# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: great circle distance
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

Roger

----- Original Message -----
From  "Zorbec Legras"
To:
Sent: Tuesday, May 23, 2000 9:40 AM
Subject: [NAV-L] great circle distance

> Van: Frank Dinkelaar
> 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
>
```
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