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Summary	Provide library for performing calculations upon measurements.
Categories	None
License	Berkeley Software Distribution (BSD) License
Owner(s)	brucehamilton, gthomps, jliu

JMCI Java Measurement Calculus Interface

The Java Measurement Calculus Interface project provides a framework for performing operations on measurements, in particular physical quantities.

Last modified: October 4, 2004

Background

Say you made a temperature measurement and measured a result of 40 degrees. Then you made another measurement and measured 80 degrees. Is the second measurement twice as hot as the first?

The answer is... You don't have enough information to say! The answer is yes if the units are in degrees Kelvin, but not if the units are in degrees Celsius or degrees Fahrenheit!

In a system dealing with measurements about the physical world, the metadata that describes the measurement value is critical to being able to make sense of that measurement value. Seemingly simple arithmetic operations may not be so simple once the metadata for the measurement, such as units or scale types, are taken into consideration.

In most of today's measurement systems, the rules for properly operating on measurement data and physical quantities are either ignored or handled in an ad hoc manner, greatly reducing the reliability of the overall system. We believe that the these rules, which we call the Measurement Calculus, should be first-class infrastructures in a measurement system rather than afterthoughts to the main program logic.

This project provides a set of APIs for representing measurements about the physical world and for performing operations on these measurements. The reference implementation provides basic arithmetic and logic operations on measurement instances taking into account any available metadata. The API is expandable through the addition of modules containing rules for other operations.

Announcements

July 16, 2004

The white paper Measurement Calculus: It's not just arithmetic on the numbers is now available in the document section. This is the electronic version of the paper that was available at the JDDAC JavaOne booth.

Project Summary

The following lists of tasks are open items as of October 4, 2004 and will be updated periodically.

Task	Description
Tidy up the Measurement class	Have its javadoc tell all about it, why it's here, how it's different from an ArgArray, etc. Give it a formatter (probably an abstract superclass, so we can have ASCII, HTML, XML, etc. versions) -- this will output value and units, and include uncertainty as number of digits or as +/- a number+unit. Make MeasMath part of the same package. Decide whether it's a separate class or a collection of methods on Measurement. I.e. do we have MeasMath.add(meas1, meas2) or meas1.add(meas2)? Rationalize the names of the operations: add/plus, times/mult/prod, div/quot, etc.
Integrate Measurement with Dimension and Unit so that one can force a Measurement to accept values only in a unit (or set of units), or only in a dimension (i.e. any unit of length).	Migrate from the String notion of units. Define the semantics of assignment to a Measurement: can I set the value of a measurement which has no unit, can I change the unit without changing the value & uncertainty, how do I state that a measurement has a preferred unit or required dimension, etc.
Write a Unit Formatter	The Unit class has only a stub of a Formatter. Write this class, and subclasses for ASCII, HTML, etc.
Extend the Measurement Calculus into Timestamp, Location and Name.	What are the timestamp, location, and name of the average of a collection of Measurements? This surely depends on aspects of the situation: the timestamp, for example, should sometimes be the average, sometimes the most recent, sometimes the smallest interval that contains all the points. The name can be computed only if certain conditions are met, e.g. "xyz length" divided by "xyz time" is likely to be "xyz speed". And similarly for locations.
Figure out the semantics of dimensionless quantities.	What is strain/time (where strain is length/length)? Is it frequency (i.e. strain is treated as "1")? Is it speed/length (i.e. factor it into (length/time)/length)? Should we leave it as strain/time? What's the right way to represent a dimension which is not in lowest terms? How can we keep strain separate from other dimensionless quantities (e.g. relative humidity), but still allow them to be cast to a common parent when needed (so they can use units like percent)?
For the adventurous: figure out the semantics of specialized dimensions.	For example, energy can be specialized into mechanical, chemical and electrical. Electrical energy / time is electrical power, which can be specialized into resistive and reactive. Having separated these, we can force different units (e.g. BTU does not get applied to electrical energy), and different arithmetic operations (e.g. reactive power requires a phase angle). This is well begun, but needs finishing. In particular, products and quotients have the same problems as dimensionless ratios, above.
Treatment of uncertainty	The treatment of uncertainty is uneven. For addition/subtraction we assume normal error distribution and do operations in quadrature; for multiplication/division/exponentiation we use confidence intervals. I'd like to have better science behind this, yet still keep the simple cases simple.
Units with fractional powers.	We don't allow units/dimensions with fractional powers, such as volts per root Hertz. We want to allow this without forcing everyone to use floating-point exponents (e.g. volume in feet^3.0). We have an idea for this, but would like to discuss it, and need help implementing it.

Participation

At this time we are currently seeking advisory members with expertise in such backgrounds as measurements and statistics. We are currently in the process of developing the framework for this API and soliciting feedback on the initial implementation. A discussion forum is available to provide a venue for communication.

We are currently seeking developers with the skills and interests listed above to work with Project Owners to enhance the current implementation.

For more information please contact Jerry Liu.

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