[Proj] Re: wrong computation of meridinal distance / pj mlfn. c

[Albert Peter]

 Noel, and all others who invested their time here ...

> The answer is that integrating over geocentric latitude is incorrect (as 
> would be integrating over geodetic latitude).

let me first thank you all for your attention and help. Sorry for the late
answer, but I have been away for a few days. And while I was drawing
ellipses over and over again, I finally also saw that the "t" in the
definition like x=a*cos(t) and y=b*sin(t) just is *not* the geocentric
latitude! This angle is always larger than both latitudes, which explains my
wrong results.

> t = atan(b*tan(phi)/a), where phi is geodetic latitude

> I derived this by differentiating your two parametric equation for dx/dt
and 
> dy/dt and then solved for dy/dx, which is the horizontal to the ellipsoid.

> The vertical is the negative inverse of that.  The arc tangent of the 
> vertical is the angle (t) that we're looking for.

Oh, my plan for today was to try to find the correct angle, so thanks a lot
again!

> 
> Let me know if you agree.

Very much.

Thank you all for your kind help,

best regards,

     Peter