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post #8 of Old 12-12-2012
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Re: Gps navigation advise

Much of the GPS accuracy is dependant upon the latest updates of the charts held within. Some are older than Methusila, dating back to the 1950s, others are older.

A great example of this was discovered when I was traveling down the Intra-Coastal Waterway a few months ago. The GPS Plotter showed I was cruising on dry ground, at least 150 feet outside the waterway, but within a margin area delineated by a dotted line. I called a friend at USGS about this and he provided me with the details. Essentially, the chart was created prior to the construction of the ICW in that area. Consequently, the location revealed by the GPS/Plotter was accurate, but the charts portrayed where the proposed ICW was - not the actual location where it was eventually dredged.

The same is true in the ICW on your charts behind Virginia's Barrier Island. All the markers are in place, but there is NO channel - it was never dredged. So, from Oyster north to Wachapreague, the ICW may be on your charts, the same charts within your GPS, but it does NOT exist in that location. Instead, the ICW in Maryland and Virginia is essentially Chesapeake Bay.

So, the argument of paper charts V/S GPS plotter is pretty much mute - the accuracy of the charts is the same. The big difference is the accuracy of the device is far superior, often with an error factor of +/- 9-feet, depending on the number of satellites tracked at any given time.

For me, the GPS/Plotter is among the most valuable navigational tools any captain can have aboard his or her vessel. THE most valuable tool is your eyes, followed by common sense.

I use the Lowrance HDS7, and it's an incredible machine.

Good Luck,

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