# Convert angles between rotation-direction and zero-point

I got angles in mathematical sense. This means 0° is east and the rotation is anti-clockwise.

I need to convert them to 0° is north plus rotation clockwise.

The first one is an easy offset but how can I build up an algorythm taking into account to rotation direction?

-
I have thought about getting X/Y in a circle with radius=1. But I dont know which angle functions will then return in the correct new system when going back from X/Y to the angle. – ZoolWay Feb 24 '12 at 18:31
If $\theta$ is the angle anticlockwise from east in degrees, then the angle clockwise from north is $90^\circ - \theta$. – Rahul Jun 2 '12 at 0:37
+1 fuer "algorythm" – Sibbs Gambling Oct 1 '13 at 3:43