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

Mikael Rittri Mikael.Rittri at carmenta.com
Tue Apr 28 02:41:38 EST 2009

```Clifford wrote:
> One minute of arc distance at what latitude?

If we can ignore the sphere/ellipsoid difference for a while,
then the latitude does not matter, since it is one minute of
arc distance, not one minute of longitude difference.

I thought "arc distance" was an established term for the length
of a great-circle arc, expressed in an angular unit (the angle
of the arc as seen from the sphere center).

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...".

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

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

I had hoped "degree of arc distance" would not be seen as esoteric.
Or maybe "degree as a length unit", or "pseudo degree" or whatever.
(The "exēntanautical mile" was meant as a joke.)

Gerald wrote:

Yes, but I am not sure I dare to answer...  The more I think about,
the more I feel that my reasons are not very good.
Actually, I was thinking about using a single class to implement
an Equidistant Cylindrical projection, which should be able to
produce coordinates both in an ordinary length unit (meter, foot, ...)
and in an angular unit (degree, minute, gon etc.).  Since most
projections should only use an ordinary length unit, I was thinking
about sneaking in the degree disguised as a length unit.
But I am starting to think I shouldn't.  I ought to follow
the EPSG Guidance Note 7.2, which distinguishes between
the Equidistant Cylindrical (codes 9842 and 9823) that uses
an ordinary length unit, and the Pseudo Plate Carrée (code 9825)
that uses an angular unit.
And if I make that distinction too, I don't need to worry about
how to disguise the degree as a length unit.  Sorry to have bothered you.

--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com

________________________________

From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Clifford J Mugnier
Sent: den 27 april 2009 16:51
To: geraldi.evenden at gmail.com; PROJ.4 and general Projections Discussions; PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Terminology: what should I call 60 nautical miles?

One minute of arc distance at what latitude?

On a sphere, "longitude varies as the cosine of the latitude" but we're not on a sphere, are we?

This stuff is not complicated enough for lay people now, so we have to add another esoteric label to shorten "60 nm?"

C. Mugnier
LSU

________________________________

From: proj-bounces at lists.maptools.org on behalf of Gerald I. Evenden
Sent: Mon 27-Apr-09 09:05
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Terminology: what should I call 60 nautical miles?

On Monday 27 April 2009 7:12:18 am Mikael Rittri wrote:
> Hello,
> I would like a term for the length unit that is 60 nautical miles.

The "nautical mile" is a rather old fashion term handy in the days of sailing
on a chart with sextant and chronometer.

I thought everyone was supposed to go metric---an underachieved effort on this
side of the pond.

> This length unit would approximate one degree of
> arc distance, in the same way as one nautical mile
> approximates one minute of arc distance.
>
> I have thought of the phrase "degree of arc distance"
> (which I think agrees, more or less, with how Snyder uses this phrase)
> but some of my colleagues dislike it.
>
> I have also thought of the phrase "exēntanautical mile",
> from Greek "exēnta" = 60, but...
>
> --
> Mikael Rittri
> Carmenta AB
> SWEDEN
> www.carmenta.com
> _______________________________________________
> Proj mailing list
> Proj at lists.maptools.org
> http://lists.maptools.org/mailman/listinfo/proj

--
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to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
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