Thanks, I forgot the transparent and draw-priority options. But even with them, the size is not changed for the combined file and I think there is a problem when I combine the contour and the osm maps. I will do other tests this weekend. Frédéric Mark :

I use the shell script below to generate my .img file (obviously, your names and map numbers will vary). Notice that I give the transparent and draw-priority options when making the contour .img.