Interesting question....how does it work
We are in the Yasawa islands in Fiji
and there is only one "official" chart Fijian F5 which is not very deatiled There are some hand detailed charts from the '70s by Picmere sold
as a set F105, but they have no soundings at all, and that's it.....
We have Navionics
in a B&G plotter and C-Map
in our Furuno NavNet
VX2 system, all up to date and they are quite different.
Navionics in particular in some places has great detail, including soundings, that has never been on official charts but in other places has no detail at all, indicates reefs
where there aren't any and worse, doesn't show a lot of the reefs
at all, and often the wrong shape and position.
seems to be based on the "officlal" charts but also has detail that's not on the paper versions...in some places.
Overall the C-map charts seem to me to be more reliable here.
I have come across "iPad" cruisers here who only have Navionics on their iPads and nothing else, no paper charts, nothing ...that's dangerous here !
We also have MaxSea
TZ on a laptop
with both Navionics and C-Map charts, and they are different again !
We also have OpenCPN
with GE2KAP satellite
pictures for almost everywhere we go...as a reality check.
The variations in all of these products means that you can't trust any of them absolutely and it pays to have as many sources as possible, AND make maximum use of the Mk1 Eyeball !