You will probably want to use GPS for
your navigation; however, using GPS afloat
is quite different from your car navigation
system. GPS is precise, and when operating
properly provides you with a three-dimensional
location in space. To relate that to
where you really are, and what is around
(and under) you, you need a chart. Your
chart and GPS become a matched pair,
each providing essential information not
available from the other.
This chapter will provide the basics of
using GPS with charts, which is a starting
point for effective navigation. At all times
you should be checking where you are by
comparing what is shown on your chart with
your real surroundings. This involves measuring
bearings both visually and with instruments.
Simply put, a bearing is the compass
direction from you to some object such as a
buoy or a lighthouse. Bearings are beyond
the scope of this introduction to navigation,
but we will get you started with your GPS.
The whole idea of navigation is to
move safely from one location to another
on the water. As you look out upon the
water, the path across may seem available
and clear. However, many obstacles may
lurk just below the surface to impede your
progress, or even damage your boat or hurt
your crew.
Navigation aids may mark some
obstacles and indicate selected channels.
You need to understand what they are telling
you and how to differentiate one from
another. Navigational aids were presented in
Chapter 2. In this chapter we will focus on
how they look on the chart and how you use
them in planning your navigation.
More often than not, these aids are
set for large vessels, and with your smaller
recreational boat you are likely to venture
outside of the channels they mark. Doing
this makes your chart-reading and navigation
skills that much more important.
Piloting consists of a set of skills. You
will learn some of these skills in this course
and will get to practice them—at least on
paper—before setting out on the water.
Some of the material and the graphics in
this chapter are adapted from The Weekend
Navigator, which is available from retail
outlets as well as from USPS.
In this chapter you will learn:
• Basic concepts in coastal and inland navigation
• Using basic piloting skills to get the most
from your GPS
• An overview of using GPS as your primary
positioning sensor
• How to measure latitude and longitude of
a point on a chart for entry into your GPS
• How to plot a position on your chart using
the latitude and longitude given by the GPS
• How to plot, measure, and label course
lines on a chart
HOW GPS WORKS
GPS has revolutionized navigation. Before
GPS, one of your primary tasks as a navigator
was just to determine where you were.
This is still an important skill that you can
learn in the USPS Piloting Course.
GPS derives its position by triangulating
on four orbiting satellites whose
positions are precisely known. The computations
are all made within your GPS unit,
which results in a 3-dimensional position
fix on or near the earth (Figure 17-1).
Your GPS then converts this fix into a
set of coordinates much like a street address
on the earth’s grid system. It has no knowledge
of what is there—that is where your
chart comes in. See Figures 17-2 and 17-3.
Your GPS can tell if you are moving,
how fast, and in what direction (Figure
17-4). This is computed in your GPS unit
by comparing your current position with
where you were just seconds ago. It computes
your “track” (direction of travel) and
“speed.” This information is continuously
updated. It is important to note that a GPS
is not a compass; it only knows direction
from your motion.
Next, your GPS can tell you the
direction and distance to another loca-
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