What happens next?

How to estimate distances (by using your thumb and eyes )
Place yourself at a good vantage point.
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. Estimate the horizontal length of the object.