Geoff;
What a great idea to E-mail ASME to see what they will say about are little discussion
But now back to the wars, There has been nothing that I have seen or read that states that a Datum surface on a part has to be perfect (flat, perpendicular, surface finish, etc.) to some other Datum surface on the same part, and yes I know surface plates, parallels, gauge pins, etc. are not perfect but they are better then the part for getting data from. By there very nature parts imperfect, So that if you take points on a part with your CMM for a part coordinate system (to level, align and origin) and then take points in a circle to get a result, (X, Y coordinates and hole size) and then I where to take the same part and inspect it here on our CMM there is a good chance that we would get different result's (I may take different points or a different amount of points for my Level, Align and Origin and hole) because all parts are imperfect and it is our job to see if the imperfect part falls inside or out side the tolerance zone that is on the drawing

Your thoughts please
John D. Ruys
CMM Programer/Inspector
Teledyne MEC