I have been georeferencing a number of historical CIR images to recent high resolution NRCS orthophotos for a project. We have been using a second order polynomial transformation with 20 control points. It works for most of the images I've been using just fine but I've had a coupled that have ended up curved and distorted once I jump from the first order to the second order transformation. The first time it happened I was able to fix the problem by using a 2nd copy of the image and starting over. I now have this one image that I have georeferenced 5 times and it ends up curved, it is not highly distorted but it is noticeable. I don't understand why the second order polynomial works with all my other images but not this one.