just guessing: record pressure at bottom of building, record at top, whip out a table and find the height. I think this is how they used to determine the elevation of mountains.

" I think this is how they used to determine the elevation of mountains."

And also by years and years of meticulous surveying,

The Great Arc by John Keay tells the story of how the height of the world's tallest peak was measured.

Hint: You start from zero and work your way up :-)