This is the line of sight the snipers had on the adjacent building.
I haven't been able to figure out the exact position of the shooter on the roof top yet, but from the videos is looks like the snipers can see him and are trained directly on his position.
As far as I can tell from Google Earth, they had the high ground from the roof of the barn and would have been able to clearly see both the shooter and the gun from this position.
Why did they appear to already be looking at this exact spot even as Trump was taking the stage?
Why did they switch from looking through binoculars to looking down their sights before the first shot but after they would have been able to see the gun?
And if they saw the gun, why didn't they fire first?