C-Map Disappearing Rocks

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daibachsail

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This summer I had a nasty encounter with a rock which was not shown on the chartplotter until I zoomed out further!
Yes, as you zoom in the rock disappears, zoom out and it re-appears!
Unfortunately I was zoomed in and made a course change with no rock showing!
Has anyone else had this experience? Or am I unique?
It was not caused by over-zooming and the other features on the chartplotter are fine.
I came later than many to the electronic versions of charts but have come to rely on them extensively.Now I am now far from sure!!
 
Think you will find that not all details are available at all levels of the chart. Also the C Map charts can have more than one data source. An example is in Fiskardo on Kefalonia where the harbour level is a different chart and you go from one level where your boat icon is exactly where you expect it to be against the quay to being behind the taverna where you are about to dine! And the building (unusually for Kefalonia) has been there since the 18c. Also if you enter Lakka on Paxos through the very clear entrance to the bay, your track shows you going over the hill on the east side, through the schoolhouse yard before mooring correctly on the quay. C Map shrug their shoulders and blame it on their source data.
 
daibachsail said:
This summer I had a nasty encounter with a rock which was not shown on the chartplotter until I zoomed out further!
Yes, as you zoom in the rock disappears, zoom out and it re-appears!
Unfortunately I was zoomed in and made a course change with no rock showing!
Has anyone else had this experience? Or am I unique?
It was not caused by over-zooming and the other features on the chartplotter are fine.
I came later than many to the electronic versions of charts but have come to rely on them extensively.Now I am now far from sure!!
Click to expand...


I've had the same thing with nav buoys with Navionics charts. It does seem crazy, but as the chart makers say, "Not to be used for Navigation". Keep your wits about you, and check other zoom levels, and also with paper charts. You can also report mistakes to the chart makers, and if you are lucky, sometimes they'll give you a freebie.
 
daibachsail said:
This summer I had a nasty encounter with a rock which was not shown on the chartplotter until I zoomed out further!
Yes, as you zoom in the rock disappears, zoom out and it re-appears!
Unfortunately I was zoomed in and made a course change with no rock showing!
Has anyone else had this experience? Or am I unique?
It was not caused by over-zooming and the other features on the chartplotter are fine.
I came later than many to the electronic versions of charts but have come to rely on them extensively.Now I am now far from sure!!
Click to expand...


The same thing happens with Portland Race, scary isnt it?
I never use CMAP charts for passage planning.
 
daibachsail said:
This summer I had a nasty encounter with a rock which was not shown on the chartplotter until I zoomed out further!
Yes, as you zoom in the rock disappears, zoom out and it re-appears!
Click to expand...
Hey, why worry about a few rocks when they are missing a whole island not far from here! Not visible at ANY scale. I might have run it down a couple of weeks ago, had I not had an old paper chart backup.

(Conway, aka Ceva-i-ra, centered at S29°4',E174°38'.)
 
The information is built up in layers, if you zoom out to look at a larger distance then you lose information, if they tried to cram all of the layers on when zoomed out then the screen would appear so heavily cluttered as to make no sense.

The bigger the chart plotter screen the better as you can work at a further zoom and still see the details with some clarity, personally i find a laptop better to use when sailing but only use it as a back up to your paper charts.

like it says on the start up screen "not to be used for navigation"
 
NormanS said:
I've had the same thing with nav buoys with Navionics charts. It does seem crazy,
Click to expand...

I have the same sort of thing on my old Raymarine b/w plotter. There are bounded areas which show up in open sea which are (one assumes) shallow patches. Zoom in to search for detail and they are gone!
Very disconcerting esp as I was going to update soon and looked forward to the end of this problem. As Fergie has said, a split screen display should help.
 
"Not to be used for Navigation"

Was there some other use for a chart plotter that I have missed???
 
This sort of issue was the reason why the UKHO were so dead set against vector charts in the first place and insisted on producing only raster. In the end, the public went wholesale for vecftor and the UKHO were left behind.
 
Bosun Higgs said:
This sort of issue was the reason why the UKHO were so dead set against vector charts in the first place and insisted on producing only raster. In the end, the public went wholesale for vecftor and the UKHO were left behind.
Click to expand...

The huge advantage of vector from the consumer point of view is that you can store much more data and customise the display to meet a range of different uses.

I think the problem with the features that come and go such as disappearing rocks is not to do with the C Map chart. The data is there, it is the plotter software which selects what to show. Also as I pointed out above the same C Map chart can have data from different sources, so the harbour chart may be from a different source than the approach chart. Some of the base data can also be very old, particularly in less frequently surveyed places.
 
Well, I have the opposite complaint with my Cmap plotter. I frequently spot the "where I'll be in 10 minutes" line passing over a little cross symbol for "underwater rock". Urgently clicking on this shows it to be anything up to 100m down. Panic over!

I do all my passage planning on paper charts, first circling in pencil anything that should be avoided. This simplifies a subsequent quick glance to check that all's well.
 
daibachsail said:
This summer I had a nasty encounter with a rock which was not shown on the chartplotter until I zoomed out further!
Yes, as you zoom in the rock disappears, zoom out and it re-appears!
Unfortunately I was zoomed in and made a course change with no rock showing!
Has anyone else had this experience? Or am I unique?
It was not caused by over-zooming and the other features on the chartplotter are fine.
I came later than many to the electronic versions of charts but have come to rely on them extensively.Now I am now far from sure!!
Click to expand...


This point has been raised a few times recently and although it seems illogical that detail should disappear upon zooming in to a smaller area however it seems it does happen and has resulted in a few accidents. It seems Jepersons
are involved in C-Maps???? and may have accepted responsibility on occasion.

On the Standard horizon plotters there is a sort of task bar along the top of the screen and one block indicates the optimum level of zoom for each chart. This isn t always the closest Zoom.!!!!! Does your plotter have this and does this bar go out when the rocks disappear????
 
we have 2 garmins (we dont leave them on either boat when we are at home ) both have options about display density .I think there are 6 setting.
I have a copy of Cmap ,old one ECS I think which i have never used for laptop. perhaps I will have a look , but I am guessing there is some option on Cmap that has a similar feature so i guess You would have to display it at max density. On the garmin when it not at max Its often doesn t display bouys etc at certain zoom levels i havent come across it not displaying rocks but it will prob be the same** so I will have to watch out for that .. I must admit tho I use them in conjunction with paper charts and also Mapsource.Thats probably whiy I have never noticed the problem with vector charts. I have been busy scanning charts for seaclear 11 to use raster charts (so i can use notice to mariners corrections on them) as I am not that confident in Updates for the garmin.

**Note I had a laugh at the garmin in the entrance to Watchet though. The garmin showed a drying height of 54 meters!!
 
As to further evidence of different levels having different data sources - I've noticed placenames misspelled at higher zooms, zoom out and spelling is correct again. Ever since I've been resisting temptation to think logically & don't assume maps layers are crosslinked!.
It's pencil & paper for a while longer!
 
I have heard of a similar problem with CMaps, and the cause of this was something to do with the depth the safety contour has been set up. Don't really understand it, but then i have 3 navigators to look after that sort of thing. But the way i understand it is if you increase the set safety depth to a deeper depth the missing things appear. A bit worrying when wrecks with 10M over them don't show and you're drawing 11.5M. Besides this i have to say that the quality of Cmaps is pretty crude.
Have just spoken to one of my navigators, Apparently if the safety contour is set to 20M all items within that contour are shown, but sometimes items with a depth of less than 20M outside the contour are not shown.
 
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