# Measurement Woes

In another thread (Scaling issues between Sketchup and .stl format), the conversion of inches to millimeters came up. I wanted to followup on that here instead of cluttering up the original thread.

The difference between 25.4000000 and 25.4000508 is only 0.0002%. Hardly significant by almost any standards. However, on a trip to Mars at its closest point, for example, this becomes about a 70 mile error. You may remember the Mars Surveyor '98 Orbiter that crashed in 1999. It wasn’t this type of conversion error that caused it, but it was rooted in the units used by the software teams in calculating the orbital values.

From Wikipedia: “[O]n September 23, 1999, communication with the spacecraft was permanently lost as it went into orbital insertion. The spacecraft encountered Mars on a trajectory that brought it too close to the planet, and it was either destroyed in the atmosphere or escaped the planet’s vicinity and entered an orbit around the Sun. An investigation attributed the failure to a measurement mismatch between two software systems: metric units by NASA and imperial or “English” units by spacecraft builder Lockheed Martin.”

Some day, possibly in the far and distant future, we will all be on the same page and the inch to millimeter conversion will be a historic note in Wikipedia.

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According to Wikipedia, the British and American inches were standardized to 25.4 mm in 1930 and 1933. 25.4000508 was the Amercan inch and the British was 25.399977.

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The Swedish mm has since its beginning always been exactly 1 mm .

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My mother always complained about my feet during the seventies, she had to buy me new shoes every two months or so.

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You should have converted your feet to millimeters.

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Would it be useful to create units multiplied up by planck lengths? Lol.

I do all my dimensions using the chinese string theory unit, the hau leong.

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That is a derived unit. The SI system is based directly on natural constants.

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