Both of the value description tables int.vdt and char.vdt are created internally by the writer and the user has no control over them. The scope of both the tables is limited to coverage level, therefore the tables are independent of Writer Mode, since feature class updating is not supported (that is, once they are written, they can never be updated).
Note that the contents of int.vdt and char.vdt for each coverage are defined by the VPF product specification. The value description tables as provided with other schema templates contain an exhaustive list of values as allowed by their respective product specifications. When a coverage is written, int.vdt and char.vdt are created with the values actually used by the feature classes being written to that particular coverage. However, these values have to be one of those provided in the list for that coverage.
Each coverage is provided with an exhaustive list of all the coded values and descriptions for each feature class as allowed by the VPF product specification. The specification does not allow creating new values (codes) in general with the exception of those attributes which have “null”, “unk” or “unknown” values in the vdt’s. Each feature class should get coded values as allowed by the coverage’s vdt. For example:
The character vdt of vmap1\bnd coverage allows only following values for f_code for polbdnl.lft
- FA000 Administrative Boundary
- FA020 Armistice Line
- FA030 Cease-Fire Line
- FA050 Convention Line/Mandate Line
- FA060 De Facto Boundary
- FA110 International Date Line
If an attempt is made to assign, say AL070, to f-code to any feature belonging to polbndl.lft, though the value gets written but the writer warns the user that it could not find it in the vdt table. Also the char.vdt never gets updated with this value.
On the other hand, polbdl.lft has the attributes “nm3” and “nm4” containing “UNK” values. These attributes can have any values, say “Vancouver”. Note these values will show only in the feature class table and vdt’s will neither have these values nor its description.
Since the description can appear in the vdt only and the users are not allowed to create vdt’s directly, any attempt of writing a description of any code (value) gets ignored by the writer. Note that the vdt’s are also provided along with the schema templates which conform to the respective product specifications. The writer actually uses these vdt templates as guidelines to create vdt for the coverage being written. The vdt which gets created with the coverage will contain only the values got used in the coverage.
For a detailed list of all possible contents in char.vdt in BND coverage, see the Boundaries Character Value Description Table in Appendix F of the VMAP1 Specification.