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If the x, y, and/or z values for the GCPs are less accurate than the x, y, and/or z coordinates that the orthorectification without GCPs calculates, then the GCPs will indeed make the orthorectification worse. ENVI assumes that the GCPs are *more* accurate, and so it takes the ortho results and adjusts them to match the GCP coordinates. So you always want to make sure that the GCP coordinates you provide are very accurate, or it's better to not use them.
Another thing that can happen is that sometimes people get confused about which coordinate system is being used when specifying the x and y coordinates of their GCPs. For example, sometimes people will not notice that the dialog specifies lat/long for the coordinate system, and they'll enter points using coordinates for a map projection. Also, if you select a point in the list, and then use the Goto button to jump to that point, the map projection in the GCP Selection dialog can automatically change to lat/long without you noticing. If you then add additional points, you need to specify their coordinates in lat/long, even if you were previously entering them in, say, UTM. To make sure something like this isn't happening, you'll want to keep a close eye on your map coordinates in the list of GCPs in your GCP selection dialog. Make sure they are all in the same coordinate system, and that it is the coordinate system specified in the GCP Selection dialog .
Many of these potential pitfalls have been cleaned up, so the whole process is less potentially confusing, in the upcoming ENVI 5 release. Right now, we expect ENVI 5 to be available for download from the Exelis VIS web site on May 1.
- Peg
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