If your image is in a well-known file format, like TIFF, for example, then a programmer does not need to know any image size information. That information is stored in the "header" of the file, and IDL routines like and QUERY_TIFF and READ_TIFF will acquire this information automatically. If the file format were something like GeoTIFF, then the formula for the geographical mapping of each pixel in the image is also stored in the header of the file and keywords to routines like QUERY_TIFF and READ_TIFF can also be set up to automatically retrieve this. James Jones