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The myth of WGS-84If a chart says it uses the WGS-84 datum, you can rely on it for plotting GPS positions, right? Wrong. We've had current paper charts that prominently state they use WGS-84 and that "satellite positions can be plotted directly" but which are off by as much as 100 meters. Now, 100 meters isn't much unless, like far too many cruisers, you're relying on GPS for precision navigation into passes or to avoid submerged obstacles. We've been in a number of places, notably Fiji, where both paper and computer charts using WGS-84 would put you on the rocks. In one case, where a chart showed several islands, I believe the error occurred because the islands aren't charted correctly, relative to one another, and therefore correcting only one known point on the chart won't correct all of the islands' positions. I keep hammering at this: Do not rely on your GPS for precision navigation, unless you have personally verified that the positions it's giving you are correct. |
