Yes Jdev you are right it does not work for all angles and i was aware of that. But it pushed me into another direction to think about it. If the bot stays almost ever on the edge or corner of the battlefield your code works very well i get some really impressive average visit counts with my radar.
Thanks Skilgannon your first approach looks interesting and i will give it a try. Haven't found the time for now. My first thougth to overcome the angle glitch was to throw in a signum() direction check and switch the angles if they don't fit. Don't know how to describe this and its again more of my gut feeling which tells me that this might work. I'm almost sure i did this once but lost it. The second one looks a little like my first try, not really sure about that to.
But anyway, thanks to both of you for bringing me out of my stubborn mind state.