quote:
Originally posted by thataway4
I don't agree with Eric entirely--but on rivers, the channel may change often, and the charts are not corrected on a regular basis. GPS chart plotters are extremely accurate--it is the charts that are not accuratet. Reliable--absolutely. I have yet to have a GPS fail, and I have been using GPS for over 17 years. The only time I have lost a signal, is the proximity of warships--the areas covered have been worldwide.
The GPS system is indeed very reliable, but ships electronics are what ships electronics are.
Accuracy of contemporary recreational GPS fixes are typically +/- 15 meters, or about 50 feet.
NOAA cites charts are accurate to +/- 0.5 mm at the scale of the survey. If you have a fairly large scale survey, say 1:25,000, then the accuracy is +/- 12.5 meters, or about 41 feet.
The expected accuracy then of a GPS chart plotter operating with a 1:25,000 survey chart is on the order of +/- 91 feet. For a 1:40,000 survey it becomes 116 feet, and at 1:80,000 it becomes 181 feet.
I am leery because the route into and out of my marina requires navigating a channel with a controlling width of 60 feet over about 3 miles. The local chart is on a 1:25,000 survey. Expected accuracy = +/-91 feet. That's a little tight for me.
And then you have Bob's point - charts get dated, channels move, etc. It's risky business to be reliant on GPS in
critical navigation situations, where the reasonably expected accuracy is not enough to assure safe passage.