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One area where you encounter different practices is the placement of the
N/S/E/W letter. Should it precede or follow the number it modifies? In
aviation the convention seems to be the letter goes first, both on
printed matter and on the machines (e.g., entering the coordinates to
initialize an inertial nav system). I think this is more logical than
putting the letter last, since you go from general to specific as you
read the coordinate from left to right.

As for signs, with most calculators you have to adopt a sign convention
for input, but a computer should accept the letter. But please,
programmers, make it insensitive to the case of the letter. There's a
Web page at the National Geodetic Survey site I use occasionally which
will not accept n in place of N!

Though I'm an American, I prefer "east is positive" for longitudes. It
makes figuring the LHA of a star simple, since you just add everything:
GHA Aries, SHA star, and assumed longitude. This is also a good
convention to use internally in software which uses geocentric
rectangular coordinates. Normally in such a system the x and y axes lie
in the plane of the equator with the x-positive axis directed to the
Greenwich meridian and the z-positive axis directed north. With this
layout, calling east longitude positive is most natural.

In astronomical work I've seen longitude measured east to 360. The
Astronomical Almanac gives coordinates of the major observatories like
that. However, in "Astronomical Algorithms" Jean Meeus grumpily refuses
to conform, saying astronomers used the opposite convention for more
than a century.

Here's a rather long but interesting document on geodetic surveying
history. About 2/3 of the way through is a little section titled
"Azimuths From the South - Why?" with a little background to this
practice.

http://www.ngs.noaa.gov/PUBS_LIB/geodetic_survey_1807.html

With respect to minutes and seconds vs. decimal, obviously minutes are
convenient for a navigator plotting on a chart. Aviation seems to use
decimal minutes exclusively. (One air-launched weapon I saw several
years ago accepted target coordinates to .00001', roughly the width of
Saddam's mustache.) Surveyors and astronomers use seconds a lot. Purely
from the standpoint of efficient packing, decimal degrees are better.
E.g., 125.4767 and 125 28' 36" have the same number of digits, but the
decimal form has 2.8 times finer resolution. If we use George's system,
expressing the angle in "milliturns", it's 348.5464, with 2.8 times the
resolution of decimal degrees.

Too bad our compasses are not marked in grads (400 grads in a circle).
Mentally figuring reciprocals, right angles, and 45s becomes child's
play with this system. Sadly, I've never seen grads used, though my HP
calculator can deal with them.

--


paulhirose@earthlink.net (Paul Hirose)
approx N39 grads W131 grads

   

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