Approaching the question of proximity to space in both directions - hiking up to the highest point I could find (http://www.norcalhiker.com/sierra-buttes-fire-lookout/) then discussing Jonathan McDowell's paper on why the edge of space might be better set at 80km rather than the current 100km. https://www.sciencedirect.com/science/article/pii/S0094576518308221?via%3Dihub