Relatively new around here, so apologies if this has been addressed and I couldn't find it...

Like everyone, I'm familiar with the chime of reaching 10,000 feet. I always assumed that this was a safe altitude to pull out laptops, start serving food, etc. because there was adequate distance (e.g 10,000 feet) between the plane and the ground. However, on a flight last week out of Denver I noticed that the chime occurred at 10,000 above sea level, rather than 10,000 above the ground. Given that Denver is at 5,280 feet above sea level, this was quite a big difference. Since then I've noticed that this chime is always based on altitude above sea level rather than above ground. It got me thinking, why is 10,000 feet the magic number if it has basically nothing to do with local geography?

Mods, please feel free to move this to an appropriate location if I chose poorly...