Some things I was thinking about in my mind were.......
* You are in pretty flat country......
*There appears to be a valley in there?? which can throw off ones view of how far it looks and how far it really is........
*I have noticed what one sees with the naked eye and what shows up in a camera/video lens are often times significantly different distance wise(when viewing longer ranges, at least for me ).................
Therefore really my uneducated guess would be around the 1200yd. range