Errors in Electronic charts

[daibachsail]

I have recently had an unexpected brush with a rock in Brittany off Pampol!
I was navigating using pilot and a C-Map electronic chart on the chartplotter.
We had passed the cardinal markers and the helmsman asked if we could turn.
I consulted the chart and seeing no problems at all said yes!
Imagine the horror a minute later when we hit a rock with our keel.
As we recovered from this shock of collision - the first after 30 years of cruising - I was amazed to find that if I zoomed out on the chart the rock was then shown!!
And to rub it in also our hitting of it was shown on the track.
However, we had zoomed in further and the C-Map software neglected to show the rock!!!!
How can this be?
This has quite undermined my faith in electronic charts.
Has anyone else had this problem?
Have they had it with C-Map?

 

[DaveS]

I've used Cmap for a number of years with few complaints, but I do occasionally get the odd "funny". Last year it refused to show any details of Dunstaffnage (only time I've ever seen this), this year, without modification, it was fine.

I tend to have the opposite problem with rocks: I suddenly spot a wee cross directly in my path, click on it for more information, get confirmation that it's an underwater rock, but with no depth listed, check the real chart and find a sounding of 40m...

While I find the plotter my most useful instrument aboard, I would be most reluctant to dispense with paper charts. I tend to go over them in advance, drawing pencilled lines round groups of nasties to aid a quick glance from the cockpit.

 

[Captn D]

I have found similar anomalies with C-Map, without your unfortunate result, where obstructions are not shown on a higher zoom level. I think it may be due to them using larger scale charts when zoomed in, which may get data from a different source. It has made me concerned and I have often gone back to check on a paper chart. I suggest you take this particular fault up with C-Map. I, and I expect many others, would be interested in their response.

 

[colingr]

I was amazed to find that if I zoomed out on the chart the rock was then shown!!

Do you mean zoomed in? I could understand that as you should be showing more and more detail as you decrease the range being shown.

If it disappeared zooming out I would be somewhat concerned!

I agree with DaveS. I tend to plan on a paper chart, then just use the plotter for general guidance. Still managed to run aground the other week though and we only draw about 0.5m

 

[LadyInBed]

Position of Rock

If you post the Lat / Long of rock we could check it against our charts / zoom factor.

 

[Tranona]

The problem with C Map (and any other Vector chart) is that it stores the data in digitised form and your chart plotter re-assembles it on the screen. This is great in one sense because it means that it can be selective about the data it uses to for example minimise clutter at different levels of zoom or by user selection.

The original data can come from different sources on the same electronic chart, although usually from good official sources. To give an example. Going into Fiskardo on Kephalonia you can watch your boat icon work its way towards the quay and tie up - but go to the max zoom in you pick up a different set of data which shows your boat neatly moored behind the taverna where you are about to have dinner!

I have stored tracks entering Lakka on Paxos by climbing over the eastern headland, behind the schoolhouse and along the quayside road - not once but twice. Came out the same way as well. Good thing the real entrance is easy to find.

Greek charting is notoriously unreliable and its a good thing that very little changes and features are easy to see. Charting in our home waters is infinitely better but still prone to glitches. C Map no longer have an office in UK, but I guess even if you get anything from Italy they will "blame" their data sources or in your case the chart plotter software.

 

[ChrisE]

Were you by any chance coming from the North, ie from Ile Brehat?

There is a very misleading set of a channel marks leading into Paimpol when coming from this direction.

You might ask me how I know......

 

[AntarcticPilot]

De-clutter

One thing to watch out for is that a lot of chart-plotters have an option to "de-clutter" the display. That isn't supposed to remove safety critical data from the screen - only stuff that makes the screen too busy. I use it routinely - in the Clyde there's an awful lot of big ship stuff not relevant to yachts, so it is handy to remove it to give a simpler display with only information I need on it. Things like harbour limits, ship channels and so on. But, of course, the database used to display the chart is pretty big, and mistakes do happen! Your rock COULD have been flagged as being suitable for removal when the de-clutter filter is on. You could check this speculation by setting de-clutter on and off and seeing if it affects the visibility of the rock in question.

AntarcticPilot really!