GPS and Nautical Charts
by Chuck Husick

The amazing position fixing accuracy of GPS can lead to serious and potentially dangerous navigation problems if the user assumes that the GPS latitude and longitude correspond precisely to the lat/long positions depicted on a chart. Charts are drawn to the many different reference systems (datum). The GPS derived position information will not match chart references unless it has been corrected to match the chart datum. The chart datum is usually printed somewhere in the title block. The most commonly used datum for US charts is NAD 83. The rule is simple; never apply a GPS position to a chart without first ensuring that the GPS datum in use agrees with the datum used to construct the chart. Many GPS units contain a library of chart datum, By entering the datum in use, the GPS will automatically correct the information it provides to match the chart. Where the datum is not in memory it will be necessary to use a reference text to manually enter the offsets between calculated position and chart position.

The position of land, channels and buoyage depicted on charts is often incorrect when compared with GPS position data. The GPS may indicate that you are safely within the confines of a channel when you note that your boat is hard aground on a shoal some distance from the channel boundary. Charts for many areas frequented by small craft are seriously outdated, major shifts in shoals may have occurred since the chart was drawn. In some areas, particularly outside the U.S. land and hazards to navigation are mislocated by hundreds of meters or more. The validity of charted depth information should always be particularly questioned. Many charted depths are based on soundings taken years or perhaps a century or two ago).

GPS will soon provide another valuable service for mariners, calculation of precise heading information. By taking note of successive position data GPS can calculate a boat's average heading and with suitable corrections display the information in magnetic or true north form. As a result of the data smoothing used in the GPS navigator, the displayed heading information will be quite stable. The GPS derived heading information is quite useful in determining the set and drift of the current as well as for correcting the compass if the work is done in a zero current, zero wind situation. Unfortunately, the GPS ability to determine heading depends in part on the speed at which the vessel is moving. At very low speeds the differences in gegraphic position in each measurement epoch will be very small, leading to increased errors in calculated heading. When the vessel is stationary the GPS is unable to resolve heading information and any data displayed will be meaningless.

GPS systems capable of providing precise heading information, regardless of vessel speed, including vessels that are stationary, will be introduced in the near future. The systems will use two or three separate GPS antenna spaced a meter or more apart. They will determine heading by comparing the position data from each of the individual antennas. Since the normal cyclic errors inherent in the GPS signal will affect the signals arriving at all antennas almost identically those errors will be largely cancelled-out in the heading data calculation. This approach to finding magnetic or true heading will join the mechanical north-seeking gyro compass as the only real time source of heading reference data that does not rely upon sensing of the earth's magnetic field. When fully developed the cost of these systems will likely be competitive with today's gyro aided flux gate systems and will be very useful in demanding applications such as autopilots and radar overlay chart plotters. Due to cost and somewhat to the need for the multi-antenna array, these new systems will initially be used on larger vessels.

In summary, knowing precisely where you are on the globe in no way ensures that you are where the chart indicates you are. Always remember the universal navigation admonition, never rely upon a single source of information. Use your eyes, your depth sounder, radar, hand bearing compass, nose or whatever other means you have to validate your assumed position, especially when navigating in areas where a position error of a few meters can make a difference in the safety of your vessel. When navigating in unfamiliar waters or in restricted visibility keep a running plot, on paper, of your vessel's position. A simple interruption in electrical power to the Loran, GPS or chart plotter can suddenly leave you wondering where you are.