We spent the last week motorsailing down the Intracoastal Waterway from Norfolk, VA to Oriental, NC and had a fine time enjoying the Albemarle and Pamlico sounds and rivers. We were navigating with a Raymarine chartplotter and the latest e-charts from Navionics, both of which are excellent nav tools.
We noticed on a fairly regular basis, but not all of the time, that the GPS position on the chartplotter was often 75 feet or so away from our actual position on the water. GPS normally has a 10-meter margin of error, and we were seeing more than double that distance. Was it the GPS, the charts or a combination of both? Interestingly, the 75-foot offset was pretty consistently to the west of our actual position, but not always.
Once you are aware of a GPS-chartplotter position variance, you can easily factor it in as you navigate as long as you have the ability to check the variance fairly regularly against real fixes. The variance won’t be there all the time and it can change direction. So, be vigilant.
The moral of the story is that in close-quarter navigation such as in the ICW, you really need to keep a good visual lookout for where you are in relation to the actual navigation markers. Plus, you really need to have paper charts for a tangible cross reference of the data on the chartplotter. On coastal runs or at sea, the small errors inevitable with GPS and chartplotters are scarcely noticeable. But in narrow channels, the variance can make the difference between running hard aground and smooth sailing.