WAYPOINTS!
GPS NEWS JANUARY 2010
**Happy New Year** to our customers! Best wishes.
A
GPS Decade Review
Last
month's feature...
MARINE
& WATER GPS.
Whatever floats your boat ...
Handheld
GPS units are generally waterproof (...iceproof,
snowproof). Some models do float, e.g. the
Garmin 70 series, which
is useful for kayaking and canoeing, and back country hiking.
The handheld mapping units
can be loaded with marine charts and topographical data, with
some lake maps also available. See Maps
& Charts.
As
for chartplotters and sonar,
they are typically mountable, meaning, they are 'fixed'
in the boat or vessel with hardware, like nuts and bolts. Chartplotters
offer GPS navigation in varying degrees of detail and image quality,
e.g. satellite, photo, colour, greyscale, etc. The cost also
is reflected in the screen sizes, types.
Safety
tips: Consider
alternate methods to complement navigation, like
sextants, the sky, compasses, conventional maps and so on. It's
safer - GPS is a powerful and useful tool, yet it won't save
your 'bacon' during solar flares. Keep it real. Also,
many
units are dual purpose, GPS and sonar in a unit. We get a LOT
of questions about sonar, so ... here it is!
All
about Fishfinders (sonar) ...
Modern
recreational sonar equipment (also called a fishfinder,
bottom machine, echo sounder, depthsounder, or just sounder)
uses sound waves to look through the water. The sonar device
then displays a picture of what's below the hull of a boat.
Sound
pulse. First it sends a pulse of sound down, via the transducer. The
sound bounces off objects in the water, like rocks, wrecks,
and fish. Second, it measures the time that sound pulse takes
to return and uses that information to display a screen.
Sound
waves reflect best off objects with densities
different than water's. So rocks, mud, metal, and the
air-filled swim bladders of fish all show up well. (There are
also side-scanning and forward-scanning sounders, normally
found on commercial fishing, military, and research vessels;
these are expensive units.)
 Whether
you use your device to navigate the depths, determine bottom
composition or to find those fish, you'll need to consider some
basic questions: Which transducer frequency should I use? How
much resolution do I need? How much power do I need? How do underwater
shapes appear on different machines?
Transducers
Frequencies
from 28 kHz to 455 kHz are common, and some provide boatspeed
and water temperature data. Lower frequencies penetrate deep
water better, while higher frequencies show better detail. Some
units are available with more than one frequency (e.g. dual frequency).
 Virtually
all displacement sailboats need through-hull or shoot-through
transducer mountings. The former will generally be more accurate;
the latter is normally epoxied to the inside of the hull, and
doesn't require drilling a large hole through the boat. Both
types require careful locating and alignment. If you decide on
the through-hull version, and need to install it in a hull cored
below the waterline, you'll need to take great care to seal the
core around the hole.
Transducers
vary from a pre-packaged unit selected for a specific machine
to many other options. There are transom-mount, through-hull,
and shoot-through units, usually in plastic or bronze options.
Screens
 LCD
screens are lighter, require less power, and are easier to waterproof
than comparable CRT screens. The latest improvements in LCD technology
such as glare-reducing coatings and internal backlighting make
these screens easy to see even in direct sunlight.
Screen
resolution is normally expressed in a horizontal and vertical
pixel count, such as 234 x 320. This means 234 dots make up each
horizontal line of the screen, and 320 dots comprise every vertical
line. The horizontal pixel count tells you how much history your
screen will show and how much screen space you will have for
split-screen functions.
The
higher the vertical pixel count, the better the machine will
be able to show a distinction between the sea bed and anything
above it, especially in deep water. If you're scanning in 20'
of water on a unit with 320 pixels of vertical resolution, each
pixel represents less than an inch of water depth. This means
the machine should easily show even a small rock or object above
the bottom profile. In 300' of water, though, each pixel represents
about a foot of depth, which necessitates the use of resolution-enhancing
split screen functions to distinguish between the bottom and
objects above it.
Functions
 Four
major functions increase a fishfinder's usefulness: bottom lock,
zoom, A-scope, and whiteline. Bottom lock and zoom increase the
screen resolution by showing only a portion of the water column
on the screen.
Whiteline,
which Lowrance refers to as Greyline, changes the way a machine
displays the bottom echoes. With Whiteline activated, the bottom
will show up as a thin black line under which varying depths
of white or gray indicate strong or weak echoes. This makes it
easier to determine bottom hardness and differentiate between
bottom and objects close to the seabed. Aside from giving a view
of rocks and obstructions, this can make a big difference in
your anchoring decisions and techniques.
The
maximum power output of most recreational depthsounders is between
100 and 1000 watts of RMS (root mean square) power, which we
can take to mean an average continuous power output. More power
generally helps a machine work better in deeper water.
There
are a couple of operational tips that can help you use your fishfinder
or sonar more effectively. "Auto" is always a good
starting point. Eventually, you'll be able interpret bottom features
and machine capabiltities well enough to begin adjusting the
settings manually. There are different ways to do this, but the
simplest is to increase gain until you begin to see "noise" in
the water column, then back off the gain slightly similar to
adjusting squelch on a VHF.
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