Re: Earth map transformation in Ada ?

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

<tmoran@bix.com> wrote in message
news:_NFQ4.29$375.13699@news.pacbell.net...
> > lat,long  <-> x,y (Polar Stereographic)
> Out of curiousity, what is that?  Some kind of geographic database?

A polar stereographic projection is a way to map the surface of a
spheroid onto a plane tangent to the spheroid surface at one of the poles.

For example, if the plane were tangent to the north pole, and the
projection point at the south pole, the meridians would be radials from
the tangent point on the plane, and parallels would be concentric
circles about the tangent point.
The size ratio of the projection at the north pole would be 1:1, at the
equator 2:1, and at the south pole, infinity.

Now, I said spheroid, because for many applications, modeling the earth
as a sphere is not good enough.  There are several spheroids in common
use -- e.g. the World Geodetic System 1984 (WGS-84) models the earth
as a polar section that is an ellipse having a polar radius of
6_356_752.3142 meters, and an equatorial radius of 6_378__137.0 meters,
at mean sea level.

The mathematics are straightforward, and the programming trivial, except
for dealing with singularities.  GNAT, for example, will return values
of NaN or +/- Inf. for some of these.  Testing for NaN, for example
requires a three-way test e.g., /=0 and then not < 0, and then not > 0.

There is a website
http://www.utexas.edu/depts/grg/gcraft/notes/datum/datum.html at which
conversion formulae may be found, to convert between various geographic
data forms.

This turned out to be more long-winded than I thought it would be when I
started to answer... oh, well.