I really like the Photomerge feature of Photoshop. 
I used it here before and decided to create a bigger image this time.
The final picture is not perfect (the hills still have some mistakes) but I was too lazy to correct all stitching mistakes Photoshop made. Here’s an imperfect 25 megapixel image (9200×2800) of a blue sky with clouds over the 19th district of Vienna (left) and Kahlenberg and Leopoldsberg (right).
Click on the image to get the original. But beware if you have a slow connection, the image is about 5.4 MB in size.
