[Proj] Terminology: what should I call 60 nautical miles?

Hamish hamish_b at yahoo.com
Wed Apr 29 03:17:38 EST 2009

[single round-up post to hasten the death of this thread]

Mikael wrote:
> I would like a term for the length unit that is 60 nautical miles. 
> Since most projections should only use an ordinary length unit, I was
> thinking about sneaking in the degree disguised as a length unit. 

> Standard Length of Degree = "SLOD"
> 60 nmi = 1 slod

> One minute of arc distance at what latitude?

(so SLOD would need to be SLODLat[^see hypothetical footnote])

> But it may be that "arc distance" isn't as easy to understand
> as I thought.

arc-sec and arc-min seem to be widely accepted, so why not arc-degree?
The trouble comes when you try to pass off a unit of angular measure as
a unit of Cartesian measure.

> For example, Snyder writes:
> "The formulas ... are practical only within a band between
> 4° of longitude and some 10° to 15° of arc distance on
> either side of the central meridian"
> (Map Projections: A Working Manual, page 48). 
> He could just as well have written
> "...between 4° of longitude and some 600 to 900 nautical miles...". 

except that Snyder's version is generalized (cut-and-paste ready for
planetary scientists mapping Mars), and yours is specific to our local
hunk of rock.

> BTW: mi indicates miles and m indicates meters, so nmi stands for
> nautical miles.

except that the common abbr. for nautical miles is "nm" (like it or not).
Usually it is highly obvious you are not talking about nanometers, so
the collision isn't a problem for human parsers.

> so we have to add another esoteric label to shorten "60 nm?"

.... if the best description of a term also happens to be the simplest,
it is harmful to complicate the matter with technical sounding jargon.

> Its purpose was probably for the navigator to always have a handy scale
> at the left or right edge of the chart for measuring his/her course.

(you answer your own question here)

> For some unknown reason it is still used by sea/air navigators.  Must be
> retained as a link to the romantic history of travel.  Certainly has no other
> legitimate usage.

> I use it all the time: is that "handy scale at the left or right edge of
> the chart" no longer a legitimate usage?
> It's certainly more useful that the statute mile!

agreed. also consider that (even in metricized countries) "knots" and paper
charts are still the best solution to the problem, and are still used both
practically and legally (again, "like it or not"). It makes it very simple
to solve the common navigational problems in your head. It's no coincidence
that "practical navigator" and "practical sailor" are popular titles.
Not to mention that just yesterday we had an unpleasant reminder from our
eeePC as to why seawater and electronics do not mix, and thus why brain-
solvable solutions are an important backup.



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