[BuleBule]

The jump from "we can compute this" to "we computed it for the entire planet" is what makes this impressive. A lot of projects stop at the proof of concept. Running hundreds of AMD Turin cores for two days straight to actually finish the job is a different kind of commitment. Curious about the edge cases, how do you handle atmospheric refraction at those distances? At 530km the curvature of the earth and atmospheric bending would meaningfully affect whether a line of sight is actually visible to a human observer versus just geometrically unobstructed.

[ryanbberger]

It was definitely quite a bit of effort to get every line of sight. We felt it was necessary if we were going to try and prove that there are none longer.

Regular viewshed algorithms really aren't built for it, and being able to do it cheaply while also maintaining a cluster of compute that can shut down at any time is also its own mess (these were all spot instance so we could get 75+% off).

The 530km number takes into account both the curvature, along with refraction. It's a simple elevation adjustment when you are doing your angle of elevation calculations.

If you want a synopsis of how we calculate lines of sight and all the parameters we use I have a shorter, less whitepaper-y blog post here:

https://ryan.berge.rs/posts/lines-of-sight/