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Subject: Re: Mercator vs. Great Circle Charts
From: Richard B. Emerson (navsys@XXX.XXX)
Date: Wed Aug 29 2001 - 13:58:01 EDT
At the risk of putting a match over an open gas tank, what was the
original question? Something is missing either in terminology or
context.
If you're working with large scale charts (lots of ocean, for
example), north is almost aways up (but *always* check anyway). Small
scale charts (for example, a harbor chart) might well not have "north
up" orientation to get the most amount of harbor or coast on the sheet
(some NOAA charts for New Jersey, for example, do this) but the
chart's lat / long grid will make this obvious (check anyway).
The problem with any chart projection is trying to make a non-flat
surface (a sphere) appear correctly on a flat sheet. And, of course,
it can't be done. It can be approximated (small scale Mercator charts
do well enough) and there are all manner of clever projections
(Buckminster Fuller, of geodesic dome fame, came up with one clever
try which looks pretty but would be impossible to sail across an ocean
with). See Bowditch for some of the better known attempts. As you
look at them, you'll see you can have "north is up" or true distance
or bearing but you can't have them all except with a globe.
The solution to this is do the math for a great circle (or modified
great circle - that is follow the great circle path to some latitude
and follow that for X distance before returning to the great circle
path) and then plot the ship's cource on a smaller scale plotting
sheet which is a Mercator projection of part of the great circle
course. There are several good sources on this process including
Bowditch. It's probably outside of the scope of a survey of
oceanography, however.
One (final?) comment re: Bowditch: it's useful as a source for
oceanography and meteorology as well as the obvious discussion of
charts, piloting, etc. (although there is, unsurprisingly, little on
oceanic life save as it pertains to navigation). The bad news is, I
think, it's gone out of print prior to the release of a new edition
(the last is now 6 years old). The good news is it's on-line, as
you've found, and I recommend downloading it if you have the time and
resources. Barring that, StarPath is still selling Bowditch along
with ColRegs (rules of the road) and Chart 1 ("every chart symbol ever
used is on this chart!") in CD-ROM form (http://www.starpath.com).
The present edition and older editions (there is still some value in
them, although mainly for changes in navigation technique and tables
for navigation) are also often found in places like eBay and used book
sites.
Rick
S/V One With The Wind, Baba 35
Carolyn Seefer writes:
> In a message dated 8/29/2001 9:05:39 AM Pacific Daylight Time,
> navsys@XXX.XXX writes:
> > The question is unclear to me. What is meant by "true direction"? Is
> > this the course to be steered or just a bearing?
>
> My guess is that "true direction" in this case means if one can gauge
> N-S-E-W just by looking at the chart. When I look at the Mercator chart, it
> seems as though I can tell directions accurately, and all compass roses on
> the chart are identical. With the great circle chart, on the other hand, the
> distortion seems to also distort the actual directions. Just by looking at
> the chart, I can't actually determine what is "true North." It seems that
> adjustments would have to be made. Again, I'm new to all of this!
>
> By the way, thanks for pointing me to the NIMA publications online. I've
> already started reading Chapter 3 of the Bowditch text. This is very helpful;
> Bowditch hasn't been mentioned in class yet.
>
> Carolyn Seefer
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