[header] How to estimate distances (by using your thumb and eyes ) [title] Place yourself at a good vantage point. [step] To estimate distance, you need a clear line of sight. Try to stand in a high place, like atop a hill or a few floors up in a building. 
If this isn't possible, try to find a spot where no other objects are in between you and the object you are focusing on. [title] Estimate the horizontal length of the object.