Procedure of Geo-rectification in ERDAS:

Select “Image→Properties”, change “Unit of length” to mm and “Pixel width” and “Pixel height” to 0.2 for the Perkin-Elmer detector (200 μm/pixel) and press “OK”. Pick the “Rectangular” selection (first button in the ImageJ toolbar) and select the ROI (Region Of Interest) to … ................
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