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Header description
The exact header definition can be found in appendix B. In
short, it is divided in sections. The first one, named leading
pseudo-section, has a special status because it contains vital information
describing the file format. It (and thus all the GDF files, V1 or V2)
starts with 12 specific characters:
- the 6 firsts are always the word GILDAS (uppercase),
- the 7
is a unique character encoding the system type and the
version of the GDF used (see Table 1),
- the 5 last are a subsequent word used to recognize the kind of data
in the file, namely IMAGE (for standard Gildas images or
hypercubes) or UVFIL (for Gildas UV tables).
In GDFV2, this last character string is only a first order information
about the file kind. Distinction between Images, Tables, UV Tables, VO-like
Tables, is provided by an integer keywords (see
Table 2). Then follows the data format, the various number of
blocks in the file, the version of the GDF currently in use, and the kind
of GDF file.
Table 2:
List of kinds available for GDFV2 files (h%gil%type_gdf). The values are integers that should not be
used explicitely in the code. The developers must use the associated
names.
Name |
Value |
Comment |
code_gdf_image |
0 |
Images or hypercubes |
code_gdf_uvold |
1 |
Old UV Data when the weights were inconsistent with the actual noise |
code_gdf_uvt |
10 |
UV Data in ``visibility'' order |
code_gdf_tuv |
-code_gdf_uvt |
UV Data in channel order (the transposed order) |
code_gdf_table |
20 |
A simple Table, with no information |
code_gdf_vo |
-code_gdf_table |
A simple Table, in the transposed way, as is the case for |
|
|
Virtual Observatory Tables |
code_gdf_xyt |
40 |
A specific CLASS table (by symmetry with code_gdf_uvt) |
code_gdf_txy |
-code_gdf_xyt |
A specific CLASS table (by symmetry with code_gdf_uvt) |
|
|
|
The following sections are standard ones, named
- dimension
- The dimensions of the data in the file.
- blanking
- The blanking and tolerance values.
- extrema
- The value and position of the minimum and maximum value in
the data.
- coordinate
- The axes definition for each dimension, encoded as a 2D
array of
(ref,val,inc) triplets.
- description
- The physical unit of the data and the names of the
axes. The first header block ends here.
- position
- The source description. The second header block starts
here.
- projection
- The projection definition.
- spectroscopy
- The description of the spectroscopic axis.
- resolution
- The beam characteristics.
- noise
- The data noise.
- astrometry
- Proper motion parameters.
- uv_data
- support for UV tables, e.g. description of extra-columns
specific to this kind of tables.
Each section has a parameter describing its length. A zero value means that
this section is not present in the file and/or not filled in memory. For
the programmer or the end-user, any other value means it is enabled (then
all the parameters in the section are expected to be filled). In details,
some of the sections have a fixed length for any file (e.g., the length of
the blanking section is
4-bytes words) and some have a
length that depends on the number of dimensions in the file (e.g., the
length of the coordinate section is
words).
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Gildas manager
2015-03-01