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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:

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 1: GDF encoding of the system and version. Most of the recent computers are little endian (IEEE) machines. The obsolete system VAX is not supported with GDFV2.
Version IEEE EEEI VAX
1 $-^1$ . $\_^2$
2 $<$ $>$ N/A

$^{1}$ hyphen.
$^{2}$ underscore.




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 $N_{dim}$ (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 $2+2$ 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 $2 + 2 \times (
N_{\mathrm{dim}} \times 3 )$ words).


next up previous contents
Next: Data description Up: Type description Previous: Type description   Contents
Gildas manager 2015-03-01