### Data Format Identification

UV Tables are now a special subset of GDF files. As any GDF file, they are identified by an integer code stored in variable h%gil%type_gdf (see Table ) which, for UV Tables must take the value code_gdf_uvt or code_gdf_tuv, depending whether the table is in native order (a visibility being contiguous in memory) or transposed. For simplicity, we have imposed code_gdf_tuv = -code_gdf_uvt.

The new layout differs in several ways from the older one. In short:

- The daps are no longer at pre-defined positions in the data file, but their positions are described by the UV table header (cf. the following section for details).
- The complex visibility elements are not necessarily (real, imaginary, weight) triplets, but contain h%gil%natom elements. For example, a zero spacing UV table from a single-dish spectrum may have h%gil%natom = 2 (as the imaginary part is 0 in such a case).
- Provision is made to handle polarization data, by indicating the number of Stokes parameters in a visibility, h%gil%nstokes.
- The number of frequency channels h%gil%nchan and the number of visibilities h%gil%nvisi are available independently of the transposition status of the UV data set.

The UV specific information is included in the uvda section of the h%gil header, as described in appendix .