Larry wrote:
Jeff wrote in :


Sounds to me like you have a boatload of people who have forgotten how
to use a compass, if they ever knew.




You mean that big round thingy we moved out of the way to install the radar
display that bobs around and spins crazy when the waves slap the hull?
What kind of reading can you get from something swinging 30 degrees off
course?

Just kidding. It's there and we use it, sort of. Over time, we made up a
compass correction chart for it using the chartplotter headings and compass
readings. The chart looks like a big sinewave. The compass reads
correctly at 2 points where the chart crosses zero. Otherwise, it's off by
as much as 13 degrees at two other points at the peaks. Totally disabling
DC currents has no effect. I have all the wiring as far from it as
practical. There's no close magnetic objects, either. All the electronics
is plastic with copper circuit boards. We tried to compensate it out with
the compensating adjustments and this chart is the result of the "best
setting" of those magnets.

Try that with your compass. Chart its error against the GPS headings over
time as you sail and plot it on a line chart. I was amazed how far off it
is. Sure glad we don't use it for NAVIGATION!....(C;

The problem here is the compass heading is the direction the boat is
pointing, the GPS heading is the direction the boat is moving. The two
are always different, always! You can't swing a compass with a typical
GPS. The GPS that give proper heading have two recievers (one at each
end of the boat) and calculate the ships heading based on the different
positions each one gives. There is stille error.

Gary wrote:
Larry wrote:
....

Try that with your compass. Chart its error against the GPS headings
over time as you sail and plot it on a line chart. I was amazed how
far off it is. Sure glad we don't use it for NAVIGATION!....(C;

The problem here is the compass heading is the direction the boat is
pointing, the GPS heading is the direction the boat is moving. The two
are always different, always! You can't swing a compass with a typical
GPS. The GPS that give proper heading have two recievers (one at each
end of the boat) and calculate the ships heading based on the different
positions each one gives. There is stille error.

At my helm I have a normal compass (deviation less than 2 degrees, as
near as I can tell), the GPS COG, the autopilot setting, and the
autopilot fluxgate compass. It is rare that even two of these will
agree, and often there are significant differences. I sort of enjoy
this seeming vagueness, but it drives my wife crazy, because she wants
to believe that one is "real" and therefore the others are "wrong"!

"Gary" wrote

are always different, always! You can't swing a compass with a
typical GPS.

Why not? Pick a slack current time and set up a waypoint to distant
landmark. Stop the boat and check the compass. You need to do it on
multiple headings or else have landmark waypoints about every 30
degrees around the horizon. If you're in a good coverage period and
the landmarks are several miles away, it's going to be very accurate.

--

Roger Long

On Tue, 14 Mar 2006 16:20:36 GMT, "Roger Long"
wrote:

Pick a slack current time and set up a waypoint to distant
landmark.

Little or no wind is also very desirable, especially for a high
profile boat. Trying to determine if the current is really slack on a
windy day is next to impossible.

Up here in Maine, at least, we are blessed with all these fixed
reference points and current direction and rough velocity indicators,
known by the quaint local term as "Lobster Pots".

--

Roger Long



"Dave" wrote in message
...
On Tue, 14 Mar 2006 13:44:26 -0500, Wayne.B
said:


Pick a slack current time and set up a waypoint to distant
landmark.

Little or no wind is also very desirable, especially for a high
profile boat. Trying to determine if the current is really slack on
a
windy day is next to impossible.

What am I missing here? You need to be able to see your compass
heading when
pointed directly at the waypoint, and the GPS reading at the same
time.
Seems to me it would take an awful lot of wind or current to prevent
you
from doing that.

Roger Long wrote:
"Gary" wrote


are always different, always! You can't swing a compass with a
typical GPS.


Why not? Pick a slack current time and set up a waypoint to distant
landmark. Stop the boat and check the compass. You need to do it on
multiple headings or else have landmark waypoints about every 30
degrees around the horizon. If you're in a good coverage period and
the landmarks are several miles away, it's going to be very accurate.

I didn't think about using the course to steer to a waypoint function.
That would certainly be much better than using the COG function. I am
no sure what the original poster used but your idea seems sound.

