[SciPy-user] Units in SciPy

Duncan Child duncan at enthought.com
Fri Apr 1 16:22:50 CST 2005


Hi all,

After reading all the emails I think the problem splits down like this:

1) Algebra of Units

It feels like you should be able to divide a distance by a time and get 
a velocity. If you then try and add a velocity to a temperature you 
should get an error.

This may be what Gary had in mind when he said "I've tried units 
packages in other systems. (notably Mathcad)  After 30 minutes I abandon 
them.  Much typing, little payback."

2) Quantities (Robert's "numerical quantities tagged with their unit 
system")

This is what I need to provide to scientists who are developing 
libraries of geophysical algorithms. They know what units they expect eg 
their algoritm needs a velocity in m/s. As long as I can tag the data 
that flows through the system with a description of what it's units are 
then I can add a 'shim' that will do the necessary unit conversions.

The problem is that some of the units packages are targetted towards 
'units algebra' and then punt on issues like temperature gradients which 
makes it hard for me to tag all the quantities. In fact, I would almost 
be better off passing around a string because that would at least allow 
me to hack around the special cases in my 'shim' code.

3) Map Projections

Describing lat lon data is harder because if you don't also specify the 
datum (effectively where you are assuming the center of the Earth is) 
your position can be miscontrued by 100s of meters. To convert from lat 
lon to xy you have to specify a bunch of parameters so this is trickier 
than converting something like meters to km. So I am not sure where this 
type of data fits in relation to the more usual conversions.

Summary:

1) no one has suggested a better approach than than the one we are 
already using for Quantities
2) Enthought has not overlooked a clearly superior implementation of a 
units package that we could include in SciPy
3) Frink is indeed very cool.
4) POSC has a fairly complete database of units http://www.posc.org/UConvert


Regards,

Duncan














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