NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: great circle distance
From: Roger M. Derby
Date: 2000 May 23, 1:12 PM
From: Roger M. Derby
Date: 2000 May 23, 1:12 PM
As you say, it comes from a time before electronic calculators. When using computers of whatever size you have to take into account the assumptions made in programming the math library. Trig functions are especially contankerous beasts; e.g. zero degrees assumed to be East instead of North, only one quadrant programmed with the angle converted in a non-documented fashion, etc. Humans look for the reasonable answer. The computer just does as it's told. I spent many hours making a VAX 11-750 do great circle calculations correctly. It was ornery enough that I finally set up a table of test cases to run after each fix, just to ensure that the thing hadn't gone awry again. Roger ----- Original Message ----- From "Zorbec Legras"To: Sent: Tuesday, May 23, 2000 9:40 AM Subject: [NAV-L] great circle distance > Van: Frank Dinkelaar > Aan: NAVIGATION-L@listserv.webkahuna.com > Onderwerp: Re: short horizon Hs correction > Datum: din, 23 mei 2000 13:28 > > > > cos D = abs(sinL1 - sinL2) - abs(cosL1 x cosL2 x cos Dlo) > > > why abs? > > look at this it comes from a time there was no Electonic calculators > > a= (sin l1 X sin l2) > ==> sign of a is + l1 l2 same name / - diff name > > b= (cos l1 X cos l2 X cos Dlo) > ==> sign of b is + if Dlo < 90�/ - if DLO > 90� > > cos D = a-b > > D in minutes of angle is the distance in Nm > > > Pm if Dlo > 180� turn the ship! > > > > Cpt. Zorbec > > > > . > ______________________________________________ > FREE Personalized Email at Mail.com > Sign up at http://www.mail.com/?sr=signup >