[Therion] Wrong average calculation from compass/backcompass data

[Wookey]

On 2018-05-17 09:25 +0200, Evaristo Quiroga via Therion wrote:
> El 17/05/2018 a las 0:36, Olly Betts escribió:

>         I think the formula is too complicated. I purpose a simpler formula, like:
>                     If bearing <=180
>                               AverageBearing =  (bearing + (backbearing -180))/2
>                         else
>                               AverageBearing = (bearing + (backbearing +180))/2
> 
>     Your proposed formula gives wrong answers in some cases - consider:
> 
>     bearing = 80, backbearing = 0
> 
>     These give AverageBearing = (80 + 0 - 180) / 2 = -50 (equivalent to
>     310), but this should be 130 (average of 80 and 180).
> 
> 
> In this case is not a problem with my formula, is a serious magnetic anomaly
> (100 degrees difference)  and the program should to stop and to send a warning.

Yes a warning should probably be issued about poor data, but it _is_ a
problem with your formula. It's just an example showing that all cases
have to be dealt with correctly, including the wrap-around at 0/360
(or 0/400 for grads) and your simplified formula doesn't.

An example with a much smaller difference between back and foresight
could still be constructed to show the issue:

Fore: 175, Back: 0 AverageBearing= (175+0-180)/2 = -2.5. That's
wrong. It should be 177.5 in this case.

Wookey
-- 
Principal hats:  Linaro, Debian, Wookware, ARM
http://wookware.org/
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