-83

The accepted height of Denali, the tallest mountain in the US, had been lowered by 83 feet.  It had been 20,230 but now the mountain is believed to be 20,237 feet above sea level.  The adjustment came from using photogrammetry, a more accurate method of measuring the height.

to read more about Denali:  http://en.wikipedia.org/wiki/Denali

to read more about photogrammetry:  http://en.wikipedia.org/wiki/Photogrammetry