"So if BSB charts are rounding the underlying data, why don't ENC
charts do the same?"
BSB charts are raster charts. That is, "paintings".
ENC charts are layers of data, vector charts
that are plotted and presented on your screen
in real time, subject to filtering rules. That's why you can display an ENC chart with "everything less than 10 feet, show as red" and make it happen. There are pros and cons to each. For instance, a BNB chart will show feet or meters or fathoms, whatever it was drawn with. And ENC chart will have the water depth
encoded in the data, but given good software
, you can display it in feet, meters, fathoms, or millimeters as you prefer. Which might require rounding errors or decimal points, again depending on software
options. Remember, "you" are creating
the ENC chart that you see to a large extent. You can make wrecks, cable crossings, bottom types, appear and disappear. Can't do that on a BNB raster chart, unless you've brought along the digital white-out. <G>
"How to read a nautical chart" or even the old "Chart Number One" will tell you how to read a chart, but I don't think either will tell you about the practical differences between raster and vector charting.