Thanks, John. A couple of questions for you; do they give you any trouble after plotting, and what causes them ?

How odd. What would cause this?

Bill, I would use electronics (GPS; radar; depth sounder) along with the paper chart. The more verification the better. And, of course, allow for error in the track. That's mainly to check for dangers in the path. These anomalies cover just a small area, so the chart note will specify the warning zone.

Thanks for another good tip.
I only use paper charts, but the other day I was asked to crew on a American flagged 42' Endeavor for a 4 hr charter in the Carib. As we approached the anchorage, I warned the skipper about a reef I'd seen when I'd been anchored there last September. It was clearly marked on my chart although without a ground swell it is covered by a couple of feet.
I could see it, because I knew where to look, and I was on the foredeck.
He looked at his expensive chart plotter, and assured me there was no reef! I suggested he zoom in on the plotter, and watched his face as he saw a 30' reef appear about 50m away on the screen. So no harm done, and I hope he has learned from this experience to trust eyeball navigation and local knowledge, rather than just an electronic aid that was not set up correctly.