| Are you aware that GPS accuracy can change! |
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| Monday, 03 November 2008 22:21 | |||
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The following article was published in the Redcliffe Coast Guard Bulletin (November edition) and I think it is an article that all GPS uses should read. A number of issues can diminish the accuracy of GPS units. The GPS Standard Position Service, made available for free to non military users by the U.S government, beams information from a network of satillites into GPS units. A world wide study discovered most of the world enjoyed GPS accuracy better than 16.4m with a 95 to 98% certainty. This means that the true position of a boat can be anywhere within a circle of 16.4m radius around the position shown on an electronic chart. In some parts of the world, this circle diameter increases to 23 m. Accuracy can be further reduced by adverse atmospheric conditions, electrical interference or obstacles such as buildings, mountains or trees. Satillites themselves can 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 such as a compass/chart or relative bearings of a conspicuos charted feature. Electronic chart displays are only as accurate as the sourced hydrographical data and should be used in a similar manner to paper charts. When agencies and businesses compile paper and GPS charts, data used to determine latitude and longitude can vary. Many older charts used datum called AGD-66, while GPS and newer charts, including those produced by the Australian Hydrographic Office and Martime Safety Queensland use datum called WGS-84. In Australia, the difference between these two datums can be up to 200m, possibly the difference between clearing or hitting rocks or a reef. By combining the factors that can diminish the accuracy of GPS units, it becomes clear that using GPS for navigating is not a substitute for sound navigational practices and should only be used in conjuction with existing aids to navigation. END Ed. As always drummed into my head by my father, know where you are at all times...... at regular intervals (depending on current, SOG, and location etc) make a mark on your paper chart in case the inevitable happens... no power for the GPS and and an emergency situation occurs.
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