2008 REED’s East Coast Edition - Navigation
GPS receivers intended for hiking or high- mismatched datums is most likely when sail-
way use make it very difficult to even acquire ing in remote areas, using older charts. Note:
a lat/long position, and may have built-in While nearly all NOAA charts list “North
databases of exit ramps and fast food restau- American Datum of 1983,” this is equivalent
rants rather than lights and buoys. to WGS 84.
Finding a Position with GPS Steering by GPS
At its simplest, a GPS receiver will provide All GPS receivers include a computer ca-
you with a latitude and longitude fix that can pable of storing information and performing
be plotted on a paper chart and used just like a calculations. This allows the GPS to take a
fix obtained from any other means. Although series of positions and convert them to speed
a GPS fix is usually more accurate than any and course over the ground. If you’ve estab-
other type of fix, it is not without its caveats. lished your course on a paper chart, the sim-
The high accuracy of GPS can generate a plest GPS units can be used with a compass to
false sense of boldness among navigators, compare your course over the ground against
leading them to pass charted hazards much your course through the water in order to
more closely and ignore the safety margins establish set, drift and leeway. If you are using
that they would have followed using other a paper chart and magnetic compass, be sure
means of navigation. With the level of un- that your GPS is displaying a magnetic head-
certainty inherent in other forms of position ing, not a true heading (a function on the unit
fixing, there was general acceptance that the will choose the proper mode).
methods used to create the chart were more A GPS unit by itself cannot determine your
accurate than the fixes produced by the user heading, only your course. However, most of
of the chart. Now however, it is often possible the more sophisticated, fixed GPS units can
that charted hazards were plotted with less receive input from a digital compass and au-
accurate navigational means than GPS and, if tomatically provide information on the differ-
repositioned using GPS, could be charted in ence between your heading and your course.
a different position. In other words, the pru-
Waypoints and Routes
dent navigator will not take the accuracy of
his GPS fix as gospel, and will employ other
Another advantage of the GPS’s internal
tools and methods to verify his position con-
computer is that it can store waypoints, or
stantly over the course of a voyage.
lat/long positions you program into the unit.
Additionally, the accuracy of the chart’s
These days most units come with long lists
physical production can affect the location
of waypoints already programmed into the
of hazards. NOAA’s official standard is that
unit, most of which are actually the locations
any charted object will be within 1mm of its
of lights and buoys, and should be checked
location. But on a 1:40,000-scale chart, 1mm
against updated charts.
is 131 feet.
The method of establishing waypoints with
Regardless of whether you are using GPS
a GPS varies from unit to unit and depends
with paper charts or electronic charts, con-
on whether the device has a mapping func-
tinue to give the same wide, safe berth to
tion or not. Many mapping units will allow
charted hazards that you would using other
you to establish a waypoint by simply click-
means of navigation.
ing on a spot on the electronic chart. Simple
handhelds may require you to enter latitude
Chart datum and GPS
and longitude coordinates.
If you are plotting a GPS position onto a When a waypoint is selected as a destina-
paper chart, it is also critical to ensure that tion, the GPS determines a course to steer,
the GPS is generating a lat/long using the and can track your actual progress compared
same datum as the chart. Most GPS units to the plotted course, providing time and
default to the World Geodetic System 1984 distance estimates to the waypoint as well.
(WGS 84), which is also used by most nauti- Some GPS units can even be connected to an
cal charts. The chart datum can be found autopilot to steer the boat to its destination.
on the chart near the chart name, and a Great care must be exercised when using
function in the GPS will allow you switch all these functions. When you establish a
datums if necessary. The chance of using route between two waypoints, or observe the
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