Supplement—Digital Charting and Chartplotting
The Global Positioning System (GPS) has
changed the way boaters navigate. Digital
charts help make your GPS unit more
practical by minimizing errors and easing
the tasks of planning and managing your
waypoints and routes. This supplement
addresses different types of digital charts.
GPS provides continuous, accurate
information on your position—a
three-dimensional point in space. This
is converted in your GPS receiver (also
referred to as GPS) into a set of coordinates—
latitude and longitude—you can
use to locate yourself on the earth. It is
imperative you recognize that GPS has no
inherent understanding of what is around
you at a given location. That information
must come from charts and your personal
observations.
You need to plot your latitude and
longitude, as reported by your GPS, on a
chart or chartplotter so you can see where
you are and get a reference for proceeding
with your navigation. This is a necessary
skill; but coordinates are just numbers, so
it is easy to make a mistake and incorrectly
plot your position.
Moreover, in order to navigate, you
need to identify the coordinates of your
destination and enter them into the GPS
as a waypoint. Your paths on the water
generally encompass a number of straightline
segments around obstacles to get you
from one place to a final destination. Each
of these line segments is called a leg and is
identified by the points at each end—called
waypoints. The coordinates for all of these
waypoints then are stored in the GPS.
First, the legs are plotted on a chart.
You need to scan along each leg on the chart
to ensure the paths are free of obstacles and
have sufficient depth to accommodate your
boat. Then, you will use your dividers to
measure the latitude and longitude of each
waypoint and record these values for insertion
into the GPS. Next, you will manually
enter the name and the coordinates for each
waypoint using the buttons on the GPS.
Finally, you will select the waypoints, one at
a time, for the paths you intend to navigate.
By staying on these prequalified
paths, you will be reasonably assured you
should not encounter underwater obstacles
while you focus your attention to watching
what is around you above the water.
This is the basic procedure for navigation
using a GPS. However, while the
GPS may be extremely accurate, there are
a number of chances to make an error that
could lead you into peril. Many of those
are related to working with waypoints. For
example, you might misread the coordinates
from the chart. You might incorrectly
enter the waypoint coordinates into your
GPS. And, once you have entered a number
of waypoints into your GPS, there is the risk
of selecting the wrong one and attempting
navigation on an incorrect path.
DIGITAL CHARTS
Digital charts greatly simplify the process
of planning for your GPS because your
selections are communicated directly to
the GPS using special software. In addition,
this digital charting software computes a
wealth of information about your plotted
courses including course direction, distance,
total trip distance, and more.
At the heart of digital charting is the
calibrated representation of a chart or map.
There are two basic types of digital charts:
raster and vector.
Most boaters are comfortable with
raster charts because their appearance
matches the paper charts they represent.
However, the scanning process takes quite
Section 18: Charts 215
1
2
3
4
5
6
7
8
9
10