If the images are georeferenced, do they look ok if you do not set an x/y offset?  I have never had to set the offset for properly georeferenced images so I am not sure what effect it may have if the values are not set properly.  If you don't set them at all, do all 3 images show up?

Thanks very much for your response. I thought the images we got were orthorectified, so I expected the mosaic to detect automatically the geo-position of them. However the three images showed up in the same position only a bit shifted to the right. So I could see the three images but they were overlaying each other when not setting any xy offset.

When you open the images in ENVI, do they have georeferencing?  Do you see the Map Info Icon in the Available Bands List?  If so, what projection does it show when you expand it?  If it shows no georeferencing, and you know your images should be in a particular projection, you should add this information to the ENVI header via File ->Edit ENVI Header->Edit Attributes->Map Info.  Then confirm the images overlap as expected by performing a geographic link on them (from the Display menu ->Tools->Linking->Geographic Link).  If this works as expected, try importing these images to a Georeferenced Mosaic (Map->Mosaicking->Georeferenced).

Many thanks mminari. I attempted to reproject my images and once I set up the first projection manually, when I tried to 'Imput Heather Info from' for the next image and then visualize it the whole image was stripped. This occured twice. I did try with other set (3) of images (different year) but this time setting again manually the Map Coordinate of Tie Point and Pixel Size, and then it worked (at least I could visualize the entire image, this was not stripped). Do you know why the first stripped image occurred? Does it make a big difference how you reproject your images (exporting info from anther file or setting the projection characterstics yourself)? I could not try the geographic link with my study area because it is a vector. Surprisingly, I also tried to do the geographical link among the three images (by error) and they did link... when they are not supposed to overlay... After all, I tried to do the mosaic of the three georeferenced images and they were not properly located in space but they overlayed... I guess I am still doing something wrong with setting up the spatial reference of the images. I appreciate very much your support. Maria  

When you import Header information from another file, it will take on all of the characteristics of the other file.  Stripping or images that come out looking like noise usually have a problem of: wrong data type, wrong file interleave or wrong byte order in the header.  Or, the images may have embedded information in the file and this is not accounted for when defining the header.  At this point it is difficult to know why the images are not overlapping.  As I stated before, if they are in a known SPOT format such as DIMAP, the georeferencing should already come with the file and not need to be specified.  Do you know if your files are in DIMAP format?  If you are unsure, you might go ahead and contact technical support for further help.

Thanks again for your support.  I haven't found yet neither how to upload the Landsat imagery by means of :File- Open external file - Landsat  It seems that the options given do not support the .bil format. Therefore I am uploading my images in ENVI by File - Open Image File. I am mentioning this in case it might an influence on the stripping effect. The Landsat imagery ( 3 images: northwest, southwest and northeast) are in .bil format and although ortho-rectified they have not spatial reference. Once I give the new Map Info ( Map Coordinate of Tie Point and Pixel Size) and then try to make the mosaic, the image of the southwest appears further right than it should be, even when its left corner is located at the same position than the left corner of the northwest image. The northwest and northeast images seem to be correctly located in the mosaic. They do also appear stripped. Best, Maria

I did contact ENVI support and they helped me out to solved this two problems. Please find their answer between quotation marks: - Striped image: "I am not familiar with .bil as a format for Landsat data.  I know that BIL is an interleave type (band interleaved by line) that many different image formats support.  But it sounds like your data are in a file format called .bil.  When you are setting the Map Info in the header for these images, I wonder if you are accidentally also changing the interleave?  If the header of an image specifies the wrong interleave, then often the image appears striped when it is displayed.  In the Header Info window (which you go to first, before choosing Edit Attributes > Map Info), check the Interleave, to see if it is listed as BIP.  If it is not, I recommend that you try changing it to BIL (which I assume is the correct interleave for files in a format named .bil)." It worked out prefectly!   - Mosaic: This is a very embarrasing one... after I send them the header files of my images: "In the map info of the SW image, the x coordinate for the upper left corner is listed as 700000.000, while for the NW image, that coordinate is listed as 70000.0000.  So you have one extra 0 in the x coordinate for the SW image" Removing that zero also worked out perfectly... All the best, Maria