GPS accuracy is often changing and theoretically ± a couple of feet may be possible. But I would never in a million years rely on that degree of accuracy. I have a GPS 76, simple hand held and it indicates varying degrees of accuracy and bounces between -+ 10 to 20 feet on average.
Converting a GPS position to a plot on a map can depend on several factors. And one of the largest sources of position error is the zooming effect. I have often been sailing over land according to my Garmin, and not by 2 feet as a result of it. There are plenty of traps in electronic charting, I’ve helped pull a few boats off rocks for this very reason.
The original surveys were often done entirely by hand, using differing measures and datums. The last time I sailed up the east side of Lake Huron the "current: chart had been done by Capt Bayfield and the paper chart showed hand written depths in fathoms, that Garmin has somehow converted to feet. So many hands have touched the that data before I saw on the GPS map. Oddly enough the area of the North Channel that was recently ( 2007) surveyed, although better than no survey is surprisingly sparely done. Often these surveys were done more with commercial traffic in mind so the level of detail and accuracy off the beaten track can surprise you and not in a good way.
GPS, digital charts, radar, AIS all great things don’t leave home without at least the first two. But if you rely solely on GSP and electronic charts sooner than later stuff will happen that a 20 inch plotter will not prevent. Not all big vessels transmit AIS, radar can be tricky to interpret. Particularity in bad weather, close to shore or in congested areas, don’t let all the pretty coloured images fool you into thinking you are safe.
Friend of mine sailing down wind lassoed a channel marker with his main sheet. It was a cute trick, worthy of Roy Rogers. He was preoccupied looking for it on his chart plotter when it happened. I missed a 2 mile wide channel once while trying to “find” it on the gps map. Sailed right by…
Another interesting trap is Nexrad Cell phone based weather. Probably not huge issue for a sailboat given we move so slowly, but the time delay on weather radar can can be up to 20 minutes. Between the time of the radar sweep, the creation of the image, the data transmission to the cell system and the upload to your device, all steps that take time…and flying a small plane at 200 knots, that storm you see on Nexrad can be a whole lot closer that it appears.
I am attaching a recent article from Yachting on e charts. Hope they won’t be too cross with me sharing it.
https://drive.google.com/file/d/1aFVMxHDi_cogP1ETzIVBIRhdhL926Fv6/view?usp=sharing
