The technology driving GPS is remarkable, but to ensure safe use, boaties should be aware of its limitations as well as its strengths. GPS should be regarded as just another aid to navigation, rather than the final solution. There have been several incidents in recent years where boaties have neglected to employ methods of safe navigation other than GPS, with disasterous results.

A number of issues can diminish the accuracy of GPS. The GPS Standard Position Service (SPS), made available for free to non military users by the U.S. government, beams information from a network of satellites into GPS units. A worldwide study discovered most of the world enjoyed GPS accuracy better than 16.4 metres, with a 95 - 98% reliability. This means that the true position of a boat can be anywhere within a circle of 16.4 m radius drawn around the position shown on an electronic chart. In some parts of the world, this circle diamater increases to 23 m.

Accuracy can be further reduced by atmospheric conditions, electrical interference, or obstacles in the terrain such as buildings, mountains or trees, and sails. Satellites also transmit errors. Therefore never take precision as a given. Always check GPS derived positions at the start of a voyage and at times during the voyage, using other established methods. e.g. compass or relative bearings of a conspicuous charted feature, radar ranges from features, transits, depth soundings, or ranges by dipping distances and vertical angles.

Difference of Datum. Electronic chart displays are only as accurate as the sourced hydrographic data, and should be used in a similar manner to paper charts. e.g. Always use the largest scale display available as it shows more detail, and any distorted errors are reduced to a minimum.
When compiling paper and GPS charts, data used to determine latitude and longitude can vary. Different systems of determining latitude and longitude used, can assume different mathematical shapes for the earth. Many older charts used a datum called AGD-66, while GPS and Australian charts compiled after 1995, including those produced by the Australian Hydrographic Office and MSQ, use the WGS-84 datum.

In Australia, the difference between these two datums can be up to 200 m., the difference between clearing a danger and disaster. When plotting GPS coordinates on to a paper chart which uses a different datum, corrections must be made according to the relavent chart notes. By default, GPS receivers are calibrated to the WGS-84 datum, but can usually be calibrated to other datums as well. If the GPS and the chart are to the same datum, GPS coordinates can be transfered directly to the paper chart. If they are to different datums, the corrections as noted on the chart must be made. If using an unfamiliar chart, always study it carefully.

Zooming In. When charts are produced, a 'capture scale' is used. This means that the chart is viewed at no more than a certain zoom, otherwise the 'capture scale' is exceeded, and the detail shown becomes grossly inaccurate. Some electronic charts, e.g. Seafarer, warn once this optimal zoom is exceeded.

By combining the factors that can diminish the accuracy of GPS units, it becomes clear that using GPS for navigation is not a substitute for sound navigational practices, and should only be used in conjunction with conventional navigation practices. Before using a GPS, you are obligated to familiarise yourself with the strengths and weaknesses of the equipment. As a starting point, it is recommended that GPS users undertake Coastal Navigation and GPS courses currently offered by Volunteer Marine Rescue.

To improve the accuracy and integrity of GPS, AMSA has established 16 Differential (DGPS) broadcasting stations along the Australian coast. These stations broadcast in the 285 - 325 khz frequency range, correcting data into suitably configured GPS receivers, to provide accuracy better than 10 m. (95% of the time). Details of these stations are available on Australian Hydrographic Office.

Common Faults in Using GPS and Chart Plotters.

1. Masters not maintaining or ensuring that a proper lookout is kept whilst the vessel is under way, to identify any approaching hazards. It is important not to lose the situational awareness of the vessel movements and course.

2. Not zooming in to the largest available accurate chart scale.

3. Not switching on the unit and ensure the correct chart datum is selected before departing. GPS units require time to initialise, and the master should assess the accuracy of the position information prior to departure.

4. Power failures or poor electrical connections.

5. Not ensuring that charts are updated with the latest corrections.

6. When going to a waypoint, not checking what is between the vessel's initial location and the waypoint, which will be in a straight line.