The accuracy of GPS can be greatly improved by using a technique called differential navigation. Differential GPS provides much higher accuracy than a regular SPS GPS signal. This results in a smaller circle of ambiguity (see First, a Few GPS Basics).
Differential GPS navigation is accomplished by placing GPS reference stations at precisely surveyed locations. These reference stations constantly compare their known position against the computed GPS position, calculate the errors in each satellite's navigational signal and transmit error correction factors to users via marine radio beacons.
In North America and many other parts of the world, publicly broadcast differential GPS (DGPS) services have been developed to provide improved accuracy on coastal waters and in harbors. The U.S. Coast Guard DGPS beacon service provides an accuracy of 4 to 20 meters throughout U.S. coastal waters, important ports and harbors, including Hawaii, the Great Lakes and many inland waterways.
Other private DGPS systems, using more expensive equipment and proprietary standards for measuring position and applying error corrections, can provide a real-time accuracy of better than Ĺ meter (about 1½ feet). They are used for high-precision applications such as channel dredging, buoy tending, seismic exploration, underwater survey, oceanography and mapmaking. But these private DGPS systems are not practical for most sailors.
Many popular GPS receivers and chartplotters are DGPS-ready. They actually contain a built-in DGPS beacon receiver using a shared antenna. These systems automatically receive and apply the DGPS error corrections to the GPS position so you see the corrected position on your screen. When using DGPS, the circle of ambiguity around your boat's position as shown on an electronic chart becomes much smaller.
If your GPS latitude-longitude coordinates occasionally differ slightly from the displayed position on the electronic chart and you are using the correct datum in your GPS, it may be due to inconsistencies among the standards and surveys used to produce the paper charts from which the electronic charts are derived. Many of the existing paper charts issued by national hydrographic authorities are based on surveys predating the availability of a precise worldwide navigation aid with a standard geographical frame of reference. Only a very small percentage of the world's paper charts have been created using high-precision DGPS surveys. (It is important to remember that should a marine accident occur, the nautical chart in use at the time takes on legal significance. In cases of groundings, collision and other accidents, charts become critical records for reconstructing the events and assigning liability.)