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Why does my plotter have a setup choice for turning “over zoom” on or off and—if on—sometimes puts up a warning message when I zoom in tight? R.B., via e-mail
To a certain degree overzoom is a hangover from the passing millennium of paper charts. Printed charts look the same to everyone and are drawn so that they work in a particular size. For instance, if a cartographer has a buoy and a dangerous ledge that, at paper size, are virtually on top of each other, he’ll slightly offset one to make the chart readable. The buoy’s safe side is obvious on the paper, but if you zoom way in on an electronic version of that same chart—and here’s where vector can be trickier than raster—you might perceive a big channel between buoy and ledge and then make an overzoom boo-boo. That’s what your plotter is warning you about.

There’s also the issue of scales, another leftover from paper. A 1:10,000 harbor chart can show a lot more detail from the original survey than a 1:40,000 coastal chart of the same spot, and items on the former are plotted more accurately than on the latter. While electronic charts have no given physical size and their scale is often not obvious, they’re really just tracings or scans of the original paper, warts and all. Different underlying scales are why the overzoomed warning kicks in at different zoom levels.

I’m hopeful that these problems will be solved as we increasingly adopt digital charts (see main story in this column). For instance, it would be great to have a circle around your plotted boat that varied with the accuracy of the original data (and your current GPS fix). But, in the meantime, I wouldn’t turn overzoom off; I’d just use it carefully. Check out this graphic example. The inset above is a mere square inch of paper chart that a sane navigator would probably not venture onto. Above right is my (careful) track on a Navionics version of the same chart greatly overzoomed by a Lowrance plotter. Tide covered those clam-flats, but I didn’t get stuck! —B.E.

Got a marine electronics question? Write to Electronics Q&A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: PMYElectronics@primedia.com. No phone calls please.

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