From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID, LOTS_OF_MONEY autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,960d1cb75613cad X-Google-Attributes: gid103376,public From: "David C. Hoos, Sr." Subject: Re: Earth map transformation in Ada ? Date: 2000/05/06 Message-ID: #1/1 X-Deja-AN: 619930051 Content-Transfer-Encoding: 7bit References: <8eudi9$ck7$1@news.uit.no> <_NFQ4.29$375.13699@news.pacbell.net> X-Priority: 3 Content-Type: text/plain; charset="iso-8859-1" X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 X-Complaints-To: abuse@earthlink.net X-Trace: newsread1.prod.itd.earthlink.net 957611807 158.252.122.167 (Sat, 06 May 2000 04:16:47 PDT) Organization: Ada95 Press, Inc. X-MSMail-Priority: Normal MIME-Version: 1.0 NNTP-Posting-Date: Sat, 06 May 2000 04:16:47 PDT Newsgroups: comp.lang.ada Date: 2000-05-06T00:00:00+00:00 List-Id: 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.