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From: Ned Batchelder <ned@nedbatchelder.com>
Subject: Re: Significant digits in a float?
Date: Tue, 29 Apr 2014 12:47:22 -0400
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On 4/29/14 12:30 PM, Chris Angelico wrote:
> On Tue, Apr 29, 2014 at 11:38 PM, Roy Smith <roy@panix.com> wrote:
>> I'm trying to intuit, from the values I've been given, which coordinates
>> are likely to be accurate to within a few miles.  I'm willing to accept
>> a few false negatives.  If the number is float("38"), I'm willing to
>> accept that it might actually be float("38.0000"), and I might be
>> throwing out a good data point that I don't need to.
>
> You have one chance in ten, repeatably, of losing a digit. That is,
> roughly 10% of your four-decimal figures will appear to be
> three-decimal, and 1% of them will appear to be two-decimal, and so
> on. Is that "a few" false negatives? It feels like a lot IMO. But
> then, there's no alternative - the information's already gone.
>

Reminds me of the story that the first survey of Mt. Everest resulted in 
a height of exactly 29,000 feet, but to avoid the appearance of an 
estimate, they reported it as 29,002: http://www.jstor.org/stable/2684102

-- 
Ned Batchelder, http://nedbatchelder.com