GPS and Compass Deviation
Al Saunders
Can a compass deviation card be established by comparing the compass
course with the GPS course after allowing for variation ?
Remember two things:
1) GPS does not give vessels heading (only course or track made good)
whereas a compass gives vessels heading, and:
2) GPS updates periodically (approximately every 4 seconds 0n some GPSs)
and the direction travelled per GPS could be (in the short term)
different from course mase good because of inherent limit of precision
(assuming no differential).
Al
Struther Macfarlane
Al Saunders wrote:
>
> Can a compass deviation card be established by comparing the compass
> course with the GPS course after allowing for variation ?
Yes... more or less... kinda... maybe....
It's a good way to "guestimate" if your compass is set "close to"
correct and a good way to "verify" your course steered is what
you "thought" you were doing.
If you find that your GPS/Loran is telling you course made good
has been 120 degrees, though, and all the while you've been steering
a course of 230 degrees (according to your compass), then you KNOW
there's a problem<g>! Of course, continuous dead reckoning en route is
a good way to ensure you're really going where you thought. Smaller
corrections are much harder to correct.
Still, the best way to calibrate your compass is to find something
that you KNOW is accurate (church steeple & smokestack, range markers,
etc...) and set your course/sompass too that.
It's a tedious task, but one well worth it
mark johnson
> Al Saunders wrote:
>
> Can a compass deviation card be established by comparing the compass
> course with the GPS course after allowing for variation ?
>
If you are in tidal waters, or water with any current, then the GPS
is going to give you your course over the ground, which may well
be different to your pointing, or your compass.
I would have thought that you would only be able to make up a deviation
card, using a GPS, in a lake on a calm day.
If you are happy to rely on GPS, which you shouldn't, then you could
do without a compass.
Regards
--
Mark Johnson
Barrie Avis
>> Al Saunders wrote:
>>
>> Can a compass deviation card be established by comparing the compass
>> course with the GPS course after allowing for variation ?
>>
>
if you stear a dead constant course (which you cannot) the COG from the
GPS is going to vary.
Its so much simpler and far more accurate to use landmarks which clearly
show on the chart so why not do that?
--
Barrie Avis
Bart Senior
mark johnson <ma...@markland.demon.co.uk> wrote:
>If you are happy to rely on GPS, which you shouldn't, then you could
>do without a compass.
>Regards
>--
>Mark Johnson
Electonic navigational aids such as a GPS are considered the least
reliable tools for the purposes of navigation. Use them to verify the
location of other forms of navigation. (i.e. to help you visually site
a buoy, and then take a bering off of it).
The advantages are obvious--an extra confirmation of your location.
This is all counter intuitive since a GPS can give you a more accurate
fix than you can get visually. Still, a visual fix is more reliable
since you do not have to repy on a piece of electronic gear that may
be malfunctioning. Many people have found GPS positions that have
placed them on land when clerly they were at sea. For example, using
the wrong set up information, crossing the equator, or in an area of
poor coverage.
-----------------------
A few years back a 6 months old, $ 20 million mega yacht that was
cruising at 50 knots, in the fog, off New Foundland, Canada, when it
went hard aground. The yacht traveled 1/4 mile on dry land before
coming to rest. It's crew relied on electronic navigation.
Total damage $8 million. No one was killed.
-----------------------
Anyone who would rely totally on a GPS and not carry a compass is
crazy. I carry two compasses on me when I sail.
A cheap plastic combination thermometer and compass--that works great
is attached to my life vest.
I also carry a flat fluxgate compass in a lanyard around my neck under
my jacket or shirt to keep it from floping around. It is so
comfortable that I often find myself looking in my sea bag for it when
it is around my neck! The fluxgate has nine memories and I take three
bearings on each reference which averaged give me a three bearing fix.
Sea Ya!
Bart Senior, ASA Sailing Instructor ARS: KC5VUO
mark johnson
In article <NUrlIRAUN$LyE...@avis.demon.co.uk>
bar...@avis.demon.co.uk "Barrie Avis" writes:
Hello Barrie
SA varies second by second, but not dramatically. It follows a pattern,
rather than a random error.
Thus whilst you are right in saying it won't be absolutely true, it will
give a good indication, otherwise the functions on the GPS will not be
much use other than position. The GPS will also average out a set of readings.
You could set a waypoint several miles away, then steer towards it, and
compare the compass readout. If the waypoint is far enough away (and still
in sight), SA will have no effect (that you can see). The difference between
the compass and GPS will be tide, deviation and / or compass error.
As a coastal cruiser, I havn't made up a deviation card. If I was planning a
long leg out of sight of land, I think I would try to make a card up by
swinging the boat in harbour, rather than at sea.
I find that trying to sail an accurate course with waves over 3 feet a bit
tricky. I would guess that at any one time my course would only be good to
maybe 5 degrees at best. !!
How does effect offshore cruising ?? 5 degrees over a 24 hour cruise say
120 miles, would put me out by over 10 miles, without tidal errors etc.
If the course (steering) error was random, then it would tend to cancel
itself out, but a compass error would be a system error and would be all
in one direction.
I'm off to Ilfracombe tomorrow (25 miles), I hope I get there !!!
Regards
--
Mark Johnson
Terry Schmidt
I guess i am the only one that has done it. I put my boat on auto
pilot under calm conditions and no current ( slack tide ) and it worked
fine. Over several outings this summer I was able to make a very reliable
deviation chart. When my boat was hit by lightning near the Panama
Canal and the hull became magnetic, I had to do the same thing.
It is far from perfect but it can be done.
Matthew Thompson
Barrie Avis <bar...@avis.demon.co.uk> wrote:
>>> Al Saunders wrote:
>>>
>>> Can a compass deviation card be established by comparing the compass
>>> course with the GPS course after allowing for variation ?
>>>
>>
>Apart from tidal waters problems, surely SA is going to mean that even
>if you stear a dead constant course (which you cannot) the COG from the
>GPS is going to vary.
>Its so much simpler and far more accurate to use landmarks which clearly
>show on the chart so why not do that?
>--
Agreed, standard GPS will not be accurate enough, use transits or hire
a compass adjuster.
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