Summary: About domains
- A domain stores the set of possible data ‘values’ that are used by a map, table
and/or column. All ILWIS data objects (vector maps, raster maps and tables) always
have a domain.
- The same domain can be used by a vector map, a raster map and a table.
- The four most important types of domains are: class domains, identifier domains,
value domains and the image domain.
- Class domains are used for data objects that contain classes. A class domain may
for instance store the names of land use units or geomorphological units.
- ID domains are used for data objects that contain unique identifiers. An ID domain
may for instance store the unique IDs of city blocks, or the unique IDs of rainfall
stations.
- Value domains are used for data objects that contain measured, calculated or interpolated
values. A value domain may for instance store the values (range and precision)
needed to define heights or to define concentrations.
- The system Image domain is used for satellite images or scanned aerial photographs
containing values between 0 and 255. This domain is a special type of
value domain, as the image domain cannot contain undefined values.
- Class and ID domains are always user-defined. Value domains can be user-defined,
or system-defined.