I have been using WRITE_TIFF to write weather satellite images in GeoTIFF that are in their native scan space, but have lat/lon tie points every eighth pixel in the X and Y directions (can be 100,000 tie points or more). However, when I try to write that more than 65536 elements (65536/6 tiepoints) into the "MODELTIEPOINTTAG", not all of them are stored into the GeoTIFF. It appears there is an integer somewhere that is rolling over. Is this an IDL problem or a limitation of the GeoTIFF format? Mark Conner