Bookmark and Share Email this page Email Print this page Print

Charts unreliable. not GPS receivers

Jan 1, 2003
From Ocean Navigator #90
May/June 1998
I enjoyed reading Nigel Calder's recent article on chart datums ("The Roots of Chart Accuracy," Issue No. 87). It was well-researched, clear, and accurate. However, I would respectfully suggest that the "unreliable navigational tool" is the chart, not the GPS receiver. Any GPS receiver worth its salt can be shifted to any number of datums, since this is a very simple and commonly available set of algorithms.

Furthermore, an offset can be entered manually by the operator to take care of datum shift problems such as the one Mr. Calder mentioned.

I note that the chart in question was compiled after 1975, at which time WGS 72 was the standard. There is no significant difference between the WGS 72 and WGS 84 datums. If the cartographer knows how much to shift GPS positions to agree with the chart, he knows how much to shift the lat/long grid to match the GPS, and hopefully will do so in future editions.