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Re: On Quantity - Measurement relationship
- From: Jean-Marie Dautelle <
>
- To:
- Subject: Re: On Quantity - Measurement relationship
- Date: Sat, 1 Nov 2014 09:19:15 +0100
Hi Martin and All,
A scalar is a physical quantity that it represented by a dimensional number
at a particular point in space and time. Examples are hydrostatic pressure
and temperature.
A vector is a bookkeeping tool to keep track of two pieces of information
(typically magnitude and direction) for a physical quantity.
From this reference book definition we see that a vector is not a quantity
(only its magnitude is). Therefore to respond to Martin (and what should be
written in the spec) is that a Vector/Tensor does not belongs to this
hierarchy and they are aggregate of quantities or aggregate of one quantity
with something else (direction)
A possible choice to enforce this notion of "magnitude" for measured
quantity would be to replace the name Measurement with Amount !
I hope this help.
Jean-Marie.
On Sat, Nov 1, 2014 at 5:39 AM, Martin Desruisseaux <
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wrote:
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Thanks Werner for the link to the Microsoft articles. I had a look.
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I'm not aware of universally accepted definition of "quantity". Some
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peoples make a distinction between "quantity" and "*physical* quantity".
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For example in an article cited by Werner [1], the author first mention
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"physical quantity", then use only the word "quantity" in the remaining of
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his article. He said "*Typically these quantities possess unit of measure*",
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but I think that he implicitly means "physical quantity".
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One could imagine the following type hierarchy:
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Quantity
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↑
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Measurement
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↑
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PhysicalQuantity or Scalar or ScalarMeasure
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But where would we put Vector, Tensor or any other kind of measurement
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that are not a scalar in this hierarchy? Putting Vector as a subtype of
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Measurement seems contradictory with the Quantity base type.
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There is my proposal:
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- I think that Measurement should not extends Quantity, because (to
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me) it would imply that all measurements are scalars, which would exclude
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other kind of measurements like vectors.
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- I think we should explain that, for the sake of simplicity, JSR-363
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merges the "pure quantity" concept with the "physical quantity" concept,
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and that we call that just "quantity". We may define our quantity as a
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"quantitative measurement".
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A Quantity in the "pure" sense is hard to fit in our type hierarchy (e.g.
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above-cited contradiction with non-scalar measurement) and make all Length,
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Mass, *etc.* subtypes useless for other purpose than marking interface.
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Furthermore the above hierarchy would allow users to parameterize Unit
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instances with arbitrary Measurement subtypes that are not something like
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Length or Mass. On the other hand, keeping the current hierarchy, with
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the idea that we merged "quantity" concept with "physical quantity" or
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"quantitative measurement", keep Unit parameterization safer (can not use
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arbitrary Measurement), and keep Measurement applicable to non-scalar
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measurement.
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In summary, I suggest to keep current hierarchy as-is but explain better
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what we means by "quantity".
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Martin
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[1]
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http://research.microsoft.com/en-us/um/people/akenn/units/RelationalParametricityAndUnitsOfMeasure.pdf
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--
It is not the strongest of the species that survives, nor the most
intelligent. It is the one that is most adaptable to change. - Darwin's
Origin of Species (digest)