I use Navionics on an iPhone and iPad,
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When using the iPhone in the cockpit, .....
................ I use it for approaching new places where I’m uncertain about the accuracy of the plotter charts, ........
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With respect, what makes you think that Navionics charts on your iPhone or iPad are any more accurate than the electronics charts in your plotter.
My plotter uses Navionics charts, but one of the first things I noticed very shortly after buying a brand new multifunction chart plotter complete with a new Navionics chart was that a wreck located a short distance outside of the harbour was incorrectly marked in that it was depicted using a combination of symbols. The references I am about to refer to are those shown in the United Kingdom Hydrographic Office (UKHO) and internationally approved Admiralty Chart 5011, namely the list of “Symbols and Abbreviations used on Admiralty Charts.”
In the instance I am referring to the Navionics chart depicted the wreck using a combination of the following symbols:-
IK28 which is the symbol for a dangerous wreck where the depth of water over the wreck is unknown, but is considered to be 28 metres or less on UKHO charts (it used to be a lesser depth on U.S. charts and on those of many other countries that didn’t have VLCCs within their national fleets), and
IK2 which refers to a symbol used to indicate a depth cleared by a wire drag sweep
Interrogating the symbol on the chart that Navionics have used, indicated that the wreck had been swept with a wire and that there was a clear depth of 14 metres above the wreck.
The symbol they should have used is shown at
IK27 which notes the clear depth surrounded by a dotted circle with the wire swept symbol directly under the dotted circle.
The combined symbol that Navionics had used was entirely wrong and misleading, and were it not for the interrogation facility provided by the plotter software it would not have been possible to figure out with certainty, what their combined symbol may have meant. It may be that you don’t have such interrogation facilities on your iPhone or iPad and I agree that a 14 metre depth in the vicinity of a port entry is hardly likely to worry you, except that without the interrogation facility there was no indication of what the depth of water was over the dangerous wreck. All you would know from the List of Chart Symbols waould be that it was 28 metres or less. How much less is the unknown bit that you would want to steer clear of.
The fact that I can find such a simple error in the production of Navionics chart software means to me that the possibility of further errors is real, but I would also like to make clear that it doesn’t mean that any of the other electronic charts sold to yachties are necessarily any better, or worse, but see the next paragraph.
Unlike Merchant Ships which have to use SOLAS approved electronic chart software or carry and use paper charts provided by an approved Hydrographic Office, our little boats can use chart software produced by anybody and irrespective of whether it is approved by SOLAS or not. Unless there has been a recent change in regard to their approval, I am pretty certain that Navionics charts fall into the “or not” area.
In regard to the accuracy of Hydrographic Office paper charts, it is highly unlikely that the electronics charts that we yachties use are any more accurate than the paper charts produced by the leading Hydrographic Offices of the world from which that data has been copied in the assembly of those electronic charts. Unfortunately in making those copies, errors can and do creep in, and without the SOLAS approval, it cannot be assumed that the electronic charts most of us use are as good as the paper charts they were derived from, but they should be better than nothing at all.
Lastly in regard to that accuracy, on most British Admiralty charts there is a small inset showing different areas of the chart along with the dates that those particular areas were surveyed. Many of those areas depend on surveys carried out many years ago, some going back to the mid 1800s or earlier.
So consider for a moment the kind of navigational equipment carried on ships back then,
A sextant - depends on having a good eye and a clear horizon etc.
A Chronometer - depends on having a periodic radio signal time check, but radio wasn’t invented until the early 1900s so back then they had to have a pretty good idea of how their chronometer was behaving, and a fifteen second error meant a one minute of longitude error in position, one mile at the equator.
A hand lead line - so any depth soundings were spot depths where it was easy to miss an under water mountain peak.
The EchoSounder was invented somewhere around 1917 and would give a line of soundings which was infinitely better than the spot depth from a hand lead line. But underwater cliff edges or mountain peaks could still be missed if not exactly on your course line, and it still had to wait until it was universally fitted aboard survey vessels.
The next improvement on sounding depth came around 1970 when side scan sonar was invented enabling a large area of the sea bed to be checked for depth.
Finally if it doesn’t say on the paper chart that full sea floor coverage has been achieved, then there may be some depth surprises to upset you.
Satellite photography will change things in due course as far as the land masses are concerned as will crowd surveying of depths from ships and yachts, but sending out a survey vessel is an expensive option that doesn’t happen all that often.
Oh, and one other point, don’t be fooled into thinking that just because you have GPS that has a read out to multiple decimal points that knowing exactly where you are puts you in safe water, it doesn’t if you don’t know if the nearby land masses and underwater dangers were accurately positioned on your charts in the first place. You could be a lot closer to something damaging than you think.
