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Re: [xml-dev] ANN: a portable data component -- length

There are other potential problems too.  Suppose, for example, that the measured distance is 1/3 mile.  What do you put down so that the equations come out exactly right?  Also, I would certainly want some indication as to which value was an actual measurement (there might be more than one) vs. which values were computed from the measurement(s), so I could distinguish between potential measurement errors and potential computational issues (roundoffs, etc.).  
On Apr 9, 2011, at 1:15 PM, Amelia A Lewis wrote:

> On Sat, 09 Apr 2011 12:00:20 -0400, Liam R E Quin wrote:
>> On Sat, 2011-04-09 at 11:10 -0400, Costello, Roger L. wrote:
>>> Here is the altitude of an aircraft, expressed in both feet and meters:
>>>     <altitude>
>>>         <feet>12000</feet>
>>>         <meters>3657.6</meters>
>>>     </altitude>
>>> It is important that the two length values are consistent:
>> The right way to mark this up in most cases is to store only one value
>> and convert as needed.
>> See Normal Form in database theory: information should never be
>> duplicated. That way it can't get out of sync.
> Agreed. My initial thought, on seeing the base post, was that this was 
> "How to generate problems in data synchronization", not "portable data 
> structures".
> By including two values in different units, rather than a single value 
> and relying upon the *well-known* formulae for conversion, all that is 
> achieved is to create doubt as to the accuracy of any reported value.
> As for the claim that the assertions will ensure that the data never 
> gets out of sync, poppycock. The initial statement is that the 
> information is 'portable', and yet this allegedly portable information 
> relies upon some external code that enforces assertion validity? And 
> what good do assertions do when these values *are* out of sync? Once 
> they've gotten out of sync (that is, at any stage of the process at 
> which someone fails to verify the assertions, or even any stage at 
> which someone uses a different degree of confidence, or uses a 
> different chain of conversions to generate values), *you don't know 
> which one is correct*.
> The following is valid:
>     <altitude>
>         <feet>12000</feet>
>         <meters>4000</meters>
>     </altitude>
> ... within a certain degree of confidence. 1 m equals one yard 
> (rounding up; in some applications this is a perfectly valid 
> conversion). Now, it's highly unlikely that such an application 
> envisions measurements in thousands (at which point the 
> less-than-ten-percent conversion error has made for very large 
> differences), but ... in the above example, *which value is correct*?
> Or, to quote the storied wisdom of the ancients: a man with one clock 
> always knows what time it is. A man with two is never sure.
> Amy!
> -- 
> Amelia A. Lewis                    amyzing {at} talsever.com
> And now someone's on the telephone, desperate in his pain; 
> someone's on the bathroom floor, doing her cocaine; 
> someone's got his finger on the button in some room--
> no one can convince me we aren't gluttons for our doom.
>                -- Indigo Girls
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