Coregistration occurs within the THOR Change Detection tool. For change detection to be effective, the images of interest must be closely aligned. The native georeferencing information that comes with the imagery is typically not accurate enough for this purpose. Instead, you must select tie points marking the same features on both images. ENVI warps one image based on these tie points to match the base image.

Using the Ground Control Points dialog to select tie points is the simplest method; however, it can be challenging in areas with few obvious features, and it is time consuming. To assist tie point selection, ENVI automatically scans both images to locate common features. For best results, manually provide three to five seed points to assist ENVI in finding tie points.

Though ENVI can select tie points much faster than a human operator, you should check automatically chosen tie points before proceeding. Automatically generated tie points may fall on clouds or cloud shadows, on rooftops, or on other elevated objects and are not suitable. Slight time differences between image collections may generate sub-optimal tie points.

See the following topics for further details: