### Dap positions and size

The list of possible daps entities is defined in appendix C, with the associated codes used to tagged them. This list can be extended as required.

The dap position as well as its length can vary. For example, the item dap would be located at h%gil%column_pointer(code_uvt_item), and it could be a real(4) or real(8) number depending on the value of h%gil%column_size(code_uvt_item).

The daps element can be located either before the complex visibilities, or after, but not intermixed with them. Accordingly, there can be h%gil%nlead leading columns before the complex visibility values and h%gil%ntrail trailing columns after. Note that these number indicates the number of 32-bit columns, so if any of the dap is a 64-bit number (h%gil%column_size(code_uvt_dap) = 2), nlead+ntrail will be greater than the number of non-zero values in h%gil%column_size(:).

The existence of daps entities of different lengths raise a specific issue when transposing a UV tables. By default in GILDAS, transpositions are applied on the global data type. For UV tables, this is fmt_r4, i.e. 32 bit entities. Assume we start with a natural (UVT) order, with for example a 64 bit quantity. After transposition to TUV order, the first 32 bit of are in the first column, and the last 32 bits in the second. This is incoherent. Thus a second transposition is required on these specific columns to establish the contiguity of the 64-bit values. This is handled transparently by the GIO library, in gdf_transpose as well as in gdf_read_gildas.

Subsections

- Restriction on the use of real(8) daps
- Are real(4) U,V,W coordinates sufficient?