|
 |
| Articles |
 |
|
To
return to the list of articles please select 'Back'
This article first appeared in
Sportsboat & RIB Magazine
|
So you
have the boat, have bought the safety kit, what
next? Simple – buy a GPS!
Paul
Glatzel reports…
Do some
basic research amongst friends with boats and you
will find the most popular ‘accessory’ is a GPS,
probe a little further to explore why they have
bought a GPS and things often start to get a little
vague with references to “ so I can tell how fast I
am going” or “it tells me my position”. Probe
further still to see if they understand the data on
the screen or how to work out where they are from
the figures on the GPS and with a fair few boaters
you may be risking your friendship!
NB: Images
for this article to be uploaded shortly
Whilst there
are plenty of us with a GPS unit we rarely use even
a limited percentage of its capability. Many of us
are unsure how to link what we see on the screen to
the charts we have or we are unsure what XTE stands
for and whether what it says matters anyway.
Certainly, when I first used a GPS XTE, BTW and TTG
meant nothing to me. Over the next page or two we’ll
look at what a GPS can do and what the various
abbreviations mean. So what is a GPS?
GPS (or
‘Global Positioning System’) was created by the US
Government for military use. A GPS receiver computes
its position relative to a number of satellites
orbiting the earth and represents its position on
the earth’s surface as a “Lat & Long”. Gone though
are the days when the sole use of GPS was to direct
cruise missles through windows on behalf of the US
Government, GPS is now endemic and is found in
taxis, delivery vans, boats and all manner of other
day to day situations.
|
What is “Lat & Long”?
Early map makers needed to
create a way to define a position on the earth’s
surface. By drawing lines between the poles and
horizontally parallel to the equator the earth
became covered in a grid. The lines between the
poles are known as Meridians of Longitude
and it is well known that the 0° line (the
‘Prime Meridian’) runs through Greenwich in
London. Every position on the earth’s surface is
either on this line or East or West of it. The
horizontal lines around the earth are referred
to as Parallels of Latitude, these lines
are measured as an angle relative to the equator
positions are either on the Equator or North or
South of it
So any position on the
earth’s surface sits on a line of latitude and a
line of longitude – giving an exact way to
define a position.
In the above example the
boat’s position on the earth’s surface would be
written as:
30º 00’.0N, 030º 00’.0W
Rarely though does a craft
sit perfectly on such a precise position. For
example CSL’s offices are at 53º 42’.5N, 001º
02’.5W. which if you were relaying verbally
would be “Five three degrees, four two decimal
five minutes north, Zero zero one degrees zero
two decimal five minutes west”.
Note:
Each degree can be divided into fractions of a
degree. Like hours, a degree consists of 60
minutes and each minute consists of 60 seconds.
Seconds are usually expressed as decimals now. (eg
42’.5 rather than 42’ 30’’)
|
A GPS is
useful for:
 | Displaying the position (lat & long) of the craft |
| Storing
waypoints that can be linked together to create a
route |
| Showing
speed and the ground track. |
There are
two types of GPS receiver typically bought by
boaters. At a simple level basic GPS units have
small screens, cost £100 – 200, and only show the
lat and long plus various other items of
information. The next level up (and an increasingly
common option that is coming down in price almost
daily) are chartplotters. A chartplotter is a
‘souped up’ GPS that contains electronic versions of
the paper charts we use and overlays the position of
the craft on those charts. One of the advantages of
chartplotters is that whereas with basic GPS units
you need you to key in manually the lat & longs of
waypoints a chartplotter allows you to position the
cursor on the chart and press a button to create the
waypoint. This can be quicker, simpler and less
prone to input error. One of the downsides of a
chartplotter though is that it can be difficult to
get the ‘overview’ that a paper chart gives. Zoom in
and you focus on a small area and cannot see much
beyond the immediate vicinity, zoom out and the
detail of the chart is less visible – have a chart
AND a chartplotter.
|
Key points to remember about
GPS:
|
Ensure
the GPS receiver is set to the correct datum –
WGS 84 (this is then the same as newer charts
meaning satellite derived positions can be
plotted directly onto a chart.) – if you don’t
there is an error between the chart and the
GPS. |
|
The
compass will only work when the set is moving
– unless the set has an inbuilt fluxgate
compass. |
|
Do not
rely exclusively on GPS as its accuracy and
existence relies on the US Government. They
may need to switch it off or create a random
accuracy error (called ‘Selective
Availability’) from time to time. |
|
Using your
GPS to navigate between two points:
| Initially
choose and input your waypoints |
| You can
then link the waypoints together to form a route.
