It is very common now to see "waypoints" (the newfashioned word for longitude and latitude) being promoted for popular cruising destinations. But just as longitude and latitude readings are worthless in the real world if the charts they are obtained from are not correct, waypoints are not absolute either. In fact, waypoints alone can be down right dangerous without copies of the charts they came from to navigate
The newest nautical charts are published with data collected
according to the World Geodetic System 1984. And most of the GPS units
that are sold today are set by the manufacturer to that system. So
mariners navigating in the continental US and Canada are fairly safe
using waypoints without having to worry about the type of chart datum
they were taken from. But if you own a GPS and have taken note of the
system your unit is set on, you will have also noticed that there are
over 100 different map datum systems to choose from.
Chart data systems that are used on US government charts that are
currently being sold for navigation in the Bahamas, the Caribbean and
Central America include: World Geodetic System 1984, World Geodetic
System 1972, Puerto Rico Datum, North American Datum 1927, North
American Datum 1983 and others. All of them are different and errors
will result if waypoints are not coordinated with them. If you enter a
waypoint from one system while the GPS is set on another, you can end up
in big trouble!
While island hopping through the Bahamas recently and referring to
various charts and guide books. I entered waypoints into my GPS memory
that I had taken off a chart of Eleuthera (US DMA 26305) while the GPS
was set on another datum system. When I was ready to navigate with the
Eleuthera chart, I noticed the datum was different and reset the GPS. I
foolishly assumed the waypoints would all remain just as I had entered
them. But after missing my mark when approaching Governor's Harbor by a
quarter mile, I rechecked the waypoints I had entered and discovered
they had all changed when I changed the chart datum setting.
When entering any waypoint into a GPS, it is mandatory that you know
the chart datum system from which the waypoint was taken and first set
your GPS to that system before recording the waypoint. Then when using
the waypoint to navigate, you must have a copy of that chart and make
sure your GPS is set to it's particular datum system in order to
correctly plot your position or else the waypoint will be an accident
waiting to happen.
Chart datum systems are printed on every chart, usually near the main
title and underneath the type of mapping system that is used (i.e.
"Mercator Projection" etc.). The best policy is to take your
own waypoints off your own charts.
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