"Both mirrors {for telescopes on the 'big island' of Hawaii} are light-years beyond everyday concepts of precision. Ground and polished for four years...the Subaru mirror surface has been smoothed to within about half a millionth of an inch. The surface of Gemini...is precise to within a thousandth of the diameter of a human hair. To make such perfection easier to imagine, if the Big Island could be leveled and polished as evenly (in proportion) as the Subaru mirror, the biggest bump would be no higher than the thickness of two sheets of paper. If the entire earth could be smoothed to the same accuracy as Gemini, the tallest hill would be less than a foot high."
Here's an example of precision for you:
Consider telescope mirrors:
"Both mirrors {for telescopes on the 'big island' of Hawaii} are light-years beyond everyday concepts of precision. Ground and polished for four years...the Subaru mirror surface has been smoothed to within about half a millionth of an inch. The surface of Gemini...is precise to within a thousandth of the diameter of a human hair. To make such perfection easier to imagine, if the Big Island could be leveled and polished as evenly (in proportion) as the Subaru mirror, the biggest bump would be no higher than the thickness of two sheets of paper. If the entire earth could be smoothed to the same accuracy as Gemini, the tallest hill would be less than a foot high."
--Howard Daniel, Pen4Rent