Care must be taken not to miss out any waypoints
from your route. |
|
GPS terminology
Cross
track error (‘XTE’):
The distance the craft is off to one side of the
line between two waypoints
Speed over ground (‘SOG’):
The speed the
craft is making relative to the ground – rather
than through the water. The difference is the
effect of the tide.
(eg
speed of craft through water – 3 knots, tide in
the same direction – 2 knots, SOG – 5 knots. If
Tide is in opposite direction to direction of
travel SOG would be 1 knot.)
Course over ground
(‘COG’): The course that the craft is making
relative to the ground, which may be different
to the direction the craft is actually heading
due to the effect of the tide.
Bearing to waypoint (‘BTW’):
The bearing from the craft to the target
waypoint.
Distance to waypoint (‘DTW’):
The distance from
the craft to the next waypoint.
Time to go (‘TTG’):
The estimated time it will take the craft to
reach the next waypoint.
GPS compass:
There are two types of compasses that could be
built into a GPS unit. Typically (and almost
always found on older and cheaper models) is the
standard compass that displays compass headings
calculated by reference to the movement of the
craft (they do not work if the craft is not
moving), newer (and often more expensive) GPS
units sometimes have an inbuilt electronic
called a fluxgate compass (which is accurate
irrespective of movement)
Selective availability:
The random accuracy error induced by the US
Government. This is presently switched off.
|
When you
activate this route most GPS sets will show you a
‘rolling road’ with the next waypoint at the end of
it. The trick is to keep your craft to the centre of
the road The distance off to either side of that
centre line is your ‘cross track error’ (XTE)
The problem
with straying off the intended track is that danger
may lurk if the XTE falls outside the acceptable
limit. You will need to decide an acceptable XTE for
your route.
|
Most GPS sets allow you to
configure the screen to show any combination of
the fields. Making the important data larger can
make it far easier to use in rougher conditions
or bright sunlight.
So how do you plot the ‘lat
and long’ that you see on the screen onto a
chart to determine your position?
Using
dividers or a ruler you will need to mark off
the ‘lat & long’ on the chart. The figure 34°
53’.866N is the latitude and is to be plotted on
the vertical scale. Plotting .866 is impractical
so round the number to something you can plot on
your chart. |
|
NB: The
Lat & Long on the simulated GPS screen is not the
same Lat & Long that is being plotted on the ‘chart’
above.
Do the same
for the longitude and at the intersection is where
you are.
GPS has many
uses and given that sets are so cheap now they are
an essential element of any boater’s toolkit. To get
the most from your GPS read the manual, consider
spending time with an Instructor to develop your
skills and invest time fiddling with it and
practicing plotting positions.
| |
More information about using
a GPSs and Chartplotters can be found in the new
RYA Powerboat Handbook written by RYA Powerboat
Trainer Paul Glatzel.
The book has been produced to
support all of the courses in the National
Powerboat Scheme and is the book recommended by
the RYA for those keen on developing their
powerboating skills.
The book retails at £12.99
and is available from the RYA website at
www.rya.org.uk or
via 0845 345 0400
Paul Glatzel runs a powerboat
school in Poole Harbour – Powerboat Training UK. |
|
| 
|
|
|
|
|
|
| New Powerboat book |
| Read about the
new RYA powerboat book
here
 |
| New RYA course! |
| For 2004 there
was a major change to the National Powerboat Scheme - read about
it here |
| Links |
-
RIB.net
-
Sportsboat
- RYA
- RNLI
-
SeaSafety
- MCA
-
YBW.com
- BIBOA
- RIBSTERS
-
Boatlaunch
-
Pathfinder |
| History |
| This site was
founded back in 1999 as a resource for powerboaters interested
in furthering their boat handling skills via training or simply
reading about how to handle their craft better. Since its
inception it remains the only site dedicated to powerboat
training and is ranked No1 for Powerboat Training by search
engines such as Google, Yahoo and Altavista. The site has been
reviewed by a variety of magazines and websites and its content
is regularly used by other more mainstream sites. |
| News |
| How familiar
are you with the new SOLAS V regulations that came into force in
2002? If you've not heard about them them you need to as they
affect your boating and probably your insurance...visit the
articles section to learn more |
|
|