"Roger Long" wrote in news3CRf.9136$Da7.2891
@twister.nyroc.rr.com:

Why not? Pick a slack current time and set up a waypoint to distant
landmark. Stop the boat and check the compass. You need to do it on
multiple headings or else have landmark waypoints about every 30
degrees around the horizon. If you're in a good coverage period and
the landmarks are several miles away, it's going to be very accurate.



Shhh...Don't confuse them when they're on a roll....(c;

"Roger Long" wrote in newsKERf.17997$jf2.16363
@twister.nyroc.rr.com:

known by the quaint local term as "Lobster Pots".


In Charleston, they're referred to as "crab traps".....or more correctly,
"goddamned crab traps"....(c;

Personally, I'm still using GPS as my backup to check my visual/radar
fixes (which I consider to be more "real world" true) until all Charts
have been corrected using GPS/DGPS readings.

Bingo!

I've been lurking on this thread waiting for someone to bring up the
key problem with GPS: sure it's mighty-fine accurate, but it's just a
lat-lon, *useless* until related to navigational features of interest
somehow, usually by consulting a chart, electronic or paper. If the
chart is not on the same datum and is also mighty-fine accurate, then
the *chart* is the weak link in the GPS/chart system of navigation.

And charts are accurate at best to a pencil width or two at the
original scale of the chart. Blowing the chart up by a factor of a
thousand on yer super-fancy GPS chartplotter so you can see your slip
makes that pencil width turn into maybe 200 feet, even on a detailed
harbor chart. A pencil width or two on an approach or regional chart
can turn into a half mile or more. So that's the level of accuracy in
a best case scenario for the GPS/Chart system.

Problem is charts are just a faint shadow of the real world:
Cartographers miss things, misplot things and mislabel things. Some
things can't be accurately plotted, some deep-set buoys can swing on
their moorings by a half mile, for example. Charts are not real-time,
as they age, changes occur in the real world which don't magically
appear on the chart. Charts are on many different datums, for example,
if you don't have the brains to switch your GPS receiver datum from
WGS84 to NAD27 when you switch to a NAD27 chart, up to 500 feet or so
of error auto-magically appears in all your plotting. Charts are
necessarily incomplete; other ships, massive breaking waves, flotsam,
commercial fishing gear, etc., don't appear on them. And, if you're
sailing in out of the way places on the planet, the chart, although
pretty, could be a P.O.S., positions off by miles, inconsistently; and
show things which didn't exist a the time of survey and omit things
which did.

The result of thinking your GPS is accurate to a few feet when you're
actually using GPS/chart system? Disaster! On a cruise in the Sea of
Cortez last month we anchored a half mile inland in a few spots,
transited breaking shoals, saw islands to port pass to starboard; all
according to our color big screen GPS chartplotter with a fresh chip.
Every year here in Southern California several powerboats run up on
Huntington Beach at cruising speed, because they zoomed in to San
Diego Buoy #1 on their whiz-bang GPS/chartplotter, set a waypoint,
panned to Angel's Gate in Los Angeles, set a waypoint, created a route,
locked in the autopilot, then went below and hit the sauce. Problem
is that route grazes land halfway through the passage! I'm sure you
can come up with your own stories; and I've got a few of my own where
I've been surprised by taking the GPS picture as gospel.

What GPS has done is allow nitwits to navigate right on the money, most
of the time, and not develop other navigation skills. Used to be
natural selection took care of them sooner or later. Cruising's going
to hell, a high-tech solution to every problem: sail handling? -
roller furling!, water? - watermaker!, batteries? - battery monitor!,
navigation? - GPS! All very alluring, but harsh mistresses when they
go south.

Now, radar, depthsounder, RDF and the Mark I eyeball and earball are
not derivative navigational tools; there is no lat-lon fixed scale
chart accuracy correct datum ju-ju going on. They have their problems,
but on a good day you are directly relating the real world to your
position, no third parties - satellites, master control stations and
cartographers involved. And, as you "zoom in", i.e., get closer to the
thing in question, accuracy gets better linearly, unlike zooming in on
a chartplotter. It's much better to know that navaid in the fog is
actually-right-now 200 yards off your starboard bow than to think the
sexy picture on your chartplotter is reality (which has it off your
stern).

