A plehtora of Quantities

[Ed Shaya]

Guy Rixon wrote:

>On Thu, 13 May 2004, Ed Shaya wrote:
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>>>I understand the concept and the starting point.  I just don't see a case for
>>>defining a new 'type' that all things must derive from. I can see a case for
>>>a 'Measure' but even that is a bit high.  For example, are there any
>>>programming languages that define such things?  No - they give you the
>>>components to assemble your own structures, because building root types like
>>>this hinder later.
>>>
>>>
>>>      
>>>
>>I don't think of quantity as a root type.  Ontologically there are
>>objects (Things) and quantities (properties).  Observation is a Thing
>>not a property, an SED is a property.
>>    
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>Yes, but by the time we get to operate on it, the property "SED" has been
>reified into "Measurement of SED recorded in a data structure" and
>we're modelling the latter.  I don't think we touch properties in the
>ontological sense.
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Yes, measurement and quantities are predicate objects.  Formally, the 
property would be hasSpectra or hasApparentBrightness (which could be a 
function of wavelength).  And SED would be the object of the property.  
hasMeasurement would be the primary branch of these hasProperty.  In the 
schema, though, one tends to skip over this and simply make the 
Measurement a child of the object (star, galaxy, etc) and then one 
loosely calls the Measurement a property. 

In OWL there are two types of properties, ones that take a simple value 
and those that take an object.  By introducing Quantity we have 
essentially eliminated the first type.

I do think that literal names like "Venus", "Sirius", "Castor", "M31" 
are literals, not quantities.  They are the object of hasName property  
but hasName should not decend from hasQuantity.  There are probably 
other TrivalQuantities that I think are not Quantities.  Here I agree 
with Martin that this may lead to a datatyping problem.  I would not 
like to see mass/Sirius crop up.

Ed


>Guy Rixon 				        gtr at ast.cam.ac.uk
>Institute of Astronomy   	                Tel: +44-1223-337542
>Madingley Road, Cambridge, UK, CB3 0HA		Fax: +44-1223-337523
>
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