02-2016, 13:25   #22
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Re: Paper chart v Plotter

Dylan has a video on KTL showing him hand charting a channel at low tide to sail it once the water is deep enough.
no paper nor electronic chart will enable you to do that.

before electronic charts (and before fancy chart plotter tables) paper charts showed you where you were at the time of your DR and you could plot your position based on speed, heading, current and wind. your next DR would show you how good your plotting skills are (in between "close enough" and "that was too close") and enabled an educated guess on where you actualy are.

since electronic charts, the educated in the guessing part has been replaced by "my iphone said so" and the convenience of having it available in a glance.

before you disagree, google USS Guardian.
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Old 16-02-2016, 14:21   #23
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Re: Paper chart v Plotter

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Originally Posted by Goodall_M1 View Post

The cheat in the UK is to order new Admiralty charts (the full size ones, and not the Small Boat Folios) from a registered Admiralty Chart Agent. They have to employ people who will apply all the published corrections to the charts before they send them to you.

Their corrections are also a lot neater than I can do when I apply any later changes.

That's getting to be "old" stuff nowadays. Most Authorised Admiralty Chart Agents carry very little in the way of chart stock.
Want a chart?, just drop in, give them the chart numbers you need, and they will print out a full size chart in a few minutes, correct to the latest Notice to Mariners, with the corrections far more accurately placed than you or I could.

The days of keeping the basement full of retired ex 2nd Mates employed in correcting the stock of charts is nearly over.
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Old 16-02-2016, 14:29   #24
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Re: Paper chart v Plotter

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Relying on a display after zooming in isn't necessarily a good idea. You need to be cognisant of the scale of the base chart from which the plotter is doing its display. The chart plotter display can be no more accurate than the largest scale chart.

Something that the navigators on the USS Guardian should have been aware of back in early 2013.

There is an excellent publication produced by the UK Hydrographic Office titled "Admiralty Guide to the Practical Use of ENC's " which is well worth a read if you can get hold of a copy. Very good explanation on the pitfalls of over zooming electronic charts which are based on small scale charts.
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Old 16-02-2016, 14:35   #25
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Re: Paper chart v Plotter

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I absolutely detest electronics. I am expecting nay hoping that someone will soon announce this world wide web thing has been a big experiment that has gone wrong and switch it off.

Mike
Mike, I wouldn't go that far. I'm afraid it could be hacked and shut down but I would be more concerned with something onboard taking a crap. The popular thing is to tie everything together. I can understand the convenience of having everything on one display or maybe the argument is it goes to my computer also?

Paper will get you home. I don't detest electronics but a backup on paper is a must.

Again JMHO
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Old 16-02-2016, 14:40   #26
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Re: Paper chart v Plotter

Your supposed plotter scale of 20m in 5" works out to 4m=1". Hard to see why you want such a scale. You are well past the accuracy of the chart. On your question, 600m at 4 to the inch = 150" = 12'-6" but why would you care?
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Old 16-02-2016, 15:03   #27
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Re: Paper chart v Plotter

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That's getting to be "old" stuff nowadays
correcting charts got me out of a lot of less desirable duties...
and it was an easy way to familiarise with the area for the next exercise.
it was also invaluable in training the new recruits in how to actualy read charts.
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Old 16-02-2016, 15:50   #28
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Re: Paper chart v Plotter

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I absolutely detest electronics. I am expecting nay hoping that someone will soon announce this world wide web thing has been a big experiment that has gone wrong and switch it off.

Mike
And yet you're posting on an Internet forum!

Pack up all your stuff and buy a cabin in Montana. Way out in the woods. Alaska would do as well. Leave your computers, phones, etc. behind.
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Old 16-02-2016, 16:47   #29
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Re: Paper chart v Plotter

The fact that you are zoomed in that tight doesn't mean the chart is accurate to that scale. Bottom contours are generalized between data points. Even wrecks are entered by coordinates (Lat/Long) and those points aren't accurate at that scale either.
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Old 16-02-2016, 22:14   #30
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Re: Paper chart v Plotter

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20m on 5inch is a 157.5:1 scale (20m/(5*0.0254))
a chart covering 53nm NtoS at that scale would be 0.33655nm or 623.3m

it's 39.37 inch per meter, which matters at that scale
Oops, is that 5" a diagonal of the screen?
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