I am reading the book A Voyage for Madman by Peter Nichols. There are some maps on some pages. On those maps, I see the acronym "SFM" and have no idea what it means.
I looked SFM up on Google and I found lots of stuff about Source Film Maker, which is a software. I don't think this is what it means here...
I also looked on AcronymFinder and I found lots of results. None of them seems to fit the context.
What does this mean?