On Wed, May 16, 2018 at 04:45:54PM +0200, Evaristo Quiroga via Therion wrote:
> I think the problem is the average calculation formula in "thdb1d.cxx"
>
> >// check backwards compass reading
> > if ((lei->data_type == TT_DATATYPE_NORMAL) ||
> > (lei->data_type == TT_DATATYPE_DIVING) ||
> > (lei->data_type == TT_DATATYPE_CYLPOLAR)) {
> > if (!thisnan(lei->backbearing)) {
> > if (thisnan(lei->bearing)) {
> > lei->backbearing -= 180.0;
> > if (lei->backbearing < 0)
> > lei->backbearing += 360.0;
> > lei->bearing = lei->backbearing;
> > }
> > else {
> > lei->backbearing -= 180.0;
> > if (lei->backbearing < 0)
> > lei->backbearing += 360.0;
> > // calculate average of two angles
> > //lei->bearing += lei->backbearing;
> > //lei->bearing = lei->bearing / 2.0;
> > double sumx, sumy;
> > sumx = cos((90.0 -
> >lei->bearing)/180.0*THPI) + cos((90.0 - lei->backbearing)/180.0*THPI);
> > sumy = sin((90.0 -
> >lei->bearing)/180.0*THPI) + sin((90.0 - lei->backbearing)/180.0*THPI);
> > lei->bearing = 90.0 - (atan2(sumy,
> >sumx) / THPI * 180.0);
> > if (lei->bearing < 0.0)
> > lei->bearing += 360.0;
> > }
> > }
> > }
>
> 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).
The therion formula is attempting to average the angles by trigonometry,
which seems a reasonable approach (though probably slower than trying to
average more directly like you're suggesting). It looks essentially
correct to me, though it's a bit oddly written since cos(90 - x) is
sin(x), sin(90 - x) is -cos(x), 90 - atan2(y,x) is atan2(x,y) (possibly
+/- a multiple of 360). But if I work it out with a calculator for
236.8 and 56.8 then I do get 236.8.
If it's still not working with the declination set to zero, perhaps
you should show us a complete small example we can process to see
what is going on?
Cheers,
Olly
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