That being said, I'm a multi-unit GPS owner too, mighty useful, but
it's more of a glance now and then and autopilot brain for me. Good
for detecting current and leeway effects, where the GPS is essentially
calculating a vector between where you are now and where you were a
short while ago, no chart referencing there. Good for calling BS on
your chart, if the Mark I's are telling you a different story. Really
good for returning to the *exact* same spot where you've taken a GPS
waypoint previously, or relating your position to another vessel's
reported GPS position (if they're dialed into the same datum). Super
accurate clock, too.

But it's no magic bullet, because of the chart problem.

Mark wrote:
Personally, I'm still using GPS as my backup to check my visual/radar

fixes (which I consider to be more "real world" true) until all Charts
have been corrected using GPS/DGPS readings.

Bingo!

-snip-

And charts are accurate at best to a pencil width or two at the
original scale of the chart. Blowing the chart up by a factor of a
thousand on yer super-fancy GPS chartplotter so you can see your slip
makes that pencil width turn into maybe 200 feet, even on a detailed
harbor chart. A pencil width or two on an approach or regional chart
can turn into a half mile or more. So that's the level of accuracy in
a best case scenario for the GPS/Chart system.

Problem is charts are just a faint shadow of the real world:
Cartographers miss things, misplot things and mislabel things. Some
things can't be accurately plotted, some deep-set buoys can swing on
their moorings by a half mile, for example. Charts are not real-time,
as they age, changes occur in the real world which don't magically
appear on the chart. Charts are on many different datums, for example,
if you don't have the brains to switch your GPS receiver datum from
WGS84 to NAD27 when you switch to a NAD27 chart, up to 500 feet or so
of error auto-magically appears in all your plotting. Charts are
necessarily incomplete; other ships, massive breaking waves, flotsam,
commercial fishing gear, etc., don't appear on them. And, if you're
sailing in out of the way places on the planet, the chart, although
pretty, could be a P.O.S., positions off by miles, inconsistently; and
show things which didn't exist a the time of survey and omit things
which did.

-snip-

What GPS has done is allow nitwits to navigate right on the money, most
of the time, and not develop other navigation skills. Used to be
natural selection took care of them sooner or later. Cruising's going
to hell, a high-tech solution to every problem: sail handling? -
roller furling!, water? - watermaker!, batteries? - battery monitor!,
navigation? - GPS! All very alluring, but harsh mistresses when they
go south.

Now, radar, depthsounder, RDF and the Mark I eyeball and earball are
not derivative navigational tools; there is no lat-lon fixed scale
chart accuracy correct datum ju-ju going on. They have their problems,
but on a good day you are directly relating the real world to your
position, no third parties - satellites, master control stations and
cartographers involved. And, as you "zoom in", i.e., get closer to the
thing in question, accuracy gets better linearly, unlike zooming in on
a chartplotter. It's much better to know that navaid in the fog is
actually-right-now 200 yards off your starboard bow than to think the
sexy picture on your chartplotter is reality (which has it off your
stern).

-snip-

But it's no magic bullet, because of the chart problem.

I enjoyed reading your dissertation. You are right about the GPS being
an enabler for folks with lesser skills. You are right about chart
inaccuracies. There is a bit of a leap when though if you don't point
out that all those same inaccuracies also apply to the traditionl forms
of navigation. The depth sounder has error and so does the charted
soundings. The RDF has bearing error (cone of position) and so does the
plotting of the transmitter on the chart. Radar has many errors; a
radar mile is 6000 feet. Depending on the range and height of tide, the
land you are looking at on radar can be completely different from what
is charted, and of course it is a skill to plot a radar fix so it is
even close to where you are. At the very best, getting a fix on the
chart tells you where you were some minutes ago. Visual fixes are the
meat and potatoes of pilotage and coastal nav but require a great degree
of skill to quickly shoot, plot and put a DR on so that you can see
where you are, not where you were. It is still an educated guess given
that tide and wind can make your position differ from the DR. Even if
the wind and tide are applied to the fix giving you an EP (estimated
position), error creeps in.

Good navigation is an art. GPS has made it much more scientific but the
interpetation of the information and the combining of that info with
other info is still an art. Practice is the key.

Gary