[ADMB Users] Using asin with df1b2variables
dave fournier
davef at otter-rsch.com
Sun Oct 20 06:20:45 PDT 2013
Hi,
The problem is that if you are "close" to north the estimation procedure may
become unstable. It is better to use atan which has a removable
singularity at pi/2 and 3pi/2. However it will still be a fiddly thing
with several cases to consider.
That is the way I would proceed to build an industrial strength function.
Something like
Start with atan(y/x)
if (x==0.0)
if (y>0.0)
return pi.2
if (y<0.0)
return 3*pi/2
else
u=atan(y/x)
if (x<0)
u+=pi
if (u<0)
u+=2*pi
then you need to deal with x close to 0.
You can use a taylor expansion for say abs(x) < 1.e-8 or whatever.
That's just a sketch.
Anyway the point is that asin has real problems near pi/2 and 3pi/2
whereas atan
is really nice.
Dave
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