Charts Puzzle Help!

CaptainBob

CaptainBob

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Hi, a little help please...

Here's a screen shot of my chart software when zoomed to 0.2m viewing a region around the SW of the river Dart (note the pink highlighted zone):

chart1.gif


Here's a zoomed in view of what's in the pink highlighted bit from the above image:

chart2.gif


Anyone tell me why there's the error where the chart boundary is? It's suggesting to me that one side of the boundary is shifted and hence not accurate... got to be at least an error in one or other of nearly 100m!!

Location is: N50 19.487 W3 35.222

Is this normal?

PS. This is from Garmin Mapsource running the latest BlueChart software for the channel.
 
Thought you'd answered it for me for a moment, but nope...

I just put a waypoint on each side of the join, same vertical position on the screen.... unfortunately they both have the exact same Latitude.... so I think you must be wrong no?
 
I don't have a chartplotter but road maps overlap so you can follow the road. If plotters overlap then you have seemless charting of your position from one bit to the next. Surely the worrying bit would be if 100m was missed out!
 
Is it possible that going to a 0.2m (did you mean 0.2nm?) scale on your vector charts resulted in serious "overscaling" of the chart information? I suggest you check the scales of the source charts that were digitised to produce your C-Map chart. I suspect there was a chart border between 2 source charts of different scales. Magnification of a smaller scale chart to match a neighbouring bigger scale chart will often result in discontinuities of charted features, simply because the smaller scale chart is being used above the accuracy to which it was drawn and was intended to be used at.

Even if one removes the possibility of the bad habit of overscaling vector charts (i.e. viewing them at a larger scale than the original data was drawn at) by looking at raster charts on a plotter system, such as on MaxSea, discontinuities at chart borders of different scales can still be seen.

One of the problems of GPS navigation is that vessel positioning is now often much more accurate than the chart data because much chart information was plotted before such accurate positioning ssytems became available. An appreciation of this might have avoided you having to ask the question about what you have spotted.
 
Re: Show me yours and I\'ll show you mine

That's where I grew up and first discovered, well, that girls are sortof 'different'.
And the earth MOVED . . . .
Relax, your gps is right on the ball!
 
[ QUOTE ]
I don't have a chartplotter but road maps overlap so you can follow the road. If plotters overlap then you have seemless charting of your position from one bit to the next. Surely the worrying bit would be if 100m was missed out!

[/ QUOTE ]

No it's not overlapped, it's offset, by 100m.
 
I was quoting the display verbatim. It's says 0.2m. I have my GPS set up for road use at present, it's referring to miles, non nautical.

[ QUOTE ]
An appreciation of this might have avoided you having to ask the question about what you have spotted.

[/ QUOTE ]

PMSL

So what you're basically saying is that had I known the answer to my own question before asking it, I wouldn't have had to ask it?

Thanks for that :P

I get your point about the scale, but it's 100m (METERS) offset... that's a pretty chunky error due to scale... more than the scale difference would warrant no?
 
It's quite common to find discontinuities in depth contours, usually because one section has been resurveyed, and the discontinuity is at the junction of the old and new surveys. I don't think that's the reson this time though, because there is a discontinuity in the HW line.
 
I just put a waypoint on each side of the join, same vertical position on the screen.... unfortunately they both have the exact same Latitude.... so I think you must be wrong no?


*************************************************

You are forgetting that what you have is a GPS with a chartplotting function....

Nothing wrong with the gps so of course both positions will appear at the same level.... The problem as was said earlier is that possibly due to overscaling there are chart errors....Ie the coastline and depth gradients are in the wrong position on one side of the shift....so yes when the boat sails across the joing although the boat moves smoothly you might hit the jagged bit of coast if rockhopping cos its in the wrong place.

Many charts are not totally accurate and I sailed with 1 second mate to made a point of recording the true position of islands we passed in the oceans of the world and wrote to the Hydrographic office suggesting corrections. Some islands could be 1/2 mile out of position where the data had not been corrected since Drake etc found them /forums/images/graemlins/smile.gif

/forums/images/graemlins/laugh.gif
 
Might I suggest that the reason is because the charts are, essentialy, flat bits of paper scanned/digitised - while the world is inconveniently round. You try to map a flat sheet onto a spherical surface, there will be errors.

I say this not from a position of expertise, just as someone who did a little maths at school. /forums/images/graemlins/grin.gif
 
A lot of the charts on the Turkish coast, particularly around Bodrum are up to 100m out because they were originally mapped in around 1900 and there simply was not the capability for accurate positioning to that degree.

Scary when you are in port and the plotter shows half way up a mountain...mind you, it would be even worse in fog if it showed you 100m out to sea...!!!

Hence the big warning message when you turn on your plotter.....mark 1 eyeball is far more accurate than a digitised chart where coastlines are concerned.
 
[ QUOTE ]
Might I suggest that the reason is because the charts are, essentialy, flat bits of paper scanned/digitised - while the world is inconveniently round. You try to map a flat sheet onto a spherical surface, there will be errors.

I say this not from a position of expertise, just as someone who did a little maths at school. /forums/images/graemlins/grin.gif

[/ QUOTE ]

The charts are a mercator projection, and so really should be correct even though flat.

They should not have 100m errors in them! I don't know a lot about sailing and navigation, but I'm currently studying for my RYA Dayskipper theory qualification, and one of the recommended ways to deal with a situation where there's low visibility is to navigate using depth contours..... I guess if I ignore the GPS and just use the paper chart, that'll be fine.... but isn't this precisely the time I should be thankful I have a GPS with me to double-check my position relative to the chart!!!!

Looking around the chart to the east in the picture I posted, all the way around it's boundary, the offset is present.... seems like perhaps the whole thing might be slightly mispositioned.

Hoping someone can shed some light on this...
 
I see the step in the online C-Map charts... it looks better than the one in the Garmin charts because the land doesn't have a step. In the Garmin charts as in my first post in this thread, the land also has the 100m step.
 
It's not just a vertical offset. The coast and depth lines would not join smoothly when offset.
Clearly there is aproblem with the chart as you have it. The electronic chart people have been a bit careless issuing the chart with this error. There may be an underlying error in the original data source - presumably UKHO.
It's probably better to have an obvious error than someone unknown doing a bit of "smoothing". At least you know you should keep a reasonably wide berth here.
 
[ QUOTE ]
It's not just a vertical offset. The coast and depth lines would not join smoothly when offset.
Clearly there is aproblem with the chart as you have it. The electronic chart people have been a bit careless issuing the chart with this error. There may be an underlying error in the original data source - presumably UKHO.
It's probably better to have an obvious error than someone unknown doing a bit of "smoothing". At least you know you should keep a reasonably wide berth here.

[/ QUOTE ]

Keep a wide berth WHERE???!!!

Either the chart on the right or the chart on the left is approx 100m incorrect... and I've no way of working out which... I'll actually have to keep a wide berth throughout both to be safe... and how can I trust all the other charts if I can't trust these two.

This isn't gonna help me very much when I'm picking my way carefully along a narrow channel (that's less than 200m wide) in limited visibility!
 
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