Well! that was a strange one.
Thanks to you all it is now solved, in the sense that I've successfully calibrated the canvas.
I followed the suggestion to try it on a new file, and of course it worked. I then tried again in my original file - and it failed again. But I don't really know why. If you're interested, here's the story.
Well actually, I still don't know why, but here's what I found out. The original file is a model of a kick-drum pedal that I'm using to create a mechanism to enable a dwarf to play the drums. If he sits at the right height to play the side drums etc he can't reach the kick-drum pedal, which has to attach to the bottom of the kick-drum. The actual pedal is inclined at an angle which I modelled by sketching its side profile on the X-Z plane. I then extruded the profile to get a basic slab onto which I sketched the actual profile which came from a traced canvas. I completed the model and made the actual mechanism, and everything was fine, until I discovered he also wanted a to use the high-hat pedal too.
So, I obtained a tracing of the new pedal profile, and decided to modify the model with the new tracing, which I placed on the X-Y plane. When I tried calibrating it, it failed as already posted. I'd forgotten that I'd placed the original canvas on the face of the extruded original pedal, so there was no construction plane for it. When I remembered what I'd done, I placed the canvas on the same plane as the original canvas, and it worked.
But I still don't know why it fails on the X-Y plane. Any ideas?
