Hello there,
I'm still struggling with that as well.
One way I found that works is to create a distance shader attached to a camera positioned in the centre above where you want the flat bit. Set the min and max range in the distance shader with the appropriate fuzzyness and feed the output into the blender of the shader you want to create a flat terrain in. Point is the flat bit will be a section of a sphere, maybe a circle or an ellipse, depending on your roll and pitch of the camera plugged into the distance shader.
One of the links above has another, ugly solution I found, but am not happy with at all.
I'm still looking for a better solution. My problem now is to "eliminate the heights in the output of the Distib Shader 4", I mean just to get a mask, yes or no, depending on the contraints.
Regards,
AM.