FITS files of the Library
23 Maggio 2022
The Library uses the "Hierarchical
Grouping" registered convention, in this way it becomes possible to
establish hierarchical relations between the HDUs of the FITS files.
Using this convention it is possible to preserve long term not only the digital representation
of the images of a manuscript, but also the combination of the images as a group belonging to a
single shelfmark, according to its foliation.
The registered convention is mentioned on UNI 11845:2022 in two different use cases in the
annex, it is also used in astronomy.
We would like to remark here that this convention allows to preserve more than one structure at
the same time, ensuring then the capability of preserving different hierarchical relations between the digital objects.
Each FITS file contains a single image, and for each digital resource (e.g. manuscript, incunable, … )
an extra FITS file, the unifying file, that connects
the images of the specimen itself is added.
The unifying file is composed by 4 HDU, namely:
- Primary HDU, it describes the content of the FITS file
- TABLE HDU, it contains the hierarchical connection between the FITS files and points to the FITS files of the shelfmark.
- TABLE HDU, it contains the full dictionary of the header keywords of the FITS file images.
- FOREIGN HDU, it contains the PREMIS file for the whole digitized specimen.
Header keywords
Each image is converted into a single HDU FITS file, a particular attention is paid to the UNI 11845:2022 homogeneity requirement: each FITS file header contains the same keywords in the same order, with the exception of the last keys BAV09_n. In the following table all the header keywords are described along with a comment about the role of the keys and explanation about the values. Custom keys are highlighted in bold and described in better detail, for all the other keys please refer to FITS 4.0 specifications.The new keywords described below come out from a revision of the digitisation project on 2022, see also The project (current phase).
|
Key |
Description |
|
SIMPLE |
First mandatory key for each FITS file, set to "T" means that the file complies to the FITS file format specification. |
|
BITPIX |
Number of bits storing pix values, tipically set to 8 for BAV files. |
|
NAXIS |
Number of data axes, tipically set to 3 for BAV files (X, Y, color). |
|
NAXIS1 |
Length of data axis 1, it matches with the X resolution of the image. |
|
NAXIS2 |
Length of data axis 2, it matches with the Y resolution of the image. |
|
NAXIS3 |
Length of data axis 3, tipically set to 3 for BAV files (red, green, blue channels). |
|
UNIKEY |
Set to "T" means the the file complies to UNI 11845:2022 standard. |
|
EXTEND |
Extensions may be present |
|
HDUNAME |
Unique identifer of the images, it matches with the filename. The goal here is twofold: to supply a minimal semantic content (shelfmark, folium, exceptions) and at the same time to supply a unique id that is linked on the unifying file. |
|
GRPID1 |
Always set to "-1" means that this file is part of a group and that the group details are in the first TABLE HDU of a separate file specified as the value of GRPLC1 key. |
|
GRPLC1 |
URI that locates the unifying file, the final part match the filename of the unifying file and the value of GRPNAME key located on the header of the unifying file. |
|
LONGSTRN |
The OGIP long string convention may be used |
|
CREATOR |
Software that created this FITS file |
|
INSTRUME |
Maker and model of the acquisition device |
|
PROGRAM |
Software that created the image |
|
DATE-OBS |
Date and time of image acquisition |
|
DATE |
Date and time of FITS file creation |
|
EXPTIME |
Exposure Time, it matches the tag exif ExposureTime |
|
REFERENC |
Publication URI. |
|
ORIGIN |
Copyright notice |
|
OBJECT |
Item identification. It matches the shelfmark. |
|
COLORMAP |
Colors mapping, the values could be "RGB", "GRAY" or "BW" for BAV images. |
|
CTYPE1 |
Linear transformation on axis 1, the value is always " ". |
|
CTYPE2 |
Linear transformation on axis 2, the value is always " ". |
|
CTYPE3 |
The value matches the COLORMAP value. BAV uses here the same convention adopetd by HST archive and the well known software Aladin. |
|
CRPIX1 |
Location of reference point along axis 1. |
|
CRPIX2 |
Location of reference point along axis 2. |
|
CRPIX3 |
Location of reference point along axis 3. |
|
CRVAL1 |
Value of the coordinate specified by the CTYPE1 keyword at the reference point CRPIX1. |
|
CRVAL2 |
Value of the coordinate specified by the CTYPE2 keyword at the reference point CRPIX2. |
|
CRVAL3 |
Value of the coordinate specified by the CTYPE3 keyword at the reference point CRPIX3. |
|
CUNIT1 |
Units of CRVAL1 and CDELT1. In BAV files is always set to "mm", millimeter. |
|
CUNIT2 |
Units of CRVAL2 and CDELT2. In BAV files is always set to "mm", millimeter. |
|
CUNIT3 |
Units of CRVAL3 and CDELT3. In BAV files is always set to " ". |
|
CDELT1 |
Coordinate increment at reference point. In BAV files is set to 25.4/XResolution, where XResolution is the value of the homonymous TIFF tag containing the dpi for the X axis. |
|
CDELT2 |
Coordinate increment at reference point. In BAV files is set to 25.4/YResolution, where YResolution is the value of the homonymous TIFF tag containing the dpi for the Y axis. |
|
CDELT3 |
Coordinate increment at reference point, always 1.0 for BAV files. |
|
DATASUM |
Integrity check of data records |
|
CHECKSUM |
Integrity check of this HDU |
|
IMGURESL |
Resolution Unit, always “INCH” for BAV files. |
|
IMGXRESL |
It matches the value of the TIFF tag XResolution (dpi). |
|
IMGYRESL |
It matches the value of the TIFF tag YResolution (dpi). |
|
BAV01 |
Contributor, the name of the institution, organization or individual that have funded the digitisation of this item. |
|
BAV02 |
Lens model, it matches the EXIF TIFF tag LensModel |
|
BAV03 |
Aperture Value, it matches the EXIF TIFF tag ApertureValue |
|
BAV04 |
ISO speed, it matches the EXIF TIFF tag ISOSpeed or ISOSpeedRatings |
|
BAV05 |
Host PC, unique identifier of the personal computer used during the acquisition process. |
|
BAV06 |
ICC profile hex bytes |
|
BAV07 |
Scanner ID, unique identifier of the acquisition device. |
|
BAV08 |
Operator, name or unique identifier of the individual that acquired the image. |
|
BAV09 |
Number of tutors in approval workflow |
|
BAV09_1 |
Tutor 1 full name |
|
BAV09_2 |
Tutor 1 approval date |
|
BAV09_{2*(n-1)+1} |
Tutor n full name |
|
BAV09_{2*(n-1)+2} |
Tutor n approval date |
The keywords of the initial phase of the project are available here:
Keyword FITS (2015)
