On 32-bit operating systems max memory that can be allocated for a single array variable is limited according to rules that are explained in ITT Visual Information Solutions' Tech Tip article #3346 ("Overcoming Windows Memory Allocation Limitations") at URL: http://www.ittvis.com/ser...echtip.asp?ttid=3346 You may find a solution to your problem in that Tech Tip. Other than that, though, I do not believe that IDL has a way to write into a TIFF file a "tile" at a time. WRITE_TIFF has an /APPEND keyword, but that keyword would force each of your tiles to be a distinct image/frame in the destination file. To reconstruct the destination image in the next program reading that file, the file reader would need to receive instructions from the file writer on how to reassemble each of those /APPENDed images into a single destination image. The image file format that, in IDL, has the most straightforward implementation of a file writer that can write one tile at a time is JPEG2000. IDL's IDLffJPEG2000 class allow the file writer to specify any tile dimension size and to run ::SetData calls using as its arguments subset rectangles of a source image and a simple tile index. Consequently, IDLffJPEG2000 images can be constructed without the same demands on memory that WRITE_TIFF might have. James Jones