Sonar charts are crowd sourced, i.e everyone that goes by uploads their lat-lon and depth to Navionics. How they deal with tide and miscalibrated transducer offsets I have no idea.
If it is a high traffic area, I guess it averages out. In a place where just one boat went by, you really are trusting that one person!
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Also note, that when you zoom in on the survey coverage there is an area that coincides with the rock circled here:
There is no sonar coverage in the dark area over the rock because it was deemed unsafe or impossible to drive a 28 foot survey launch over it to acquire data. That’s a pretty good indicator that this area should be steered well clear of by anyone in a vessel bigger than a dinghy.
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In Maine those rocks are called “sinkers” IIRC.
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I had moment of enlightenment after we had our ECDIS installed but before we figured out how to set it up.
We were leaving Singapore westbound, second mate was on watch, I was in my office doing paperwork while watching using an Ipad running Navionics. We were maneuvering to avoid vessels being overtaken and I noticed the mate was headed for a spot with insufficient under keel clearance (UKC).
On the paper chart that spot was tinted blue but on the ECDIS the same area was white.
I went up to the bridge, showed the mate the shallow spot on the chart and they altered a bit to avoid.
After that the second mate and I figured out at what depths the contours should be and made some how-to on the proper set-up and some step-by-step instructions.
From the NTSB report:
The ATB crew stated that they did not use the contour depth feature on their ECS because “for most of the spots we operate, the soundings don’t really mean anything.” The company’s SMS did not specify the configuration of the ECS or require any vessel-specific preconditions, such as vessel draft or contour depths, be entered into the system.
Apparently contour depths wouldn’t have helped in this case but they are still useful.
Rosepoint navigation software does have the option of displaying the “Isolated Danger Symbol”, which would have made the rock they struck much more visible on the display.
Rosepoint software also has a “route check function”. Rosepoint calls it “Route obstacle checking”.
Sorry for the late reply, I was underway.
@Rock_Hill Thanks for the timely caveat regarding Navionics “sonar charts”, the crowdsourcing nature of which @yacht_sailor already ponted out. The quality of the sonar chart layer varies widely.
I personally keep it turned off in general, because I would not trust random data sources, however, I sometimes find them useful for information purposes. Since many recreational fishers nowadays have elaborate 3D sonar equipment, the bathymetric data often are quite detailed, as I have stated, however they may not be valid (lack of calibration, user input errors etc.), thus they should imho not be used for navigation. I found the Navionics layer useful to get an idea of how isolated that rock is.
@Rock_Hill Thank you very much for the very interesting and useful link. I envy you in the US for the government policy, that most or all of publicly funded unclassified data have to be made available to the funding party (i.e. the public). This holds true for many fields (e.g. complete deliberations, meetings, submissions, comments of FDA’s medical drug review process). In most European countries, in comparison, “open data” is a lip service or in fledgling status.
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Good term, analogous to the early days of radar in the commercial fleet, the so-called 'radar-assisted collision."
Electronic Navigation Systems (ENS) can greatly enhance situational awareness during navigation because there’s no need to shift attention back and forth to the task of plotting.
During the planning stage however laying down tracklines manually with a pencil on a paper chart, while slow and tedious, did make it much more likely that depths and obstructions on track would be noticed. With ENCs the ability to add legs with a click means that process doesn’t happen. This is where the route check or route obstacle check come in.
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I had a friend miss a buoy by about 5 feet and then complain loudly my plotter was terrible. I had to explain to him that zoomed out as far as he was the boat symbol was taking up about 1/4 mile and covering the buoy 
At least on a paper chart that covers Norfolk to the C&D Canal it is really obvious that there are a lot of things you can’t see at that scale and it isn’t going to work for missing channel marks.
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One of the digital chart apps for iPhone, iPad, mac and PC, widely used by recreational sailors in parts of Europe and the Carribean (NV Charts) did, until recently, not have any overzoom warning. However, they were very responsive, when I pointed out that critical ENC functionality to them and submitted a feature request, which was incorporated into the next software release.
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In 2021 the UK (MAIB) and Danish (DMAIB) Marine Accident Investigation Boards issued a succinct report to point industry attention towards the real life problems of ECDIS implementation, training and application which can be found here: https://dmaib.com/media/8502/ecdis_application_and_usability.pdf
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