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Could you explain
all this “chart datum” business in the back of my GPS manual?
HR, via e-mail
Best termed “horizontal
datum,” the concept relates to the fact that our planet is not a
perfect sphere but rather a slightly lumpy ellipsoid. Traditionally national
cartographic offices came up with their own formulas to compensate for
the irregularities as they laid out a precise lat/lon grid for their region.
The resulting variations in international chart mechanics weren’t
important because almost all navigation was done relative to a single
chart system or, as with celestial navigation, was too inaccurate to matter.
Times have changed,
and datum differences have become a little messy because our super-accurate
GPSs also use the formulas when they calculate lat/lon positions. If you
take a fix from a GPS set to one datum and plot it on a chart drawn to
another datum, your location on the chart could be as much as a half mile
off, even though your fix is accurate to 30 feet! But that’s an extreme
example. In fact, most of us American boaters will never experience datum
problems because there is an international reference called the World
Geodetic Standard 1984 (WGS84), which is both the GPS default and also
the horizontal datum used on all American nautical charts. Plus almost
all worldwide electronic vector charts have been corrected to WGS84.
However, if you cruise
foreign shores and you’re using GPS with paper charts, you should
check the chart’s datum—it’s in the title block—and
adjust the GPS datum to match. You should also be aware of this issue
in regard to foreign raster charts, though most of the PC navigation programs
that use them will alert you to a mismatch. Modern GPSs support dozens
of datums, as apparently you discovered in your manual’s appendix.
Also note that it is
still possible to experience datum problems here in the United States—if
you use GPS ashore with paper topographic maps, which are still drawn
to North American Datum 1927 (NAD27). I experimented, and at least where
I live, a WGS84 GPS fix plots 400 feet west of where it does using the
proper NAD27 datum. I imagine (hope) that eventually all maps and electronic
nav systems will be coordinated, but in the meantime datums are another
pesky item for a globe-trotting navigator’s checklist and another
reminder about the value of universal standards (see main story in this
column).—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@primediamags.com.
No phone calls, please.
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