Which ENVI version are you using? I had the same message while trying to mosaic ASTER imagery in both ENVI versions 3.5 and 3.6. The problem was solved in version 3.5 by simple rotating the images using "Basic Tools" -> "Rotate/Flip Data". Follow the steps bellow: (i) you must go to "Edit ENVI Header" in the main menu (under "File") and select the image file you want to work with (ii) click in the "Edit Attributes" button in the box that will pop up and select the Map Info option (iii)in the box that will open you will have to copy the map rotation value as it is shown. (iv) get back to "Basic Tools" and select "Rotate/Flip Data". Choose the appropriate file name and enter the value you copied into the "angle" field (case the value has a negative sign, disregard it; case, not, make 360º-value). (v) select the output filename and click ok. Do this same procedure with all other ASTER scenes you want to mosaic and you should be ready to go now (run the mosaicking tool). Case you are running ENVI 3.6, you may not be able to do this same operation. For some reason, ENVI 3.6 keeps showing a message saying there is still a very small map rotation factor even after you run the sequence above (probably some floating point issue). I would advise you to contact RSI's support service in that case (and share the result of you inquire after that - it would be nice to learn the answer). Let me know if you have problems. Wilfrid.