Standardising units and formats (and ref frames?) in transmission

[Alberto Micol]

On 18 May 2009, at 11:58, Anita M. S. Richards wrote:

> For example, I publish data in Galactic coordinates. If someone wants 
> to search in RA and Dec then a rough conversion is quite quick, as 
> long as they don;t mind getting back a region which may be slightly 
> larger than they asked for. But, we have had repeated complaints from 
> users in the past who want to search Galactic plane surveys in 
> Galactic coordinates, since a simple box (from an image or catalogue) 
> will give them what they want, but in RA and Dec it is horrid.
As Francois puts it, a galactic search box is nothing else than
a equatorial box plus an angle. Not that horrid.
But that goes to the unification of reference frames (which is in parenthesis in the subject exactly because I know how difficult it
would be to agree on that), while the main subject is about units and 
formats.
> Or, I want data with a certain spectral resolution which I specify in 
> wavelength unts, but the data are in frequency units with a non-linear 
> conversion - i.e., the spectal resolution at one end of the bandpass 
> is different from that at the other, if the units are changed.
I'm asking for unification of units and formats: wavelengths always in meters, frequencies always in Hz. I was not pushing to have frequencies 
always in keV.  No complex conversions, no re-gridding, nothing subject
to interpretation, nothing that can corrupt the original piece of information. 

> My suggestion would be that we should support a limited number of the 
> commonest alternatives fully - my impression is that this is a 
> modified 80-20 problem, i.e. we can satisfy maybe 50% of the users 
> with a single flavour of coordinates, and another 30% or even more by 
> supporting just one or two alternatives (Spatial: RA/Dec, Galactic, 
> Healpix; Spectral: wavelength, frequency, energy...).
I'm fully with you here.

> The default should be that the user gets the data back in its native 
> coordinates (regardless of the search coordinates). 
Yes! Though, I'd like to get back also a new field with the "standard"
interoperable representation of that. 
> If they want to perform a coordinate conversion then they should have 
> the tools to do it, but this should be explicitly requested and they 
> should be aware of the native units in case re-gridding introduces 
> errors (e.g. converting a data cube with a frequency axis to 
> wavelength is often a very bad idea).
Again, I was not proposing any re-gridding, just fixing units and 
formats for the most common quantities.

> Even where coordinate conversion is linear, e.g. milliarcsec to 
> decimal degrees of arc, or wavelengths in the X-ray regime to metres, 
> we have to make sure that all the stages involved have adequate precision.
Absolutely, this is critical.

> [...] whatever our standards for units are, we should always label 
> units. That way, if I search for data in a radius of 0.1 degrees and 
> get nothing back, it makes it easier for someone to establish whether 
> that is because the data provider interprets search radii in arcsec. 
> Happens regularly!
Yes. Fixed (units) does not mean implicit.

I hope to have clarified things now.

Alberto