40 seamanship secrets
I mentioned vector charts in Chapter 1.
Unlike raster charts, which are simply
scanned versions of traditional paper
charts, vector charts pull information
from a database within their software.
Many chart plotters and GPS systems
use a vector display, but it does take
some getting used to. The fi rst thing
you will notice is that the display
may show much less detail than a
nautical chart or raster display of
the same area. Here’s a summary of
the pros and cons of this remarkable
technology.
PROS
Razor-sharp adjustable display. The
display appears with unmatched clarity
but looks a bit “computerized” to some
mariners. Land profi les and imagery
appear with less detail than a raster
scan but retain the same positioning
accuracy. GPS plotters normally carry
a menu function, allowing users to
adjust the screen for viewing in bright
sunlight or darkness.
Selective layering. The user retains
control over how much data he wants
displayed on the screen. Don’t want to
see depth contours or landmark names
cluttering up the display? Simply open
up the Chart Display Options menu
and deselect that function.
Depth unit selection. Choose from
feet, fathoms, or meters, and the display
instantly converts every sounding and
depth curve to your selection.
Text font consistency. Raster chart
text enlarges when you zoom in for
detail. This sometimes obliterates
important information such as wrecks,
navigation aids, or soundings. With
vector displays, the text stays the
same size when you zoom in or out.
Vector Charts for Precision Electronic Navigation
Memory storage. Vector compression
technology allows storage of massive
amounts of data using a mere fraction
of the memory required to store the
same amount of data in a raster
display. In his book GPS for Mariners,
author Robert Sweet states that a
set of raster charts requiring 200
megabytes of memory only requires
20 megabytes of memory through
vector compression. Using this
technology, every chart in the world
could fi t onto one tiny CD-ROM disc!
Info dialog box pop-ups. Do you want
to know the hours the fuel pier is open at
the marina 5 miles ahead? Or how about
the range of visibility on the lighthouse
ahead? Simply move the cursor, place
it over the object desired, and a
pop-up dialog box shows amplifying
information, similar to that of a cruising
guide, Light List, or coast pilot.
CONS
Display limited by screen size. As with
all displays, the monitor-screen size
limits the amount of screen real estate
available. When zooming in, you’ll need
to scroll right, left, up, or down to move
the display. It’s easy to lose your place
when zooming in. If this happens, zoom
out, find a reference landmark, and
then try zooming in again.
Cursor movement. With layering
technology, you need to move the
cursor over areas of interest for more
detailed information. Save time by
doing as much of this as possible before
casting off. For instance, check marina
data along your route and make note
of information such as the hours of fuel
dock operation and the ship’s store, or
radio contact information. Enter this
into a logbook for instant reference.