# RE: What is this principle called: "I' send data in my UOM and

• From: "David Lee" <dlee@calldei.com>
• To: "'Frank Manola'" <fmanola@acm.org>, "'Costello, Roger L.'" <costello@m...>
• Date: Fri, 9 Sep 2011 09:05:33 -0700

```Another way I think of this (not to answer the question for which I agree I
have never heard a precise term).
I thinik "What is the purpose of the measurement".

For example what is  the *intent* of sending "1/3" rather than the decimal
approximation ?
This is the key.  There may in fact be legitimate purposes where decimals do
not fit.
For example a CAD drawing constrain may contain a "rule" that is expressed
as a formula.
Or maybe a MATHL construct where you want to present "1 / 3" as text not
.33333x   Or a textbook on fractions.
Or a recipe book  "1/3 cup of milk".
If the *intent* is to literally send fractions in fraction notation  (note
this is not an issue of Units it is an issue of mathematical notation) then
sending "1/3" may be the right way.  So might  "pi/3+sqrt(-1)"

Intent and context are key.

Generally industry specs have a purpose for every field.  And define the
expected format for that field.
If its length it will not be factional, due to these problems. It will be
decimal with a possibly global manufacturing level definition of precision.
If its part of a constraints rule it may be in some symbolic language.
If its text then it is not intended to be calculated. (say a recipe book).

----------------------------------------
David A. Lee
dlee@calldei.com
http://www.xmlsh.org

-----Original Message-----
From: Frank Manola [mailto:fmanola@acm.org]
Sent: Friday, September 09, 2011 8:53 AM
To: Costello, Roger L.
Cc: xml-dev@lists.xml.org
Subject: Re:  What is this principle called: "I' send data in my
UOM and you convert it, as needed, to your UOM"?

Roger--

If "UOM" in the subject line means "unit of measure", then one possible name
for this principle is "Babel", since it requires (for data exchange) every
member in the community to understand the units used by every other member
(and if a new member with a new unit joins the community, every other member
has to be altered).  But that's not exactly the principle you describe in
the text.  In the text, everyone's agreed on decimal values in centimeters
as the exchange standard.  The difficulty you cite comes in not having
worked out the standard so as to cover all possible values (so there is
uncertainty about what the original system should send for the value of 1/3
meter, and how receiving systems should interpret whatever the sending
system decides to send).  If it's really true that "exchanging the wrong
amount of precision could be catastrophic" you need to work that out.  It's
not properly handled by some general principle.  Recall, though, that
arbitrary-precision arithmetic is needed in a relatively few applications.
Recall also that if some member of the community literally *requires* data
with a certain precision, then the data exchange contract needs to take that
into consideration.  In that case, you may not necessarily be able to
literally apply the principle "a sender should transmit its original data;
it is up to recipients to convert the data, if necessary, to the precision
they require" because there may be no sender capable of providing that
degree of precision (note that I'm not talking about how many digits are
sent, but about the actual precision of the data;  if some recipient system
requires 33.33333 and the sender sends 33.30000, that doesn't cut it).

On Sep 9, 2011, at 10:15 AM, Costello, Roger L. wrote:

> Hi Folks,
>
> Suppose that every member in a community is required to exchange data
using one unit of measure. For example, suppose length is required to be
expressed as a decimal value in centimeters. Consider a system that
internally deals with length measurements in meters. Suppose the system
wishes to exchange the value "1/3 meter." Recall that the system is required
to exchange values in decimal centimeters. However, there is no exact
conversion of "1/3 meter" to centimeters. Should the system send the value
33 cm, 33.3 cm, 33.33333 cm? In the general case, the sending system has no
insight into the precision required by recipients. Exchanging the wrong
amount of precision could be catastrophic. There is a principle that a
sender should transmit its original data (e.g., 1/3 meter); it is up to
recipients to convert the data, if necessary, to the precision they require.
>
> What's the name of the principle? I remember someone on this list
> talking about it long ago. Perhaps Len? Perhaps Walter Perry? (BTW,
> anyone know the whereabouts of Walter?)
>
> /Roger
>

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