[QUANTITY] Quantity "arguments"

[Ed Shaya]

Dave,

>So what do you do if the client is interested in a
>position which is not in your set of discrete axis values? For instance, a
>client asks for the image value at a particular RA and Dec which turns
>out not to correspond to a pixel centre. How do you find the correct pixel
>coordinates at which to interpolate the image data? Do you just do linear
>interpolation in the tabulated values?
>  
>
The Argument/Quantity/Accuracy holds a BinWidth for binned quantities 
and/or Error, positiveError, or negativeError for measurements with 
errorbars along the coordinate.  These may be  arrays or single values.  
BinWidth and Errors can be combined to describe errors in the 
calibration of the coordinate bins.  If the coordinate requested is near 
a coordinate and within the Accuracy tolerance of it, then the value at 
that coordinate is returned.  If there are overlapping error bars, then 
several points should be returned.  If the requested coordinate value 
lies in the gaps between the errorbars or between truly discrete 
measurement values then noData should be returned by the core API. But,  
we can also have a mode that allows for the two (or more) adjoining  
points  to be returned along with the noData  flag.   This mode may be 
useful also when the request is out of range and you want to know  the 
value on the first data point that is within range.

If the application wants to do interpolation despite there not being any 
appropriate data (say we have nuclear cross-sections in the lab at 
T=1E9,3E9,1E10 K and one wants to interpolate to 6.3E9 K), then the 
application may interpolate the "off" points. Or, the service that 
provides the data may include interpolation service when appropriate.  
In this example, however, such interpolation is dangerous because major 
cross-section "resonances" can fit between the discrete measurements. So 
this option should be used with caution.

Ed