Entering the coordinates for the four corners in the header of the image doesn't truly georeference an image in ENVI.  If there is no other map projection information in the header, ENVI will use the corner coordinates to estimate the locations of all of the pixels on the earth, but it's a pretty approximate calculation.  So, ENVI calls this a *pseudo* projection.  It sounds like your image is in a true projection already, though.  So you should not need to warp it to get it georeferenced.  You just need to figure out the parameters of the projection that it is already in, and enter those into ENVI, then provide the map coordinates for one pixel into the header.  Here are the steps: 1.  Set up the projection for your image in ENVI using Map > Customize Map Projections.  It looks like you will want to set the projection type to Sinusoidal.  Then you will also need to enter any false easting or northing that the projection uses, and the longitude of the central meridian for the projection.  From the URL you sent, it looks to me like the central meridian will be 0.  I didn't see any mention of false easting and northing, so those must be 0 as well.  You will also want to specify the datum, but I didn't see any datum info at the URL.  It's possible that the data don't use a datum.  Give your projection a name, and then save it. 2.  Figure out the coordinates in this projection for some pixel in your image.  Typically people use the upper left corner of the upper left image, but you can choose anywhere, as long as you know both the pixel coordinates and the map coordinates.  If you have those coordinates in lat/long but not the projection coordinates, then you can use the lat/long to calculate the map coordinates using a tool found in ENVI under Map > Map Coordinate Converter. 3.  Open your image in ENVI, and then select it in the Available Bands List, and right click then choose Edit Header.  In the Header Info dialog, click Edit Attributes > Map Info.  Click the Change Proj button to change your projection to your newly defined Sinusoidal projection.  Then enter the map coordinates for your reference pixel, and at the top, the image (pixel) coordinates for the same location.  Enter the pixel size, then click OK. At this point, your image should be georeferenced. Peg

  Thanks for your help. Your method does indeed georeference my image, but I have yet to get it correct. I found the pixel size, which gets me to the right scale. But my original image is upside down, leading to an incorrectly georefrenced result. I suppose I could just rotate the image (I haven't yet tried that...), but I wasn't sure if it will be a simple 180 or some other, specific rotation. Also, you mention that simply using the four corners leads to a "psuedo" georeference. I can see how four points wouldn't be sufficient- but if that's the case why is "map info" able to use only pixel as a reference?!  

I'm sorry, but I can't tell you what the proper rotation factor would be.  You can enter a rotation in the map info in the ENVI header.  The metadata for the image should tell you what the map rotation factor is. Map info can use just one reference pixel to understand the location of every pixel, because you've also supplied the map projection and pixel size information.  If the complete projection is known to ENVI, along with the pixel size, then ENVI only needs one point to place the image in the world.  Then using the pixel size and the transform equations for the projection, the coordinates for every other pixel in the image can easily be calculated. Peg

Assuming your SPOT image is in DIMAP format, you should open it using the 'metadata.dim' file.  Then simply use the Georeference SPOT ->Build Geometry File (to use a GLT type registration) or Georeference (to use internally stored points).  You could also use Orthorectification ->SPOT.  The metadata will allow you to perform any of these options.