Manuel, I did not understand the meaning of your question... But, about GCPs position, the GCPs must be well distributed on the analyzed surface. On a rectangular surface, they should not be aligned or too much at the edges. In the case of a corridor (survcey of a road for example), you must try not to align them. 3 points is the minimum. I get better results with 5 points. In the case of a corridor, 3 points per km is a minimum. More than 5 points per km doesn't change anything.
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