1. The charts aren't always correct.
2. Not all obstacles are charted.
3. Depths haven't been surveyed at every 20 ft lateral.... in some areas, in can be hundreds-thousands of feet between surveyed depths, everything else is inferred.
4. Navionics can use user derived data (bathymetry). That data is susceptible to error as users must correctly set their transducer depth offset in Navionics (which very few do). I trust it for seeing the contour profiles, but not the specific depths.
5. Sandbars move
6. River outflows will move sand / silt. The chart may reflect where the bottom was 30 years ago.
7. Aids to navigation can be moved, you are responsible for updating the chart (via notice to mariners), Navionics can be months or years behind on updating your chart (if at all) depending on where in the world you are.
8. Channel ranges are valid when they were installed. They are not moved, the bottom though can move. When was the channel last dredged? Go slow, trust your eye and depth sounder.
9. The user needs to understand the datum for the water levels, and for the bridge clearance heights! This is not an international standard, there are marked differences between Canada and the US... boater beware.
Assume that the datum is different for tidal and non-tidal waters.
Remember that charts and aids to navigation are intended to support commercial shipping, thus the level of details and everything else isn't designed for us pleasure craft users. Up here in BC for example, there are areas where I can be anchored in 15 feet of water that the chart says I'm on land, and areas that are marked with 20 feet of water that dry at low water.
I trust my eyes 1st, my depth sounder 2nd, and my chart 3rd.
If you really want to get good at leveraging charts, take a power squadron navigation course.
