Using GPS with Electronic Charts

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Using GPS with Electronic Charts
By Aaron Burke

Editor's Note: Aaron Burke serves as marketing manager for Navionics, a vector-based electronic charting company. When not working or attending boat shows, he enjoys sailing and fishing for striped bass in Buzzards Bay.

Global Positioning System (GPS) and electronic charts are a powerful combination. GPS is the first electronic navigation aid offering accurate all-weather, real-time, continuously updated position fixes with global coverage. Electronic charts provide the ability to view your constantly updated position superimposed on your nautical chart, sparing you the work of plotting latitude and longitude coordinates and laying out your projected courses on a paper chart. At a glance, you can see where you are, where you've been and where you're going.

To use GPS chart plotters safely and effectively, however, it is important to be aware of a number of factors...

GPS Accuracy
The GPS Standard Positioning Service (SPS), which is made available, free to non-military users by the U.S. Government, provides guaranteed accuracy of 20 meters (about 66 feet) with a 95-98% certainty. That means that if you draw a circle with a 20-meter radius around your actual geographic location, 95-98% of the GPS position measurements for your location would fall within that circle. What this means to you is that your boat's position, as shown on your electronic chart, should be viewed as an approximate location, not an exact one. To be safe, you should draw an imaginary circle of ambiguity with a radius of 20 meters (or 66 feet) around the on-screen position, recognizing that you could actually be anywhere within that circle.

GPS accuracy is also affected by factors such as multipath propagation (the tendency of radio signals to bounce from trees, buildings, terrain features and other obstructions), and atmospheric distortion. These effects can be minimized by GPS receivers to some extent through advanced filtering and processing techniques.

Differential GPS
GPS accuracy can be improved using a technique called differential navigation. This 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 mobile GPS navigators via a data link. In North America, and many other parts of the world, public broadcast differential GPS (DGPS) services have been developed to provide improved accuracy in coastal waters and harbors. The U.S. Coast Guard DGPS beacon service provides accuracies of 10 meters (33 feet) or better throughout all U.S. coastal waters and many inland waterways. The DGPS reference station measures errors in the GPS satellite signals and transmits error correction factors to your boat.

Other private DGPS systems, using more expensive equipment and proprietary standards for measuring position and applying error corrections, can provide real-time accuracies of better than one-half meter (about one and one-half feet). These are used for high-precision applications such as channel dredging, buoy tending, seismic exploration, underwater survey, oceanography and mapmaking, and are not practical for most boaters.

Many popular GPS receivers and chart plotters are DGPS-ready or 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 the screen. When using DGPS, the circle of ambiguity around your boat's position shown on the chart becomes much smaller.

Wide Area Augmentation System (WAAS)
Like DGPS, Wide Area Augmentation System (WAAS) increases the GPS signal strength using a series of land-based beacons and provides accuracies of GPS positioning to about three meters (10 feet).

GPS and Electronic Charts
If your GPS latitude-longitude coordinates occasionally differ slightly from the displayed position on the electronic chart, it may be due to inconsistencies among the standards and surveys used to produce the official paper charts from which the electronic charts are derived. Many of the existing paper charts issued by national hydrographic authorities such as NOAA 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.

In some areas, like the Bahamas and Caribbean, some of the paper charts are extremely old and contain errors of 500 feet or more, and these are the most detailed charts available for the area. In other areas of the world, the errors may be several miles and positions of islands may be shown miles off station. Unfortunately, in areas like the Bahamas, the official paper sources are sometimes inconsistent with each other, and the same latitude-longitude plot may locate your boat in two different locations on two overlapping paper charts.

As more of the world's surface is resurveyed with DGPS, many of these inaccuracies will disappear. Navigators must be careful in depending on GPS and electronic charts. It is always good navigation practice to utilize all available aids to navigation and never to depend on any single source.

See also Electronic Charting 101.