Introduction

With the advent of the Navstar Global Positioning System (GPS) and, in United States waters, the U.S. Coast Guard's Differential Global Positioning System (DGPS), mariners are now capable of navigating ships with much greater accuracy than previously possible, especially now since May 2000, the GPS signal is no longer being degraded.  As a result, it is very important that mariners understand the inherent limitations of nautical charts when plotting vessel positions using GPS or DGPS.

Before GPS, mariners realized that their geographical positions could be over a nautical mile in error when ship's positions were derived using various electronic instruments such as LORAN and celestial observations.  With this error of uncertainty, mariners gave a wide berth to hazards depicted on charts, including shoals and obstructions.  There was general acceptance that the available navigational information and cartographic processes used by the chartmaker to position the hazards were more accurate than the navigational means available to the user of the chart.

With GPS providing such an accurate fix, the mariner now needs to pay closer attention to the reliability of the chart because accuracy limitations of charts will be critical to ship safety when GPS is used.  For example, to save steaming time, mariners may become more daring and rely on their GPS to pass hazards depicted on charts much closer than prudent.  However, the plotted hazards may have been positioned by less accurate navigational means than GPS and, if repositioned using GPS, could be charted in a different position.

Chart datums can also play a role in plotting GPS positions or transferring positions from one chart to another.  When transferring positions from one chart to another navigators have always had to determine if a correction to adjust for different chart datums was necessary.  To plot correctly, GPS positions must be on the same datum as the chart being used.  Most GPS receivers default to the World Geodetic System 1984 (WGS 84); if the chart being used is on another datum, the GPS receiver must be set to that chart datum or a corrector applied before plotting the position.  Ninety-nine percent of  NOAA nautical charts are on the North American Datum of 1983 which, for charting purposes, is considered equivalent to WGS 84.