[obs-tap]access_format

[Mireille Louys]

Dear all,

Here is the short text I suggest to have for access_format.

4.7.	Access Format (access_format)
The access_format column emphasizes information about  the format of  
the data product if downloaded as a file. The values should describe  
(in increasing detail) the overall file format as well as the  
structure of the data within the file.
This data model fields is important to evaluate for data discovery and  
data retrieval.
MIME types can be used for that in existing protocols ( like http).  
However, when dealing with observations as in ObsTAP service, more  
information about the astronomical arrangement of data into predefined  
formats is very useful . For instance we want to distinguish between  
various formats like aedm (ALMA) , evla, MUSE multi-extension fits  
files( IFU) etc?
Providing this information speeds up the interpretation step for  
client application consuming these files on one hand , and improves  
data selection in the discovery step on the other hand.
Some data collection offer data sets are as multi-files and distribute  
them as  as directory or  tar format
Here we consider a list of possible MIME-type strings for various  
types of observational datasets
This could be extended or modified after implementation experience.

MIME-type	    Shortname	     Definition
application/fits	  fits
application/xml	          xml
application/x-votable+xml votable
image/fits  	          fits.image any multidimensional
                                     regularly sampled data cube, etc?
image/jpeg  	          jpeg
application/x-aedm 	  aedm	    ALMA Export Data Model
application/x-ms  	  ms	    Radio measurement set
application/x-euro3D 	  euro3D    imaging spectroscopy
application/pdf 	  pdf	     graphs stored as document: filter curve,
                                      footprint, spectrum profile, etc.
application/dvi	          dvi	     graph
text/csv	          csv	     comma separated value
text/tsv	          tsv	     tab separate value
text/html  	          html	     texte in HTML format
application/x-tar	  tar	     multiple files archive in tar format
application/x-directory+text dir     multiple files set as text list

Compression may be applied at different levels:
?	after the data file is built
?	after binding a bunch of files into an archive file (like in .gzip,
         .7zip, .gz, .tar.gz, etc.)
?	directly on the file content (jpeg, hcompress in fits images,  
multi-resolution compression (.MRC files as in MR/1 application)
In this case extension file name conveys the information directly on  
the file content.
No suffix means there is no compression applied.
Example of combined access format could be a concatenation of mime  
short name , with compression suffix.