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