Let me preface this by saying that this shit clearly shouldn't have happened, and I'm pretty interested to see whatever version of the report that's made public. But after reading some preliminary information and taking a look at the area on maps & streetview, I get how this probably happened.
That's an overhead of the location. The shooter fired at Trump from the nearest roof of the AGR International factory, Trump was located about 120-130m away adjacant to the 3 barn type structures. The sniper team we've seen in video is located on the roof to the furthest north, closest to the AGR roof.
There's a scattered bank of mature trees directly in line of site between the sniper team and roughly half of the roof Crooks fired from. Streetview has good access to the AGR parking lot to the right of the building and the trees are big 30-40+ feet.
The AGR parking lot is big (rough guess, but a capacity of 100+ cars) with no gate access at the front.
Roof access is pretty hilarious
2 big ass cedar trees to the right side there would give good cover to get up on the roof on the side obscured by the large trees. He could have been up there for a while unspotted before moving over towards the other side of the roof for a clear shot.
Even if there was a local cop stationed in the parking lot (and there probably should have been someone covering the easiest access/egress points to and from that roof...fml), big potentially busy parking lot isn't crazy hard to sneak by one cop there.
I've heard it mentioned that the secured perimeter was 100 yards, the location in that photo is 180ish away.
Also a quick note on Secret Service protection. Ex Presidents and Presidential Candidates do no receive full secret service details. Ex Presidents are assigned 4 USSS agents at a time (3 shifts x 8hrs, so 12 agents in the detail in total). Couldn't find what the standard compliment is for presidential candidates, but it's not like what we see the President having in movies, they don't have infinite resources and as for local/state cops...well fuck, we've been hearing stories about how poorly trained they are across america for yeeeears. Occams razor gets all the chicks, but Hanlons Razor is more useful in a lot of cases. Never attribute to malice that which can be adequately explained by incompetence. Again, this is just a guy with some experience and training in incident investigation who used to investigate industrial incidents for a living taking a quick run at how this could have potentially happened and in no way is comprehensive. But a lot of the shit that should be in place for incompetence to adequately answer this appears to be there.