By Ben Ellison
Chicken and Eggs
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.
This article originally appeared in the July 2003 issue of Power & Motoryacht magazine.