A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Don Seltzer
Date: 2016 Aug 10, 17:27 -0400
>>GPS is more statistically likely to be a human error hazard, as we get so pampered by its amazing properties we may get complacent with our interpretive skills, or we fail to confirm that the charts and data in it are properly updated. <<
I know I am saying nothing new here for NavList readers, but I continue to be impressed by Nigel Calder's "How to Read a Nautical Chart" - and in particular, Part 1 of his book: "The Limits of Accuracy." Though GPS (very) occasionally has accuracy issues, by and large, our assumption that it is enormously accurate is well-founded and quite correct.
But a significant portion of the world's charts reference some locally defined ellipsoidal model of the earth that is not the same as the WGS 84 model used by GPS (and I am supposing this is part of 0° longitude not quite going through Greenwich Observatory, as we discussed some time ago). To quote the author:
The half-mile difference between our actual and our plotted position we once experienced in Cuba is an example of the type of differences that can arise when satellite-based navigation equipment is operating on a different datum than the chart datum. Neither the GPS nor the chart was "wrong"; in fact, the chart is a very good one. The two were simply referencing latitude and longitude to a different set of ellipsoidal assumptions....
Therefore, for a considerable period of time, it is going to be essential for navigators to check the datum of every chart used...and, when using a GPS, to ensure that it is operating on the chart datum. Remember that if the datum is not WGS 84 based, the GPS software may introduce an unknown degree of error in the datum-conversion process. Even if the chart is based on WGS 84, the GPS mucst be checked to ensure that it is also on WGS 84. It is only a matter of time before someone playing with the buttons on a GPS accidentally sets it to some obscure datum with a considerable offset, resulting in the boat running aground. (pp. 21-23)