A domain defines the type of the information available in maps, tables and columns. A domain consists of a set of thematic names or IDs, a range of values, colors or range of colors, a sequence of time. All elements in a map or column must use one of the classes, IDs, or values of the domain that is used by the map or column; if not, these elements will appear as undefined. All ILWIS data objects (maps, tables and columns) have a domain. One domain can be used for several data objects. Domains provide the relational link between maps, tables and columns. A domain may also provide a link to a default representation, so that the colors etc. for the classes or values in the map(s) are known. When you create a map or column, you also have to select or create a suitable domain for that map or column. When you create an attribute table for a map, the table must use the same domain as the map to which the table refers.
The main types of domains in ILWIS are:
Note, when working for instance with multiple land use maps of different years, the idea is that you create only one thematic domain Landuse in which you enter all possible land use classes of all years. This domain can then be used for all your land use maps and related attribute tables. It is not necessary that all classes in the domain actually occur in each map. As a domain is linked to a representation, the use of one domain for maps of the same theme ensures that these maps are also by default displayed in the same colors.
Domains for attribute tables and columns
Maps with thematic or ID domain can be linked to an attribute table as long as the table uses the same domain as the map. In the table, the class names or IDs of the domain appear on the record buttons. You can add attribute columns to the table which contain additional information on the classes or IDs in the map. When you create columns in the attribute table, you may create suitable domains for each column; for general use of values however use the system domain Value.
Numeric domains for maps and columns
When implementing calculation on numeric maps and columns, you can usually accept the default system domain Value and adjust the value range and precision for the output map if necessary. You only need to create a user-defined numeric domain when the maps values represent e.g. Height, pH, Yields, or concentration values. in other words, when the output values have a specific 'standardized' meaning that you want to document or when you need a specific representation for values of the domain (colors are linked to values). A number of numeric domains are available in your ILWIS\SYSTEM directory, namely:
Value, Count, Distance, Min1to1, NILto1, NOAA, Perc, Radar.
Other domain types supported in ILWIS
Bit; system domain Bit is available for raster maps and columns that have to store values 0 and 1.
Bool; system domain Bool and system domain Yes/No are available for maps and columns that have to store True, False or undefined values.
Picture; for raster maps which contain merely colors, the pixels have no meaning, for instance color composites or scanned pictures.
Color; for raster maps that are stored in 24-bit colors.
Group; a user-defined list of values and class names that can be used to classify a value map (similar to the ILWIS 1.41 classify tables).
String; system domain String is available for columns that contain text.