By using a differential GPS unit corrections are made for this and the fix becomes accurate to within one meter for a fixed receiver. When passing under the George Washington Bridge the unit showed my vessel directly under the bridge on the chart - that is exactly where it was. Keep in mind, however, that this does not occur all the time. There are just too many factors to allow for exact accuracy without the differential corrections. Electronic navigation has come a long way but anyone who relies solely on it is in for trouble.
Any prudent mariner knows that learning the essentials of basic navigation is important.
When all else fails having a compass, a chart and knowing what to do will get you home. While en route I will fix my vessel on the course using the GPS . If my electronics fail I will at least have a fix within a reasonable time and can then continue using traditional navigation. I often wonder how many fishermen miss out on some great fishing because they feel limited in their navigational skills or, for that matter, venture farther and end up in trouble for the same reason. Being a prudent and knowledgeable mariner with the basics in navigational skills and electronics can certainly expand your fishing horizons.