Thanks for the info. I will work it some more and see what I can do with the affine. I've been using the 2nd order polynomial because that's what the person I am doing the work for requested, per methods used by some other groups. I am digitizing river channels on each image and using geoprocessing methods to come up with river migration rates over time. We are attempting to keep the RMS low to reduce the error in the migration rates. The second order polynomial was effective and worked well for all the other historic images I've used. I think this one has presented problems because it is in a slightly different area where the satellite imagery has been taken from different angles and I'm working in Albers EA instead of NAD83. I can keep the RMS low with the affine/1st order but only with control points near the center of the image, points closer to the edge are not referencing well.
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