@eteq: yes, right, I think I mean frame in your definitions. But I'm not even talking exclusively about physical/World co-ordinates -- it could be a transformation from pixel indices with one zero point to pixel indices with another (eg. a subarray). Or it could be from pixel indices to ra,delta,lambda. Or wavelength to more-accurate wavelength (applying a flexure correction). I think I like your ```represent_as``` as it is for your examples, though it doesn't seem much different from turning pixel co-ordinates into equivalent World co-ordinates without actually changing which set of points they represent, which I was viewing as a step in a chain of transforms along with others that actually correspond to resampling steps and the like. I may still need to give this some more thought (if Nadia hasn't already).