Paper chart v Plotter

[Mike1956]

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

[four winds]

Twenty meters top to bottom on the screen.

What are you looking for at that zoom level? Would only show a boat length or two ahead. I think I'd probably look up. No, I'd certainly look up.

I'm confused, unless you're just trying to "start something".

[Mike1956]

Wreck fishing (angling) some wrecks are only the size of a family car.

Come on someone get your calculator out. I had a mental block & do not trust my own calculation.

Mike

[Mike1956]

Had another look at the calcs as follows...

53M x 1852 = 98,156m divided by 20m = 4,908 x 5 = 24,539 (inches) divide by 40 (+/- inches in a metre) = 613m

Chart would be 613m N to S

Anyone agree?

Mike

[Alberg 34]

The chart plotter will zoom in ,so you are only looking at that particular area. Don't trust them 100% as I have hit bottom . Chart plotter said 12 ft ,shoal off to port (300 yards away) 5 ft I draw 5' 3" .what a bump!,
Charts come in different sizes . For Lake Ontario ,there is one for the lake,then there are ones for specific areas which will overlap . If you are sailing a area you should have the charts . Learn how to read them or take a course .Some day your electronics will fail.
I use my charts to plan my sail ,then plotter while sailing and will verify against the charts on longer passages. Some times just use the charts and dead recon ,then verify how close / accurate I was practice makes perfect

[barnakiel]

Originally Posted by Mike1956

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

;-)

3 boat lengths? You will have to scroll that screen pretty fast now.

Few CHARTS have this level of detail, Google sat pictures perhaps in some areas.

Have fun zooming in!

b.

[Mike1956]

Thanks,

Sounds like you have it sorted. No chance fishing wrecks using paper charts.
Regarding you hitting the bottom, what was the date of the last survey in the area you were in?. Many surveys were carried out donkeys years ago, not to be trusted.

Now has anyone got their calculators out?

Mike

[Simonsays]

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

[reed1v]

And your point is? Scaling up is not linear. Navy found that out when the Enterprise hit land. Simple transfer from screen to paper good only for DR.

[Mike1956]

Originally Posted by Simonsays

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

Thanks Simon,

Thought someone would be along sometime who knows what they are doing.

Mike

[Mike1956]

Originally Posted by reed1v

And your point is? Scaling up is not linear. Navy found that out when the Enterprise hit land. Simple transfer from screen to paper good only for DR.

Just serves to illustrate the difference in scale. I have come across people who think they can do a 3 point fix, plot it on a paper chart with the fix landing on a charted sounding and they actually think they are exactly where the sounding is plotted on the chart.

Mike

[grantmc]

Originally Posted by Mike1956

Thanks,

... Many surveys were carried out donkeys years ago, not to be trusted.
Mike

So what do you think chart plotters base their display on if not from those same surveys? In essence the same chart that was produced (in the largest scale for the area in question) from the last survey with notices to mariners applied, forms the basis of the chart plotter's data base?

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.

[mikecambrai]

Care needs to be taken when using plotters due to the way that they are made up. Raster charts are scans of original paper charts and contain everything which is on the original. Vector charts are made up of chart layers whereby as you scan out you lose the detail. So if you scan out too far you will not have the shallow water on your chart display and you will go along merrily until you hit the object. There are many groundings on super yachts and others because of this fact.
The best way to work a passage plan is to read your paper charts and cross hatch the areas where you do not wish to be. Then you can be anywhere on the chart except those areas.
Put on the chart your destination then a track to that point. If there are waypoints available then plot the courses to those waypoints.
Write up those way points onto a voyage plan indicating courses, areas of interest, points that you may be able to get a fix from etc.
Then you can put that information into your plotter.
Never plot from buoy to buoy. A buoy is where you do not want to be - they can weigh 2 tons or more and are solid metal! Plan to pass at least a cable off.
Now the carrying out of your passage is via the written plan, marked off as you get to a waypoint. The plotter will be backing you 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.

[barnakiel]

Originally Posted by Mike1956

Had another look at the calcs as follows...

53M x 1852 = 98,156m divided by 20m = 4,908 x 5 = 24,539 (inches) divide by 40 (+/- inches in a metre) = 613m

Chart would be 613m N to S

Anyone agree?

Mike

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.

[Rockyscience]

Originally Posted by grantmc

So what do you think chart plotters base their display on if not from those same surveys? In essence the same chart that was produced (in the largest scale for the area in question) from the last survey with notices to mariners applied, forms the basis of the chart plotter's data base?

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.

Sound advice Grantmc. Good thing Mike's not using old Loran charts in microseconds.